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Saturday, March 20, 2021

Building a Curve Tracer - Version 3

                                                              Mark's contraption

                                                                And this is mine.


Version 3 of the Curve Tracer Project

Based on the experiences we collected with Version 2, we made some significant changes to make it a lot more reliable, hopefully easier to build with commonly available parts, and with greatly increased usability and safety. In addition, we would like to see if we can make more measurements.

More information about this overall project can be found here:

The building of the first version, actually a working prototype with a detailed Theory of Operation.

There is also a description of the second generation based on the first prototype. This is a fully functional CT but has some problems and shortcomings that we're addressing in V3.

During the development and testing phase for Version 3, we use 10x10cm boards that contain the required functionalities so we can easily swap them out with newer revisions. I will describe them below.

The pictures above show that we're working hard on the completion of this version.We now have a fully functional V3 Curve Tracer and we can make measurements. I have started a dedicated Blog with the measurements that we are currently able to make:


This is not a project to fully replace or replicate a Tektronix 577 or 576 Curve Tracer. We do however hope to replace these units as they are currently still used in workshops, laboratories or high-schools/university classes but are dying of age or due to unavailable replacement parts.
We can do many measurements these very versatile instruments can do, but not all. Most likely never will. Not yet anyway. 😏


Completion date

The current goal is to have a finished and published instrument by the end of the first quarter of 2022.



Here is a list of the specifications, but note that they are not final yet!

Voltages/Currents (Collector or Drain)

There are three main ranges that can be selected.

  • 0-35V @ 0-2A
  • 0-75V @ 0-1A
  • 0-200V @ 0-100mA

The voltages and the currents of any of these ranges can be adjusted from 0-100% of the maximum values with full load. The voltage output can be selected between a triangle-based Sweep, which is the default, and also DC for leakage measurements.  The maximum current in the DC mode is half of that for the triangle based sweep mode. An indicator will be added to the x1 current range setting to warn the user not to use this range.

The frequency of the triangle-based waveform is set at about 500Hz but can be tuned by changing a capacitor inside the unit. It can range from 150Hz to 500Hz and can be used to tune the rate of flicker on the display of the oscilloscope.

An indicator will warn the user when voltages higher than approx. 50V are present on the DUT outputs.

There are 5 current ranges to limit the maximum currents in the three voltage modes:

  • x 1
  • x 0.5
  • x 0.2
  • x 0.1
  • x 0.05
  • x 0.02

This means that in the 35V @ 2A range, the maximum current can bet set to 2A, 1A, 400mA, 200mA, 100mA and 40mA. A Current Limiter can be used to further adjust the maximum current from 0-100% in any of the selected Current Ranges.

An LED will signal the current limiting mode, ie when the current to the DUT is exceeding the set current by the Current Range and the Current Limiter (CL).

Step Generator

The Step Generator feeds the Base or Gate of the DUT and can be set to output a current for BJT devices or a voltage for FET devices.

The Step Generator output can be set to generate 0-7 steps.

A Step Delay can be selected to delay the time between complete step cycles to reduce the thermals developing in the DUT and prevent it from over-heating. When activated, the delay is approx. 40mSec between step cycles and can be increased to more than 250mS. The step cycle can be set from 1 to 7 steps.

The output of the Step Generator can be set in a 1-2-5 sequence.

The selections are: 

  • 5V or 5mA *)
  • 2V or 2mA *)
  • 1V or 1mA *)
  • 500mV or 500uA
  • 200mV or 200uA
  • 100mV or 100uA
  • 50mV or 50uA
  • 20mV or 20uA
  • 10mV or 10uA
  • 5mV or 5uA
  • 2mV or 1uA
  • 1mV or 1uA
  • 500uV or 500nA
  • 200uV or 200nA
  • 100uV or 100nA
  • 50uV or 50nA

Note that with the 5V, 2V and 1V ranges, you must reduce the number of steps to avoid clipping against the supply rail of the step amplifier.

An offset can be selected with either a positive (aid) or negative (oppose) offset to the Step output. The offset range has a maximum range of 10V in either direction. Depending on the selected DUT polarity and the selected offset direction, the maximum number of steps in the V-mode will have to be reduced to avoid clipping against the supply rails. For N-channel devices, the positive offset will be limited due to clipping while the negative offset can go the maximum voltage. For a P-channel device the offset will be full in the positive direction, but limited by clipping in the negative direction. 

Both the current and the voltage outputs are corrected for DUT junction voltage drops and other current/voltage drops so composite devices (Darlington) can be tested without influencing the Base or Gate step settings. 

The Step Gen output is protected against damages by high voltages in case of a shorted DUT socket, or a shorted/damaged C-B or D-G junction. A Fault signal will warn the user of this condition as long as it is present and the voltage at the Collector/Drain output will be removed for the duration of the fault condition.

A switch determines the polarity of the DUT for PNP/P-channel or NPN/N-channel type devices in such a way that the origin of the I/V curves always start in the lower left-hand corner of the oscilloscope.

Device Under Test (DUT)

A large variation of DUT's can be connected to the Curve Tracer by means of test sockets or 2mm Banana sockets. Two DUT's can be compared by means of a DUT selector switch that can be off or power the left or the right DUT socket to make comparisons and matching of devices possible.

Many different 2 or 3-pin DUT's can be measured or characterized with the instrument.

Display Device

An analog or digital oscilloscope is used in the X-Y display mode to show the I/V curves of the DUT. There are two BNC sockets available on the back of the instrument to connect the X and Y outputs to the input channels of the scope. An optional Z or blanking signal can be made available through a BNC connector on the back for analog (phosphorous) oscilloscopes.

A multiplier for the measured DUT current can be used to select between x1 and x10 to amplify small DUT currents on the scope and raise the signal above noise levels.


A mains switch and mains fuse is available on the back of the unit. An indicator on the Face Plate will show that the instrument is powered on.

The overall current consumption of the instrument still needs to be determined.

The instrument can be used with 115V 60Hz or 240V 50Hz mains voltages by means of a selector switch inside the unit.

The plastic enclosure measures 25cm x 18cm x 8cm. The overall weight still has to be determined.

The Major Building Blocks

Below is a description of the basic building blocks of this CT. Several of the diagrams and schematics posted are not the latest version or revision. Eventually, that will be the case, and we will publish everything on a Github site, but for now, this shows our work in progress.

After this section with the description of our progress and struggles, there is now also a section where we show the measurements we can currently make with the CT.


1. The auxiliary supply

This is the supply for all the voltages for the Op-amps and other parts. There are actually three segments, all fed by one transformer that has dual primary windings for 230V and 115V based main voltages, and equal dual secondary windings. We need dual secondary windings that are isolated from each other because we need a fully isolated supply for the Step Generator.

One separate winding from the transformer is used for the Step Generator, and the other winding for the rest like the Triangle Generator, the Collector/Drain Sweep supply and the XY amplifier.  This supply generates voltages of +10V, -5V and +24V. The +10V is used as a reference for a few critical circuits, so with the next revision we are going to make this supply adjustable with a trimmer.

The -5 and +10 voltage regulators get a little too hot. This is mostly because of the voltage drop over them. The transformer we currently use is over-dimensioned, but Mark had a number available we used during the testing. In the next board turn, we're going to a lower voltage, lower wattage version.

The Step Gen supply section is the +15V (called plusStep in the schematics) and -15V (minStep) for the Step Generator circuits. This supply is floating (isolated) from all the other supplies and uses a separate winding from the transformer to accomplish this.

The third section is for the X-Y amplifier. It needs -5V (minusXY, the same as minTri)  and +24V (plusXY). We use a multiplier and a regulator to get this voltage. The 24V is needed because we need a minimum deflection of 20V to show the 200V Collector/Drain supply on the DSO, and also enough steps that are measured with the Collector/Drain current shunt.


We've made several changes to this portion of the supply that will be reflected in the next revision.

2. Triangle Generator

The Triangle Generator is the hart of both the Collector/Drain Sweep supply and also the Step Generator. It creates the triangle wave form that is the basis for the Collector/Drain Sweep supply, and it triggers the Step Generator so the steps are aligned with the triangle signal.

This circuit is quite simple, Two Op-amps and two gates create a free running oscillator that is also an integrator created by the feed-back capacitor of the output Opamp. The value of this capacitor determines the triangle waveform shape and the frequency. The R/C delay between the two gates is there to compensate for all the propagation delays in the Step Gen circuits, mainly the opto-coupler, and is needed to synchronize the triangle transitions with the Step Gen step transitions at the DUT. We're currently looking into making some changes to the circuit.
The output of the first Gate is used to create a trigger signal for the Step Generator. With the remaining two Gates we can create a single trigger pulse, either positive going or negative going, or both. It is not shown on this earlier version of the diagram, but this can be selected by jumpers. This allows us to synchronize the beginning of the Step sequence by either the top of the triangle, the bottom of the triangle or both. The consequence has been explained in the other two Blogs and will be explained later in the Step Generator circuit.

3. The AC Collector/Drain Sweep Supply

The AC supply section is the first stage for the regulated Collector/Drain Sweep supply. 
It contains the two transformers for the voltages, the switching of the voltage ranges and the transformer winding switches.
We are using two transformers to create the 35V, 75V and 200V ranges. The ranges are selected by a special on-on-on DPDT switch on the front panel that is wired to create three positions. The 35V switches the two secondary windings in parallel, to double the current. The 75V position switches the windings in series, and the 200V position adds the high voltage transformer "on top" of the main transformer to create a unregulated voltage of about 230VDC.
Bleeder resistors are used to make sure that high voltages are quickly brought to safer lower levels. We use a beefy relays to switch the secondary windings of the main transformer from serial to parallel, and use another relays to switch-in the 120VAC transformer for the 200V supply.

Adding Voltage and Current ranges

While we were going through the redesign of the Collector supply controller, I was experimenting with a 120V transformer to see if we could reliably go up the the 200V that I set as the goal for Version 3. Adding that transformer into the mix would cause more switching issues, and I was concerned about the safety aspects with these high voltages. Richard who was continuing to profile Version 2 of the CT was also experiencing problems with the voltage and current settings of his version. He and I continued to blow small signal transistors that we used as the DUT, mostly due to thermal issues when the voltage or current became too high for the poor DUT. This is very easy to do when you try different step settings and the variable current limit is set too high. With the current configuration, the current limit can be set from zero to 2A and the voltage from zero to 75V both using normal potentiometers. Extending the voltage to 200 V would make it even more difficult to adjust the voltage. These two adjustments make it way too easy to select a higher voltage or higher current than the DUT can handle when you are searching for the best step generator setting.

After many deliberations and tests, we decided to offer the following main voltage and current ranges: 

  • 0-35V @ 0-2A
  • 0-75V @ 0-1A
  • 0-200V @ 0-100mA.

The voltages are available with the maximum current load. These ranges are created by switching the AC side of the main transformers, and also by limiting the triangle waveform at the input to the Sweep supply. We use MOSFET's that are activated by the Voltage Selection switch to limit the voltage by limiting the input voltage by resistors and trimmers to ground so we can calibrate each of the three voltage ranges.

We also added a current range selection that will allow you to set the maximum current in any of the main voltage/current ranges, so you can more easily protect the DUT by selecting a maximum current.

The current range attenuation selections are:

  • x1
  • x.5
  • x.2
  • x.1
  • x.05
  • x.02

This means as an example, that when you select the 75V @ 1A range, you can set the current selector to 0-1A, 0-500mA, 0-200mA, 0-100mA, 0-50mA and 0-20mA, and still use the current limiting adjustment to go from 0-100% within any of these ranges.

These ranges are created by a rotary switch located on the front panel that changes the reference voltage for the current limiter circuit. They work in tandem with the already mentioned Voltage range shunts and this is why they have a multiplier and not an exact number.

High Voltage Warning Indicator

We wanted to warn the user when dangerous voltages are present on the DUT output connectors. We determined that this is at about 50V and higher. Originally, we used a more complicated circuit that was part of the original range switching of the transformer. Mark came up with a simplified method that just uses a single transistor to drive the LED on the front panel when the 50V threshold is exceeded. After trying that on the new AC supply, we could even make it more simple and avoid having to use a trimmer. This circuit now works very well.

There is also a Current Source circuit on this board. It provides a 12.5mA load on the Collector/Drain supply for regulation stability. See the next section for details.



We've made some more tweaks and changes and are currently on Revision 7 for this board. 

4. The Collector/Drain Sweep Supply

The Collector/Drain Sweep supply is one of the two main sections of the Curve Tracer, the other one is the Step Generator discussed below.

The Collector/Drain Sweep supply is made up of two sections. One is the AC supply section supplying the raw DC voltages, and the other one the regulated triangle based buffered output section that we call the Sweep Supply.

Initially, we used Opamps to control the Voltage and the Current regulators, and power transistors or Darlington types for the regulation but we were having all sorts of problems to provide a clean triangle waveform at voltages ranging from 0-200V and with currents ranging from open circuit to 2A.

While we were having these issues with the stability, I decided to call in the help from my friend Bud, an ex chip designer from LT. He and I worked together remotely as mouse-pal's on a few other projects, most notably on the UPS power supplies for the Raspberry Pi Model 3 and 4 and a differential scope probe, both described in different Blogs on this site.

Bud didn't have our hardware at his disposal, but used LTspice extensively. In simulation, it turned out that due to the Opamps we selected, in combination with the series transistor types, we badly needed different compensation configurations. Over the course of many weeks, investigating, discussing and trying things out, we eventually arrived at a different design that uses MOSFET's as the series regulator. However, we were still having difficulty taming this tiger and were spending a lot of time trying different methods and circuit variations with an attempt to get it right in all conditions.

Bud's Wild Hair idea.

Bud could not leave the challenges the Sweep Supply circuit posed out of his mind, and started working on a novel and different solution that would accomplish a better regulation transition from voltage regulation into current limiting. He called it a "Wild Hair" idea.

This plus a number of other developments and additions resulted in the busy diagram below.
This circuit looks pretty complicated but I will break it up into a number of sections and explain it in more detail.
This new circuit uses discrete trans-conductance amplifiers for the voltage and current controllers. This type of amplifier converts a voltage at the differential inputs into a current at the output. This is what we were trying to do, kind of, earlier but in a much more complicated way. The two regulators are now fully discrete using matched transistors (Bud was a chip designer after all), and the LTspice results looked very promising. Gone are the troublesome transitions from voltage regulation to current regulation and back and no more desperate hugging of the Opamps to the rails.  

The Sweep Collector/Drain Voltage Regulator

The major difference between our Curve Tracer and many others, like the bench Tektronix units, is that we do not use transformers to create a half-sine wave sweep supply, but use a triangle based wave form. This was also done with the Tek 5/7CT1N scope plug-in units that I was very familiar with, because I built one myself, described in another post on this Blog.  
In the bench designs where transformers are used, they used a large number of taps on the custom made transformers to create different voltages. It is relatively easy to limit the current from these taps by switching a resistor in series. We use a different method to create the voltage because we do not have access to these multi-tapped transformers and we do not want to be limited to twice the mains frequency of 100 or 120Hz for the Sweep frequency.
The other limitation is that you need very complicated and sturdy custom made rotary switches and many relays for switching the transformer tab selections and also for the many resistor selections. These complicated switches are one of the major reasons that many of the Tek CT's are no longer serviceable.

We use an electronic method to generate different voltages and also to limit the current of the triangle based voltage. This is analog to a Lab power supply with the major difference that a Lab supply is only needing this for a DC voltage, and we need to do the same for a triangle waveform. This means that the current limiter needs to limit the highest voltages of the triangle waveform, and that with the frequency of the triangle, because it is repetitive. The transition from voltage regulation into current regulation is a touch challenge if you also consider that the voltage can range from 0 to 200V, and the current from 0 to 2A.

This is why, after going through many different variations, we decided to use discrete trans-conductance amplifiers for both the voltage and the current regulator circuits.

The Voltage Regulator

Below is a reduced schematic just dealing with the voltage regulator portion.
The triangle signal from the triangle generator comes in on the left. The input voltage is about 7Vp-p. The trimmer in series is used to calibrate the maximum output at 200V in that range. Next are the two MOSFET switches that will limit the triangle voltage going to the amplifier. There are two switches, one for the 35V range and one for the 75V range, each with their own trimmer to calibrate these two voltages. The two MOSFET's switch the triangle signal with a resistor to GND to reduce the voltage. One is used for the 75V range and the 35V range switches a resistor in parallel to reduce switching glitches. The signals that drive the MOSFET switches also drive the relays for the three voltage ranges.

The triangle input signal is entering a trans-conductance amplifier created with matched transistor pairs. The input is protected with a set of limiting diodes. The output of the amplifier is protected with another set of diodes and is used to drive the Gate of the main MOSFET, Q1. The Drain is connected to the positive DC supply. The Source connects through a little balance resistor via a current sense resistor to the main GND.  

Driving the parallel MOSFET's

Quite novel is the circuit Bud designed to drive the parallel MOSFET's. In most circuits that I know, there is a separate Opamp to drive the second (or more) MOSFET and it must make sure that the load is in effect really shared. That kind of a circuit is a little more difficult to realize in our setup. Good load sharing is not so simple to do in reality, because with the MOSFET's in the linear mode, a minute change in the Gate drive will cause a major change in the conductivity and hence the temperature. Bud came up with a "current duplicator" circuit where he uses an Opamp that measures the current through the main MOSFET, and drives the "slave" MOSFET to conduct the same current. This works really well.

We have seen situations by which the Opamp has been blown during a fault situation, so we added diodes to the rails to protect it.

Voltage Regulation
The feed-back loop to regulate the voltage is going through a 100K Sweep Voltage potmeter located on the front-panel and that connects to a summing point to the regulator Opamp through a 1K5 resistor. This resistor sets the minimum output voltage to about 2Vp-p to aid in the regulation and also protects the potmeter when it is in the lowest setting with the highest output voltage of 200V.

More details about the regulation principle can be found in Blog 1 that deals with the Theory of Operation and it explains how the regulator is tied to GND, and regulates "downwards". This is analog to the Harrison Labs (HP/Agilent) method of designing Lab Power Supplies. (Tech Letter)

The Current Regulator/Limiter

The current controller seems to be a little bit more complex, but is actually not much more complicated than the voltage controller.

 In principle, the current controller measures the current through the main MOSFET and compares that with a set reference. If the measured current is larger than set, the Gate of the main MOSFET is pulled down to reduce the voltage and hence the current. The second MOSFET will dutifully follow along.

The output current is measured by using shunts of different values to make them equal in the three voltage/current ranges. There is a 10R shunt for the 200V range, this will produce up to 1Volt across this resistor at 100mA, the maximum for this range. We switch other shunt resistor values in parallel to the 10R to create the 1A and the 2A ranges to reduce glitches when switching. The shunts are switched in circuit by MOSFET's that are activated with the same switch that controls the other voltage range settings. 
The reference voltage for the current controller Opamp is set at 1V by a voltage divider located on the left side of the schematic. The different current shunts for all three ranges will produce a 1V level at the maximum for the ranges, 100mA for the 200V, 1A for the 75V and 2A for the 35V range.

We create current range attenuation for every range by changing the reference voltage through resistors to GND. The attenuation values are x1, x.2, x.5, x.01, x.02 and x.05. This selection is made by a rotary switch that is located on the Front Panel. The reference voltage is buffered by a unity-gain Opamp and fed into the trans-conductance amplifier.
The trans-conductance amplifier is also made-up of two pairs of matched transistors.  The output of the amplifier drives a 2N3904 transistor that is pulling the Gate of the main MOSFET down to regulate it. The output is also going to a pair of transistors that switch the Current Limit LED so there is a visual indication on the Front Panel when Current Limiting is in effect.
Below is the current limiting in effect with a 500Hz triangle waveform in the 200V range. This frequency is the practical upper limit we need, although we can go higher. Note that in this particular screen shot, the triangle wave-form is going negative, with the zero Volt at the top of the display.

The current limiting keeps the triangle voltage from reaching higher levels (here going down) by simply "stopping" it, creating the horizontal line.
Note the clean edges top and bottom and the utter lack of wiggles and glitches during the transition from voltage regulation (the triangle slopes) and the current limiting taking over (the flat portion). This is a far cry from our earlier versions so we made the decision to go forward with this design. 

The Current Source & Compensation

To increase the stability of the regulation, we use a Current Source that pulls about 12.5mA from the supply, regardless of the voltage range. This circuit is part of the AC supply. In the 35V range with 2A and the 75V range with the 1A maximum, 12.5mA will not be noticed. In the 200V range with only 100mA however, this will create an error so to speak of 12.5%, because it is part of the 100mA budget, which is too much.  In order to compensate for the approximately 12.5mA, we need to keep the 1V current reference at a maximum of 100mA. We do that by adding a parallel resistance circuit that can be used to calibrate and subtract the current of the Current Source from the output so the maximum net current is 100mA. This is the circuit around R47, R52 and Pot4 in the middle of the diagram. BTW, R46 was an earlier attempt but is no longer used.

Output and thermal stress test

In order to verify that the unit can provide the maximum voltages with the maximum currents while all thermals are OK, I ran a few stress tests with the maximum load for every voltage range with the maximum current, by using (cooled) load resistors across the output. I ran the tests for about 10 minutes each so see if we had any issues.
I used an IR-camera ( a new toy for me) to have an overview and make detailed measurements.

Obviously, the two MOSFET's are the most involved. The picture above shows that the thermal balance between the two is excellent, and I also measured that the temperature of the hot-spot on the device package itself was not above 50C. The three tests passed with flying colors, although this was with everything in free air. We need to do the tests again when everything is inside the enclosure, but it looks like we have the thermals under control with a normally operating CT and don't need a fan. More extreme/fault tests are described below.

DUT protection circuit

During my extensive testing of DUT's with the previous version CT, I had a major disaster happening that blew out many parts on the Collector/Drain supply and also the Step Generator. We needed some sort of protection from this happening again.  More of this is described in the Step Generator section, but the solution we adopted was to disable the Collector/Drain output that could find it's way into the Step Generator circuits. In my earlier case, it turned out to be caused by an almost shorted C-B junction, but can also happen by a pilot error creating a short on the DUT socket.
We now have an input coming from the detection circuit on the Step Generator and that feeds-in to this supply to kill the output by disabling the triangle input the same way as the Voltage ranges do. We use a discrete SCR to control the triangle voltage and use an R/C filter to keep the voltage off for a few milliseconds before it releases the triangle again, making it fully automatic and dynamic.

At the same time, an LED is lit on the Front Panel to indicate the Fault situation to the user.


We need to do a lot more testing but also the other changes and features we put on this circuit seem to work really well. Do we have a winner? It sure looks like it, but we're not done testing yet.
We are now at Revision 8a for this board.


5. The Step Generator

The Step Generator consists of two parts, first the digital to analog (D2A) section that uses the signal coming from the Triangle Generator and creates the stepped waveform. It also has the ability to set the number of Steps and has the Step Cycle Delay circuit. The second part has the Buffered Output section with the output step size selection in Volts or Amps, the offset and the polarity switching.

The D2A section


The optical isolator in the top left of the diagram is where a pulse comes in that is generated by the triangle wave form generator. The edges of the pulse are aligned with the triangle transitions. The signal goes through two gates and enters a CMOS 4040 counter. The counter outputs a count for every transition of the trigger pulse. The resistor ladder network at the output sums the counter outputs and thereby creates the stepped staircase waveform, which is the basis for the Step Generator. The output of the resistor ladder network is buffered by an Opamp to preserve the equality of the steps, while the gain can be set by a trimmer to calibrate the individual step size. Another Opamp creates an inverted signal and together they are used to activate NPN or PNP type DUT's.

The output of the 4040 counter is also going to a CMOS BCD-decimal counter that is used to set the number of steps per cycle. The output resets the 4040 counter, starting a new cycle. The selection of 1 through 7 steps is done through a rotary switch located on the front panel. On the next turn, we will also create a 0 step setting, useful for leakage measurements.

To control the thermal heat that can develop in a DUT during a measurement, a delay circuit around a 555 timer is used and driven by a potentiometer with a switch on the front panel and is used to turn the delay feature on and make the delay variable. This function sets a delay between complete step cycles, which can be from 1 to 7 steps. See a more detailed description below.

The Buffered Step Gen Output Section

 This is the Rev3a that we currently use, but note that we are already on Rev4.

The input for this section comes from the previous circuit. The connector in the top left section is where the positive NPN or negative PNP stepped waveform is entering this buffered output section. The switch that is located on the front panel selects the polarity. This input signal is summed by an Opamp together with the Volt output feed-back signal, coming from an Opamp buffer.
The output goes to another Opamp that is used to create a positive or negative offset to the stepped wave form. The offset controls are on the front panel. The output goes to a complimentary transistor buffer. The output from this amplifier goes through a resistor, selected by a rotary switch that sets the output current or voltage going to the DUT Base or Gate.
In the case of a current, there is a feed-back to the Opamp that also deals with the offset. The feed-back is needed to keep the output to the DUT linear and compensates for the junction voltage drop. A similar feed-back loop is needed in the voltage mode to create steps of the selected voltage size. The Voltage mode is selected by placing a 1K resistor from the output to ground. This will also create uniform range settings between the current and voltage mode setting.

The above Rev3a diagram shows the latest version of all the protection circuits we added, which is needed when the Collector/Drain voltage accidentally connects to the DUT Base or Gate connection, potentially injecting up to 200V into this circuit, and destroying it.
In this latest version, we used two optical isolation circuits that fire when voltages beyond the +/- 15 Volt supply rails are detected. The optical isolation is needed to keep the Step Gen circuit floating from the rest of the CT. This fault signal goes to the Sweep supply and is used to "strangle" the triangle waveform to the input, protecting the Step Gen from harm.

More details can be found below.


This is a picture of the earlier Rev2 board. The white PCB in the overview picture is the current Rev3a.

The Rev2 version has been verified and tested. It had a few issues, like an incorrect foot print for the package holding the two MOSFET's.  In this version I used a jumper to select the output current or voltage settings to save a rotary switch. The sliding switch is used to select the voltage or current mode. 

A newer revision (Rev3a) has been designed that has the new protection circuit on it and implements both the "old" offset circuit and the new one, designed by Mark. His version allows for a larger adjustment range in the Volt mode. We also added the circuits to add a few more outputs and used a 1000x multiplier to allow the use of an inexpensive rotary switch, rather than the expensive and single source 18-position one we initially selected. That idea did not work and the board has some more issues and we're now at Rev4.


Protecting the Step Generator

In the post about the Version 2 CT experiences, I already described the massacre that happened when there was a major catastrophe with the Collector supply. This event showed that the Step Gen was not protected from the high voltages that could make their way into the Base/Gate circuit and cause havoc in the Step Gen circuits

As I already mentioned in that post, Opamps have a really hard time dealing with voltages on the inputs that are greater than the supply voltages. In our case, they are +/- 15V, while the Collector supply can be as high as 200V.

If you realize that there is only a single N or P-junction of a few microns separating the Collector from the Base on a DUT, its easy to see that this can go horribly wrong. If you blow the Collector-Base junction, you have a serious problem. When I examined the 2N3904 or 2N3906 transistors that I blew up, there were several that suffered from a blown C-B junction for the NPN or a blow E-B junction for the PNP. In those cases, that resulted in a low junction resistance, putting the full Collector voltage through the Base back into the Step Gen output and blowing-up parts.

The protection we already added as a modification by using a 100K series resistor and clamping diodes to the rails will help to protect the voltage feed-back Opamp, but that still leaves the rest open for destruction.

I looked for days and studied other CT designs and looked for possible protection circuits for high voltage protection for Opamp inputs and did not find any protection methods for voltages over say 40V that could be used in our application.

Not knowing how to go further, I called in the help from Bud again, and after some brain-storming and long days, he eventually came-up with a clever circuit that uses MOSFET switches to disconnect the Step Gen output section from harms way when the voltages go beyond the supply rails.

Bud found out by using LTspice that there is still an issue when the output of the MOSFET supply gets shorted. This can easily happen at the DUT connector, when the Collector gets shorted to the Emitter somehow. This can be caused by a pilot error, or a blown DUT.
He came-up with some more measures that will add better protection to the supply. At the same time, we found that we needed even better protection for the Step Generator Buffer circuit due to the changes that were introduced by the new offset functionality. There is an additional Opamp that we now need to protect.
After a lot of trying things out with LTspice, and discussing and trying different methods, Bud came-up with -in hindsight- a quite simple method to add much better protection, at the same time eliminating a number of parts. Instead of protecting the output of the Step Gen to voltages up to 200V coming from the Collector/Drain supply, we now simply clamp the input to that supply and remove the output when a problem is detected. 
The Step Gen circuitry still sees the 200V in case of a short, but only for a very short time, until the protection circuit gets activated and is eliminating the output. The circuits must still be able to withstand the 200V, but aren't required to withstand high power dissipation. The protection is active, meaning that as long as the fault is there, it is active. When the fault is gone, everything goes back to normal. No manual reset required, and no fuses to replace. A fault indicator LED will be added to the front panel to warn the user.

Because both supplies are on different and floating rail supplies, the detection signals that need to drive the Collector/Drain supply need to be isolated with optical-isolators.
This means that both the Collector/Drain supply board and the Step Gen Buffer board needed yet another board turn to accommodate these changes and also add all the other refinements and changes that were made. 
Below is a rudimentary schematic showing this new output protection method for the Step Generator circuitry.

This circuit provides some of the protection to the Step Gen Buffer output against lethal Collector/Drain supply voltages that could find its way to the Step Gen output. This can happen by accidental pilot error or a destructed DUT device. As I stated before, Opamps have a hard time surviving when voltages are present at the inputs that are beyond the VCC and VDD supplies. The trick was to detect that situation and prevent damage. The circuit above does that.

The output of the Step Gen Buffer circuit, also going to the Base/Gate of the DUT, comes in on the left hand side of the diagram. Diodes D1, D2 and the two opto-coupler diodes work together to create a fault signal. When the Base/Gate voltage is going beyond one of the +/-15V supply rails, actually at +/-18V,  one of the opto-couplers will fire and turn on the dual transistors configured as an SCR. The SCR flips and will turn on both MOSFET's. Q3 will strangle the triangle signal input to the Sweep supply and completely remove the Collector/Drain output voltage. Q4 is used to turn on a fault indicator on the front panel so the user is alerted.

The R/C set by C1/R9 will release the SCR after about 15mS. When the fault is no longer there normal operation continues otherwise the output remains clamped.

After I got a working board, I collected enough courage to test the safety feature. I used my lab supply to inject a DC voltage with a low current to the output of the Step Gen, and slowly increased the voltage. At +/- 18V the safety circuit cut in, and eliminated the triangle signal going to the MOSFET output section, reducing it to zero and lit the warning LED. Success!
Now I need to find enough courage to really put this circuit to the test by applying the full 200V triangle waveform the the Base connector of the DUT. I also need to do some short tests on the 35, 75 and 200V volt ranges. Initially, I avoided doing these tests in fear of blowing something up that I didn't have the replacement parts for in case something went wrong.

Step Gen protection Test

I finally found the courage to do the real Step Gen protection test. To make the test, I added a small jumper wire between the C-B connections on the DUT test socket. I monitored the X-output with my DSO and used the CH2 connected the the Y-channel, or monitored the Fault signal with a 10x probe.
I first selected the 35V range, and selected a few volts for the output so I could see it on the scope. I also selected the X1 current range, but set the CL to minimum.
I first tried the Voltage mode because this has a different feed-back loop. I slowly increased the CL setting so I had a signal and increased it further to just before the Fault kicked in. 

The triangle is at 17.6V, just below the tripping point. The Y-amp shows the Base voltage.
I then increased the sweep voltage until the fault signal came on. This was at 18.4Vp-p. Further increasing the voltage only changed the shape of the output, but not the voltage.

Switching to the 75V range or as the acid test, the 200V range and turning up the voltage to the maximum 200V did not change the situation. The output and feed-back loops of the Step Gen are now fully protected.
I also used a 10x probe to look at the Fault signal in relation to the sweep voltage and this is the result:

As soon as the triangle waveform goes beyond 18V, the protection kicks-in and that turns off the triangle feed to the Collector/Drain voltage output. It keeps the signal off for a little while, and then frees it again.

In the Current mode it looks a bit different, but works the same.

There is no Base signal displayed in the current mode. The sweep voltage is again clamped at 18V.
This protection circuit works really well. Kudos to Bud!

Extending the Step Gen attenuation ranges

It had always been difficult if not impossible to measure high gain devices like Darlington transistors with the previous versions. The lowest setting of 1uA/step was still far too high, so we added 4 more settings by switching from a 12 to a 16 position rotary switch. The added settings will be for 500, 200, 100 and 50nA/Step.


New offset circuit

Mark was not very impressed with the way I implemented the offset feature for FET's. I simply used the same circuit for both BJT's and FET's, but that's not ideal. The trouble is that with a normal step voltage for FET's, almost any offset will drive the output into the supply rail. You can circumvent the problem a bit by lowering the internal step voltage, reduce the number of steps or increase the supply voltages, but that's still a work around and not a very good solution.

Mark figured out a way to create a dedicated offset circuit for FET's. The BJT version will stay the way it is. The user will not even know about this, because the switching from one offset circuit to the next will be accomplished by the same BJT to FET selection switch already on the front panel. We just changed it from a SPST to a SPDT version to activate either circuit.


Dealing with thermal issues

When you're testing devices with higher currents, there are two effects you have to keep in mind. One is that the self-heating of the device while you are testing, can distort the I/V display because of an effect called looping. This is caused by the way the DUT is activated. With the single step level at the Base, the Collector is getting a raising voltage due to the triangle based supply. The higher the voltage becomes, the higher the thermal heat will be. This typically results in a gain change, so the curve will bend up a little. If the triangle voltage now goes down during the same step, the heat dissipation gets less and the gain changes again so the curve will bend down a little. This causes the typical elliptical looping of the traces. Below is an example of a very minor case on a Tektronix CT.

The thermal heat of the DUT die can increase very rapidly and can get very hot, so much so, that you have to stop the test to let it cool off again. If you don't notice it in time, your DUT may have been damaged or even died of a heat stroke already. With high currents needed for power devices, you can't even run the test for more than a few seconds and the looping of the trace can become so big that it will be impossible to interpret.

There is a way to cheat however. When you select only one half of the waveform per step, as you can do with a triangle waveform, you can eliminate the looping effect. This is what we do now standard with this version 3. When you use a half sine-wave, as the Tek 576/7 do, that trick is not possible. Look at the first Blog of building the CT project to see a more elaborate description.

Professional Curve Tracers allow you to use a pulsed step mode, by which there is a pause after each complete step cycle. This gives the DUT some time to cool down before the next step cycle arrives.

I implemented a similar functionality for Version 3 by adding a few components to the Step Gen circuit. With a potentiometer adjustment, it adds several Milli seconds of delay between step cycles. The delay is synchronized with the end of the step cycle, and can be applied for every number of steps from 1..7.

Below are two screen shots that show this feature in operation on a prototype. The first picture shows the normal operation but shown in the time-base mode of the DSO.
The second picture shows the delayed step function. It starts with about 40mS delay between the step cycles. This can be extended to about 150-200 mS by using a potentiometer on the front panel. This is the practical maximum because the trace starts to "flicker" due to the pause between the X-axis acquisitions. The display can be adjusted somewhat by changing the time/div. setting of the DSO to make it as smooth as possible.

We are currently at Revision 3a for this board.


6. The X-Y Output Amplifier and DUT circuit

This circuit contains the X-Y amplifier section together with the select-able IC current shunt and also the DUT test section.

The ZIF socket is used to try various DUT's during the testing and verifying of the CT with the 10x10 boards.

Dealing with the XY display noise level

Because I'm using a relatively inexpensive DSO, a Rigol DS2072A, most of the X-axis displays for small signal transistor currents are very noisy on my DSO because I have to use V/Div. settings that are in the mV area and they show a lot of noise. It's not so much the DSO itself that is noisy, but the combination of the DSO input circuitry and the pick-up of noise makes the traces very fuzzy.

Both Mark and Richard use professional scopes and they don't have this issue.  There are two solutions. We can add another Opamp with higher gain, but that will also amplify the noise from the source. The other solution is to use a higher value shunt. Both solutions will allow you to avoid the lower level Volts/Division settings of the DSO. 

I decided to use two different IC shunt resistors, because using a single 1 Ohm resistor does not make sense. (pun intended)

By adding a toggle switch to the front panel, the user can now select a 1 Ohm and 10 Ohm shunt resistor values, in effect multiplying the IC current by x1 and x10. This will allow you to use V/Div. settings that are a factor ten higher and therefore more free of noise and we will have a better sensitivity at lower currents as well.

Here is a measurement taken with the 1 Ohm shunt, and a 2mV/Div. setting on my Rigol.

Here is the same measurement, but now with the 10 Ohm shunt, allowing you to go to a 20mV/Div. setting.

Note that this was made with the "dual triangle sweep" per step. Also, the small "opening" in the trace for the second step is caused by the period the DSO needs to process the acquisition of the collected data. This "hole" only shows when you make a screenshot. Look at the first post of building a CT for more information.

Small offset on the Y-axis

Richard has found that on his Version 1 CT there is a slight offset of the X-Y picture. This is probably caused by part tolerances. He fixed it by shifting the output of the X-Opamp a little with a high value resistor to a rail. We're going to add a trimmer that can be populated when required that solves the problem by adding a pos or neg offset calibration to the Opamp itself. 


We have a fully functioning Version 3 Curve Tracer

Mark has finished populating the latest revisions of the Collector Supply (rev 8a) and the Step Gen output board (rev 3a), and also added the DUT/X-Y amp board to the collection. He now has a fully operational V3 Curve Tracer spread out on six 10x10 boards.


Here is a measurement of a 2N2904 transistor using the dual slope triangle method:

And here a 2N7000 MOSFET, using the Step offset feature to "lift" the first step to conducting.

Due to my vacation, I was behind the curve, but Mark sent me the Rev8a Collector/Drain Supply and the Rev3a Step Gen output boards with the SMD parts already populated, so I was able to quickly add these two boards to my own setup. This has been done and after a few small fixes, the two boards are functional. I also populated the DUT/X-Y amp board to get a fully functional CT as well.

This is my own fully functional setup: 

Here is what you see on the picture:

Top left with the transformer on it is the Aux Supply. To the right of it is the AC supply withe the green color and with the very large capacitor. Above that is the main AC supply transformer mounted on the back-panel of the enclosure with the main socket, fuse and main switch. To the right of the AC supply is the 120VAC transformer. On the top right is the red board for the Collector/Drain Supply. It has the large heat-sinks on it where the MOSFET's are mounted on. The two potmeters are the Voltage selection and the Current Limiter.

In the middle, you see a blue board that is the Triangle Generator and Step Selection. The number of steps is selected by a wire jumper.

To the lower left in black is the Y-Y amplifier board with the DUT section. There are two small coax cables going to my DSO. I have a power transistor in the ZIF socket.

The white board is the Step Gen Buffer output board. The potmeter to the left of it is for the Offset adjustment. Mark had an idea to use a divider jumper to reduce the Step outputs so we could use less expensive rotary switches.  With this 10x10, we can use a simple jumper on a row of pins to select the output settings.  Unfortunately, Mark's idea did not work in reality, so in order to add the 8 lowest settings, I used a small test board with THT resistors and a 16 position rotary switch instead of the jumper selection to create the full range. It is not pretty, but functionally does the job although, not surprisingly,  there is a lot of hum and noise in the lowest uV/nA settings.


First measurements to test the functionality

Below is a measurement of a 2N3904 as an example (20uA/Step) using the single slope triangle method.

First is the traditional Time-Base picture of the Collector voltage and the Base current and below it the I/V plot in the X-Y mode. The Step is not flat because the Collector voltage will change the gain, and hence the current. You can see that with higher steps (current) the flatness changes quite dramatically. This shows up in the X-Y plot with upwards going slopes at higher Base currents (the Early effect). This is quite typical for a 2N3904. There are other transistor types where the lines stay horizontally flat for every step, meaning that the gain or beta performance is more uniform across the Base current or Collector spectrum. 

You need a Curve Tracer to see this effect.

Also note how much cleaner this X-Y plot is with the single slope method compared to the Version 1b and also the Version 2.

We identified a few issues that we were able to resolve or fix already and are working on a few things we need to try, possibly involving yet another board turn, but the CT is now fully functional and a lot of testing and profiling still lies ahead of us.

The BJT dV/dt problem

While profiling the CT, we stumbled on a rather strange phenomena that we're trying to understand and see if we can explain it, or better yet, design it into oblivion.

This phenomena shows up at very low Base currents for BJT devices, and also with higher Collector voltages. For reference, look at the screen shot above made with a 20uA/Step and a 12V Collector voltage. When you reduce the Base current to lower levels, we see a change in the Step function. There is a sudden drop when the Collector voltage (the triangle) changes direction. This drop in the current creates a double line display for every step. Below is the Time-Base picture to show the situation at 1uA/Step. Below that is the X-Y plot with the resulting double lines due to this drop.

BTW, the "funny" transition of the first step in the lower left corner is due to the turn-on point of the B-E junction (at around 600mV) and that is always there on the first step, seen with low step currents. Look again at the Time-Base picture. Note that the very first edge of the first step in the cycle, is a little bit to the right of the start of the triangle. When the triangle voltage goes beyond the junction voltage, the device turns on, but that is with a bit of a delay due to the triangle shape, and that's what you see in the X-Y plot. That's not the issue at stake though.
With the help from Bud we now understand that we seem to have introduced an extra stray or "rig capacitance" that is introduced in the C-B junction, which changes the dV/dt constant and creates a charging/discharging phenomenon at the turning point of the Collector voltage, causing the transition. 
The size of the the drop is related somewhat to the triangle frequency (dV/dt : the "t" makes it timing related) but even more to the C-B capacitance. Every BJT device will have some capacity, called the Miller capacitance, but our circuits may add about 10pF in addition, which Bud's simulations in LTspice proved. We're now trying to find the source of this "rig capacitance" and may have to go to yet another board turn to see if we can eliminate it.
When I started to test more DUT variations, I ran into related issues that are most likely caused by the long wiring and long interconnects with the current setup, and the modifications we had to make to get all steps functioning.

I'm pretty certain this is all caused by our current setup.

To continue to test and profile while trying to minimize this effect, I lowered the base frequency to about 155Hz. My DSO display is still very nice at this repetition rate so no harm is done, although it may be different on an analog scope. 

Moving forward again

Mark and I made the decision to not go to two separate 10x10 board turns again, but to go straight to the layout for the front-panel PCB we will need eventually. This board will combine the Step Gen buffer section, the DUT section and the X-Y amplifier section. When we do that we can make sure we eliminate long connections and avoid adding parasitic/stray capacitance as much as possible.


The Front Plate

I had made a front-plate design earlier that we will now start to follow. It is hopefully the final design. (it is not) 

VBA stands for the initials of our surnames. The face plate will be mounted just in front of the front-panel PCB, and it will be a PCB by itself using black as the color and with white silkscreen for the text.

This is taken from an uploaded Gerber set and an online viewer. The plated mounting holes serve as a  connection to the also plated back side and will reinforce the board and also act as a shield for the Front Board that will be sandwiched at the back where many of the components are mounted on. All the plated holes have metal parts in them, mostly the switches and they will be connected to earth ground for safety. The potentiometer for the voltage adjustment is specially selected because although it has a metal fastening part, but the rest is plastic. It can carry voltages up to 200V so we want to be careful.

The lower six holes with the E/S, B/G and C/D names will be for 2mm banana jacks that can be used with flying leads, or with a selection of PCB boards with different sockets for all sorts of DUT's, including one for a ZIF socket that we already use now. We'll leave it to others to design these boards for different DUT's.
The two large holes just above it are for two TO220-TO92 burn-in and test sockets.

[UPDATE]  Verifying the sources for these test adapters just showed that they are no longer available anywhere. Bummer! I noticed this just after I finished the design of the PCB.
Fortunately, there are two good alternatives, one available through Digi-Key, but with a potential short-coming. It will not accept larger size leads from power devices. This one is also pretty expensive at € 14 a piece.

There is a better alternative, available through AliExpress. They sell a set of two for $16.

I've ordered a set of these. Unfortunately, they don't show the dimensions so I need to wait until I have them so I can modify the Face Plate and the Front Board. I also contacted the manufacturer so hopefully they are willing to send me the mechanical details. In the meantime, we can continue and can make the changes for the Front Board and Face Plate later.  The reason I selected 5-pin sockets is that you can wire them for a variation of DUT pin-outs as indicated on the Face Plate.

More stress tests

With the unit now fully functioning, we also needed to make a few more tests to see if it survives a higher than planned current and even a downright short of the Sweep Supply. This can be caused by a pilot error, or by a failed DUT when there is a short from the E-C/S-D or A-C junctions.

The first series of tests were done with a 10 Ohm power resistor across the Sweep Supply output terminals. All three voltage ranges passed with flying colors and the thermals were not out of line.

I then proceeded with a straight wire to replace the resistor and create a real short. Again all three ranges passed with flying colors and thermals were not out of line. The current limiting circuit works well to avoid these issues.

However, when I tried a jumper lead to create a sudden short, there were some sparks at the connecting point, which is no wonder with currents up to 2A, but I also saw smoke coming from the Sweep supply. 

After running some tests, it was clear that it broke down, but what was the root cause? After several days trying to fix the unit and finding the root cause, we think we now have an answer and a solution. 

The root cause was the failing of U2, the current multiplier that drives the second MOSFET. The failing of U2 was determined to be caused by a too large of a current spike in one of the protection diodes and that took out U2. That in turn caused a thermal imbalance between the two MOSFET's and caused a breakdown of the main MOSFET. The stress on it caused it's Source resistor to smoke. There was also another resistor starting to smoke, but that was also due to the MOSFET failing.

After replacing the two damaged resistors and U2, it looked like that fixed the problem. However, I still had a pesky problem in the 200V range. The current limiting was no longer working as before. After several more days of chasing the problem and searching for the cause, I found that the MOSFET's failed. They seemed to work well, but brake down in the 200V range. Initially testing the two MOSFET's on our CT using another Sweep Supply did not reveal the problem, but at the time I was looking for a failed unit, not a damaged one. Replacing both MOSFET's fixed the problem.

Here are the two MOSFET's that failed in the circuit, tested with the now functioning CT again. 

They were all taken with 50mV/step, 4 steps, and a positive offset.

First at 35V, then at 75V and lastly at 200V

There is definitely a problem with these two MOSFET's at higher voltages. They seem to work, but got damaged. Good luck trying to find that without a CT, or on one that only uses 9V or so.

Bud came-up with a solution that hopefully solves future problems by moving the protection diodes from the output of U2 directly to the Gate of the MOSFET and also increasing the value of the series resistor between the output of the Opamp and the Gate. This should now better protect U2 from failing again and preventing damage to the MOSFET's.

I will need to do some more testing of this potentially destructive method to make sure we have fixed the problem. However, I need to wait a little because I'm running out of spare MOSFET's and I don't want to be without a functioning CT while doing more work.

DUT Breakdown/punch-through effect issue

I have found that during a VCEO breakdown test or an ICEO leakage test, the breakdown or even punch-through of the DUT causes a nasty glitch on the Step Gen supply rails. The reason is that in the NPN-mode, the Emitter of the DUT is connected to the isolated GND of the Step Gen, and this is connected to the negative Collector Supply. The very fast glitches that are caused by the breakdown event are so severe that they can be found in many places. 
These glitches trip the Step Gen protection circuit and turn on the Fault condition. The Fault detection switches off the Collector supply for a few Milli-seconds and then turns it back on. This will prevent you from making these two measurements. 

The top trace shows the glitches on the isolated GND of the Step Gen, and are also visible on the +/-15V rails. Note that the CH1 scale is incorrect, it should be 1V/div. (wrong multiplier) The glitches turn on the Fault detection system, shown here in the bottom trace.
After I found the problem, Bud proposed a few potential fixes. Unfortunately, after trying these out, the problem remains. We will now wait until we have a new layout that will eliminate all the long leads currently inter-connecting the six 10x10 boards and hope this will fix the problem.

Triangle Sweep Generation fix

We are currently revising the triangle sweep generation circuit. The original circuit dates back from the original ELV design, but Bud found a few smaller issues that we'd like to address while we still can. Mark proposed a different method and Bud further designed and verified the changes with LTspice. I will try a few things with my hardware to see if I can verify some of it. In any case, Mark will need to do another board turn for the sweep supply and also the triangle generator so we can fully test and evaluate these (last?) changes.


Making more measurements with the CT

I have created a dedicated Blog post for this to keep this one to the design and testing of the CT itself.



With these measurements, and the ones listed in the dedicated Blog, I hope to have demonstrated that we are finally getting very close to the completion of the Version 3 Curve Tracer. We still have some work to do with some revisions and part value changes, but these are all fairly minor now.



Mark is now working on the layout of a new board that combines the Triangle generator, the Step Gen Buffer and the DUT/X-Y Amp functionality on one board as it will be more close to the final form. It will sandwich behind the Face Plate PCB shown above. We hope to be able to eliminate stray capacitance and the inductance of long leads a bit better. We're also going through an Auxiliary Supply board turn that uses a smaller VA transformer to reduce the physical size, and that will also reduce the heat developing on the regulators due to the excess voltages. Finally, it will have two different versions of the multiplier circuit to get the +24V supply.

We've decided to add an additional switch on the Face Plate that will allow the user to switch from a triangle based sweep supply to an adjustable DC voltage supply. It will allow us to do a few more measurements, mostly leakage ones. It works well, but there are some limitations we are now investigating and documenting.

While using the CT and trying to make as many measurements as we can, we're also still finding some things we need to address or like to fix so we are still dealing with minor changes in the design. The good news is that the CT is getting more versatile, more reliable and better all the time.

When we have verified the correct working of the fixes and changes and when that all works as advertised, we will start to work on creating a hopefully single main board with the remaining circuits on it so we can start to put it all in the enclosure.

It all takes a lot of time and it's slow going at times, but we're making very good progress.

Stay tuned for more information and updates...


  1. Hi!

    It's as well the four PCBs I was making at the design stage are still only about halfway through tracking, as it looks you've identified very serious problems with your first design Paul!

    I'll wait for you to post the final V3 Circuit Design and then, if I may, rewrite Chapter 150 of my book
    based on the new circuits!

    I don't want to write up something in my book that has a risk of repeatedly damaging the D.U.T., after all, the readers of my book may be wanting to test irreplaceable OEM devices!

  2. PS!

    I did buy a pair of Mark Allie's Rev 1B PCBs, but they're so cheap to get made from JLC it's no great loss, I'd gladly pay the same over again if you get the V3 design finalised to your satisfaction Paul!

    1. Hi Chris,
      Yes sorry for this but we did find a few serious weaknesses and also sufficient room for improvements. Keep an eye on the blog for version 3, although not all is in there yet with all the final details, but we're making very good progress.

      Stay tuned!

  3. Hi!

    Even if the new Mk. 3 version board designs aren't released straight away I'm more than happy to have another attempt at my own personal PCB designs, so I look forward to the final schematic designs!

    Chris Williams

    1. Hi Chris, I appreciate your interest!
      The process that we're following is first to get the circuits working on 10x10 boards so we have a fully functioning CT. As a next step we will go to a final layout trying to cram everything into the enclosure that we selected earlier. This will also include a PCB face plate with all the markings on it. Only after that process is finished will we consider releasing the schematics. The final schematics are not a secret, but we want to avoid getting various designs "out in the wild" that we have no control over, in terms of corrections, improvements and updates.

    2. Chris, if you can let me know your email address, we may be able to work something out for you.

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  7. Just some house-keeping information.
    I will remove all comments that try to plug a service, so please don't pollute this Blog.
    I will also remove all posts that contain personal information like an email address, for safety reasons.

  8. This comment has been removed by a blog administrator.

  9. Hi Paul, Great work, to say the least. Re safety, something else to consider: Some products have a clear plastic cover with a microswitch or reedswitch to that senses that the cover it closed. The cover encloses the DUT. When voltage is high, the test won't start unless the cover is closed. Also good to do for high currents, since I've had a few devices blow up on me, with projectile particles :-)
    Regards, Rich S.

  10. I am into curve tracers myself and have designed and build a few myself. It is surely great to see all the passion and engineering work that goes into your project! Keep going and all the best.



  11. Hi Rich, yes I know, that's the Tektronix CT method, although they also post the trick to defeat the safety cover action. We're catering to engineers that should know what they are doing. The high voltage warning is there in case they forget what the voltage is. Designing a cover is beyond our project, but can certainly be done when a PCB that taps into the 6 2mm Banana plugs is designed with that cover on it. We'll leave that to others to design.