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Tuesday, August 22, 2017

DIY build of my Tektronix gear and a 5CT1 Curve Tracer

When I worked at Tektronix in the 70's, I build my own 5440 Oscilloscope and TM503 mainframe.

What that means is that I collected every part and soldered each one of them myself. Some boards came from scrapped units, so I also scavenged parts.

This in itself was not a very unusual project at Tek, but I believe I was the only one at the time to build a 5440. Several others build portable scopes, but since I repaired the 5000's, I was very familiar with them. I purchased many parts from the Tek factory in Heerenveen, and in the employee shop in Beaverton. Some very good friends at Tek helped me to get the printed circuit boards, and several special hardware items, which were otherwise not available. I remember that one time I went on a business trip to Beaverton with one suitcase, and came back with two. Imagine the explanations I would have to do at customs at Schiphol...if I was stopped. The CRT was a reject from a customer (there was a dust spectacle on one of the plates that caused a funny light reflection effect. A colleague of mine spend days ticking on it and shaking the tube to get it off the plates. Eventually he gave up, and it was decided to scrap it. So, I had my tube. When I build my scope, I never saw that effect again... go figure)

I took the transformer from a unit that had been on fire, so that was another item I got. One of the 5000 frames was bent when a customer dropped his unit. I replaced the frame, and was lucky to be able to bent the old one in shape again without breaking it. It is now my TM503 frame.

The list goes on and all in all it took me several years, but here it is:


The measurement system consisted of the following parts:


Tektronix 5440 Plugin Oscilloscope System.

D40 Single Beam Display Unit
5403 3-slot Mainframe with Display Readout Option
mainframe is typically 70MHz, could display up to 100MHz.


5A48 Dual Channel Amplifier with 50Mhz bandwidth

5B42 Delaying Timebase, to zoom in on a specific area of the trace.



5A22N Differential Amplifier (down to 10microVolt), with LF & HF filters and DC offset.



TM503 Mainframe
This is a TM503 with original parts, but build in a 5100 lower section scope frame.
It has no middle section, and therefore no handle. I usually have the scope on top of the TM503, so I don't miss the handle.
One the left:
DM502 Digital Multimeter, VAC, VDC, ACmA, DCmA, Ohms, Temperature(C/F), comes with 2 temperature probes and a spare tip
 
In the middle:
PS503A Dual Power Supply, (0 +20V, 0 -20V with tracking, 0-400mA and fixed 5V 1A)
This is a self-build copy of the original unit with lots of original Tek parts.


To the right:
SG502 RC Oscilator, 5Hz to 500KHz low distortion (<0.0035%) sine wave and square wave output
This is a fantastic unit for audio work, which is what I did most of the time, back then.



 

5CT1 Curve Tracer
After I left Tek, I had no time to spend on my electronic hobby, so everything was mostly stored, and followed along with the several moves we've made. Somewhere around 2000, I wanted to finish an instrument that was on my to-do list for quite some time, a Curve Tracer. Some parts were still waiting for this project, but since I left Tek, I had no access to parts anymore, at least not for reasonable prices.

I had no access to a front panel either, so I made my own, but missed the engravings for the settings. I also didn't have the customary drum rotary switch that Tek uses a lot, so I used normal single deck rotary switches. These were connected through diode matrices that drove little 5VDC DIP relays, and these mimicked the finger switches. I used the same technique to add a read-out capability for the Vertical Amperes/Div. and for the Step Amplitude. The ./. 1000 switch was added to the mix, so the display also changed from milli to micro etc.

The instrument uses a transformer to obtain voltages up to 300V. At first I used a normal 220V transformer in reverse, but I was not too happy about that. This transformer had no shielding either, so I got some extra switching noise. When I had an opportunity, I had a special transformer made. There was no information about the transformer Tek used, so I had to do some reverse engineering and the transformer company did the rest. In the end this trick worked really well and the unit is within specifications.

This is how it looks like (left compartment):
Note that since I added the readout capability at a later date, I should have renamed the 5CT1N to 5CT1. The "N" suffix in Tek parlor means "No Readout".

BTW, in case you wondered, the 7CT1N is exactly the same unit, but has a mainframe adapter board to make the circuit fit in the 7000 mainframes.






 Here I'm measuring the voltage drop of a 1N914 diode, showing 0.7V (at .5V/Div)


This is the breakdown voltage of the 1N914 at 20V/Div 


This is the voltage drop of a Schottkey diode.


Here is a trace of a 12V Zener diode (at 2V/Div)


This is the characteristic of a 2N2219 transistor.


This is the breakdown voltage of the 2N2219 at 20V/Div.
Note that the traces show in different intensity, but is caused by the sweep rate of the scope and the way I took the picture.


This is a screen shot of a tunnel diode, used quite a bit by the Tek designers in the 60's and 70's.

This is a BS170 MOSFET


Right now, I'm selling all my Tek equipment and parts. I don't have the space to keep them on my bench, and it's too cumbersome to create the needed space to use them for measurements. I have a new scope now, and I have no need for the DM501 and the PS503 anymore.

I will miss the SG502 because of the good quality sine wave forms (0.0035% THD). I may rebuild that instrument in a different enclosure at a later date, maybe with OpAmps. (one more item added to the already long Bucket List)
Update, I just finished that project, look for it on my blog.

And I will also miss the 5A22N differential amplifier capabilities and the Curve Tracer, although I'm using less and less older parts that need checking or verification. Parts are so inexpensive these days that swapping them out is a no brainer, certainly compared to the old days.

Update : I also started a project to replace the 5CT1/7CT1 Curve Tracer


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Sunday, August 13, 2017

My New Power Supply Design Project Part 4

Before we dive in to the measurements, here are some pictures of the PCB and finished product.



As you can probably see, I used a combination of through hole parts with SMD. The reason is that I have a lot of THT parts, and I'm just starting with SMD.

Here are pictures of the finished supply for the 3A version:



The fan is mounted such that it will suck the air out. There are holes in the bottom of the enclosure, below the heatsink. It's a little hard to tell, but I added a number of holes in the top enclosure half to get as much airflow out as possible.




The parts on the backpanel are, from left to right, top to bottom:
Black negative supply, 3 pin DIN connector for the 9-0-9 VAC supply and the Red is the positive supply.
Yellow is the 12 or 15VAC or middle transformer tap for the panel meter supply.
Below it is for the 12VDC wallwar plug for the fan supply.
In the middle is the main supply fuse. (3.15Amp for the 3A version, which this is)
The two empty holes where used with the previous supply, but the thicker fan got in the way, so I can't use them anymore.



Here you can see how I mounted the two TIP142 transistors. In the middle is the NTC for the fan, it has a heatshrink to minimize effects of the airflow from the fan. This NTC is screwed into the heatsink with an M3 bolt.
Below the NTC is the 0.22Ohm shunt. The two green resistors are the 0.6Ohm balancing resistors for the TIP's. Sticking up are the 150 Ohm resistors in series with the Base. On top of the left TIP142 you can see the second NTC mounted on a lug, which is used for the thermal protection. It has a good heat transfer from the actual transistor die temperature.



Excuse the glue gobs for the LED's. I had dried out glue, which was needed to secure the 3mm LED panel holders. These holders suck, and the only way to secure them is to glue them. I ended up using super glue. Top right is the switch and the LED for the main power cut off.


And here is the frontpanel. I make a design in PowerPoint, and print it with a photoprinter on HP Premium Plus Photo Paper in the glossy type. I use double sided tape to secure it to the frontpanel, and carefully cut the openings with a small blade and very sharp knife.




Profiling and Measuring the Power Supply

Besides the obvious measurements, there is more information available that can be used to profile a lab power supply. After some searching around, I found a good application note that I'm following as much as I can, besides, it's a good read to get more information. The app note is from Agilent, number AN 372-1 and called Power Supply Testing.

One of the most important elements of any power supply is the dynamic load regulation, or as it is called in the app note, Load Transient Recovery Time.

In order to measure that, I first had to make some modifications to the pulsing of my Electronic Load. It did not have an adjustable base current load setting, so after adding that feature, the DC Load pulse has a DC offset capability. This is used to set a minimum load current, and the pulse itself raises it to the level you set with the output adjustment. You can follow the DC Load modifications in my post about that topic.

Before I took measurements, I twisted the leads to the DC Power Load as much as was feasible to reduce noise coupling. This is the setup I used for many of the measurements:



The DC load is connected to the supply output, and I use a 6digit bench multi meter to measure the output fluctuations. I use a scope to see any effects on the output itself.

When I finished the 3A version of the supply, my T3 box was still missing the 9-0-9 V transformer, so I did the following tests with the 1.5A T1 box first, and later with the T3 box.

Load Transient Recovery Time.

To  see the effect of the load transients on the output of the supply, I used a shunt resistor in series with the negative lead going to the DC Load. Accross the resistor is an (isolated) BNC connector that goes to another channel of my scope. The resistor is in a metal box to shield it from noise. The resistor is small (0.1 Ohm), so the volts across it are in the milli-volt region, so noise is a factor.



Here is a picture (taken with my new scope) that shows the effect of pulsing a 2.0A load at an output of the supply of just 1.8V. I used a setting of 1.8V, which realistically speaking, is the just about the lowest voltage you will encounter.  It is also the most strenuous on the supply because it means that the pass transistor is almost fully closed and the maximum voltage is across its C-E junction. It has to dissipate this large difference in heat.




The yellow trace (top) is the actual load pulse measured over the series shunt of 0.1 Ohm. The pulse is about 2V which means a 2A load. The blue trace is the transient measured on the supply output.
In the upper screen shot, the load is applied, which means that the current demand jumps from about 100mA to about 2A. It takes the supply about 30 uSec to react to this load change. At first the output goes low (blue trace), and then the regulation kicks in. This could be improved by adding a larger output capacitor, sometimes a few 100uF's is used, which is commonly the case.

Note that the output level of 1.8V after the initial transient is dropped a bit. I measured this to be 25mV with a more sensitive measurement. At a 1A load, the drop is only 10mV.



In the above screen shot, the load is removed, so the voltage over the shunt goes from about 2V (2A) to about 100mA. The blue trace is the measurement on the output of the supply. The output shoots up 300mV for a duration of 12uSec before regulation kicks in again. Note that this pulse is also an effect of the output capacitor dumping it's charge. This is why I keep it as small as possible. I personally believe that more damage can be done by the output capacitor dumping it's charge into the DUT, then the small sagging as the result of adding a load very quickly.

As you can see here also, when the load is removed the output level raises by about 50mV.

If you are looking at this for the first time, the pulse shooting up above the set output level may scare you. However, this is a normal behavior, and because the duration is so small, at about 12 uSec, it will cause no harm to the attached circuit.

The good news is, that the 300mV peak is about the same regardless of the output level. When I put the scope input to AC coupling, I can measure the pulse across the output range. With a 1.8V output (with AC coupling) the scope reports a pulse of 200mV, at 3.3V 195mV, at 5V 190mV, at 10V 186mV, at 15V 177mV and at 20V 173mV, etc.

For these measurements, I'm using the T2 transformer, because I don't have a 9-0-9VAC transformer for the T3 box. Initially, I used an old 6-0-6VAC transformer. The headroom for the 5V aux. supply is too small, and I need to change the zener diode value in the supply switch off transient circuit. This will stop me from interchanging these transformer boxes, so although thr supply is configured for 3A, I'm using the T2 transformer, not allowing me to go up to the full specifications at this moment.

One caveat though.

At this moment, I'm not sure if I'm looking at the maximum edge speed of the DC Load, or the Power Supply. The 555 Timer pulse that is driving the MOSFET's in the DC load has a rise time of 500nSec. I need to profile the DC Load a bit more to see where the limitations really are. For starters, it was never designed for speed, that requirement and modifications were added later on. For now, the Power Supply Transient Recovery Time looks pretty good to me, even though it could be limited by the DC Load. The Agilent app note mentions that this is typically measured in milliseconds or microseconds, so with 30 uSec rising and 12uSec falling, I can't be that far off.


Load Regulation
We already saw an example of the load regulation, where the output dropped 50mV from a set level of 1.8V, when a 1.5A pulsed load was applied.
I'm not sure that this is the correct way to measure load regulations, so for this measurement, I'm going to use my DC load in the static mode (DC only), and just dial in the current at various voltages..

The load regulation I measured is as follows. Although the panel meter is very precise (more later), I'm going to use my 5 1/2 digit DMM as a reference.

At an output of 1.8004V with no load, a 1 Amp load, dropped the output voltage to 1.7975, a drop of 2.9mV, or 0.16%
With a load of 2.0A, the output dropped to 1.7967V, 3.7mV, or 0,20%.

At an output of 24.998V, with no load, a 1 Amp load dropped the output to 24,997V or 1mV, or 0.004%.
With a load of 1.5A, the voltage dropped to 24,995 or 3mV or 0.011%


Current Limit Characterization
The Current Limit Characterization showed that this is a true Constant Current supply, as opposed to a less precise Current Limit supply. The set output current by the DC Load stays constant when the voltage is changed from minimum (at CC) to the maximum. The current deviates less than 1 mA (the least significant digit did not change) with a 5mA, 100mA, 500mA, 1A and 1.5A loads, while the output voltage is going through the range. Once set, the current limit staid stable.


PARD (Periodic and Random Deviation) Measurement
At this moment I don't have the ability to really measure this.
I can only use my scope, AC coupled, at the most sensitive vertical setting (5mV/div) and measure the noise on the output. I first established the "noise floor" of my measurement, by connecting both the tip of the probe and the ground lead to the negative output. The scope then tells me I have a 12 mV p-p noise level in the peak-detect mode.

Measuring directly on the output, I get 10mV p-p at 0V output,  and I get approx. 14 mV p-p at all levels from 1 to 30V.


Turning on (Startup) Transient
The output of the supply should not have any transients when it gets turned on by the main power switch.
Normally you should not really do this, but if there is a brown out event, the supply may turn off and on again.
There is a provision in the bias circuit (the 220uF capacitor) for the series transistor, that will delay the bias current, in effect holding off the series transistor from conducting.

Here is a screen shot of the turn on transient at an output voltage of 4V (bottom trace) with a 500mA load. The top trace is the raw positive auxiliary supply. The output gets turned on when the aux supplies are in regulation. There is no ringing or overshoot on the output.





Turning off Transient
If the supply gets turned off by the main switch, or a brown-out, there should be no transients at the output, to prevent damage to a (sensitive of expensive) DUT.
I concocted a little circuit that gets triggered by the sagging auxiliary supply, before the main regulators that are driving the Opamps come out of regulation. As soon as the unregulated supply gets below 10.6V DC, the bias for the pass transistor gets turned off. You can see that in the simulation result shown in part 1.
Here is the real thing:



The top trace is the raw positive auxiliary supply at about 15V.
Note that when that voltage is dropping to the 10.6V trigger level, the bias is cut and the supply output is gone before the series regulators are running out of headroom at about 7V. This results in a very clean turn off at the output.


Line Regulation (Source Effect)
I don't have the means (a variable transformer) to test this, but because our electric grid in the Netherlands is very reliable and stable, I'm not worried about this aspect, although a big factor is the transformer I use. The T2 transformer I used is at the limit, especially drawing higher currents at a high voltage, because the supply can get out of regulation. The good news is that the CV LED will go out as soon as the series transistor is out of headroom, so you have a visual warning.


Overvoltage Shutdown.
This is not implemented.


Short Circuit Output Current
I tried this three different ways. First by the Electronic Load by setting it to the maximum current draw. Then I used my DMM in the 10A current mode and put that across the output.
Finally I used a lead to short the output.
In all cases, the supply went straight into CC mode with no side effects.
I also did the "sparky" test by rapidly shortening the output and drawing sparks. No problem!


Drift
I did a little of that already, but only for relatively short periods of time. After a sufficient warm-up period, the output stayed at the set voltage level to within about 1 milli-Volt. This is without the 4.096V reference in pace.
From the 5V regulator, I used a series resistor of 10 Ohm and a 47uF Tantalum cap to create the reference voltage.

I just did another measurement, now with the 4.096V reference and the fully build supply. The supply was set at 5.0025V, and 4 1/2 hours later it is still at that level. So it stays within a 100micro Volt. I did not make a plot over time, still need to learn how to do that with my new scope, but I checked the value regularly. Only the last digit (100micro Volt) is sometimes flipping. For a power supply, this is more than adequate.


Measuring with the T3 box main transformer

OK, same stability measurement. Output at 1.8048V, with a 3A load, the output dropped to 1.7995, or 5.3mV, so a regulation accuracy of 0.29%.

With the output set at 20.005V, and a 3A load the supply is on the verge of current limiting, at which point the output is 19.750V, or a drop of  255mV, so a regulation accuracy of 1.27%.

With the output at 30.009V, the supply is getting out of regulation just over 2.6A.
At 2.5A, the output is at 29.910V, or a drop of 99mV so the regulation accuracy dropped to 0.33%, still respectable.

I should point out that the inability to get the full 3A at 30V is more a function of my transformer than the supply itself. Although I also should point out that the heatsink is not large enough to supply 3A for longer periods. The enclosure has no room for a larger one, but that was part of my constraints.

I seldom if ever need that much current, and if I do, I have other means. This supply will be used most of the time below 20V and well below 1A. Mission accomplished as far as I'm concerned.


PCB
I'm expecting this question so let me be frank and up front. The current PCB has some issues with the fan supply and the panel meter supply, as I pointed out earlier, and needed some surgery with trace cuts and wires. So, although I was initially planning to, I'm not going to publish the Gerber files, so please don't ask. I also used a lot of THT components, and that makes the board size larger than it needs to be, and therefore more expensive.

With the details in the schematics I provide you can build your own, it's not that difficult and not that critical. Just make sure you have a main center ground point to avoid loops, and keep things apart that needs to be separate.


Building the second supply for the 1.5A version
In the meantime, I finally found some time to scrap the old 30V 1.5A supply kit connected to the T1 box, and started building-up my second supply. I did some more work on the PCB trace cutting and isolation of the Fan Supply, so it's much cleaner.

With the exception of the maximum current resistors, all the parts are equal on this version. Obviously, I only needed one TIP142 series resistor. Look at the diagram in part 3 for details.

After carefully soldering all parts on the PCB and creating wire harnesses from all parts not on the PCB, I wired it all up and everything worked as it should. As mentioned in the testing section above, I now have a 30V at 1.45A version as well.

I hope you enjoyed this journey as much as I did.

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Tuesday, July 18, 2017

My New Power Supply Design Project Part 3

Updated text and diagrams after I fully built and tested the supply.


The Auxiliary Power & Reference Supply

Enough with the simulation, I'll move over to the schematic capture of the complete project.
First of all the Auxiliary Power Supply and Reference Voltage.

Here is the schematic, using Diptrace:


There is really nothing special about this, but let me go through the design. I use a 9-0-9V AC transformer I salvaged out of a alarm radio a long time ago. I have no idea what the maximum current is, but that hardly matters. I'm only pulling a few milli-Amps from it. This transformer is located in the transformer box that I'll cover later.
The 9V AC as a minimum is important, because it creates enough head-room for the regulators, and helps with my "early warning switch off system" that I'll cover later.

The 9-0-9 V lines come in the power supply enclosure through a 3-pole DIN connector, like we use a few decades ago for audio equipment. From the chassis part, it will go to a connector on the main PCB. The above circuit is just that section of the main PCB.

Because I only draw a few milli-Amps, I use 1N4148 diodes for rectification, they can handle 100mA. Next to the filter caps, I use 18K bleeder resistors to quickly remove their charges. This helps with the turn-off/ back on of the supply. I use TO-92 low power linear voltage regulators to get plus and minus 5V. These supplies are used for the Opamps.

The +13V tap is used in my early warning signal that the mains is going down, and this voltage is used to turn the supply output off right away, to prevent mishaps at the DUT. As I mentioned in the simulation earlier, the trip point is at 10.6V, this is 3 volt above the regulation headroom margin for the 5 V regulators, so before they start to cave-in and cause problems with the Opamps, the output has been shut off long before.

I could have used the 5V supply of the positive regulator as the reference voltage for the volt and current setting. The PCB is made such that I can easily bridge the LM317L, and leave the series resistor in place. Because I want to be able to use this supply with a controller, I will use 12-bit DAC's to drive the volt and current settings. Using a reference voltage of 4.096V makes the settings and calculations easier, allowing 10mV steps in the volt setting. Also, the output stability and noise specifications are largely depending on the reference voltage source, so I'll build at least one supply with the 4.096V reference voltage. Note that the resistors for the volt setting and the current setting in the V/I control section need to be adjusted to get 30V and whatever Amps you want at the output.

I use a small trimmer to adjust the reference output to exactly 4.0960V. During my tests, this reference voltage turned out to be very accurate and stable with a fluctuation of only +/- 0.0002V over a period of several hours. This is a mixture of my 5 digit DVM (still in calibration) and the reference itself. I cannot measure the noise level other than with a scope, so I can't give you any numbers, but it's very low.


Transformer Box

As I mentioned before, the space on my "bench" is precious and limited. This is one of the reasons why I decided a long time ago to separate the bulky transformers and even the rectifier bridges and smoothing electrolytes to another enclosure, that I can put in the background. An other benefit is that I am much more flexible with this relatively expensive equipment. As an example, I added 4mm binding posts to the AC outputs as well as the rectified outputs so I can have multiple uses out of these parts. 

Right now I have three transformer boxes, of which I'll cover two here. One houses my 15-0-15VAC 3.3A transformer, rectifier and 2 x 10.000uF capacitors and a small 9-0-9VAC transformer. The second one houses my 12-0-12VAC 2A transformer, and also a smaller 9-0-9V transformer. Both transformer boxes will be used with this supply (I'll build two). The output voltage of the supply fed by the 12-0-12VAC supply will go to 30V, but the current will be a bit limited at higher voltages. At the maximum 30V DC output, I can pull 1.1A before the output starts to drop. At the realistic minimum of 1.8V at the output, I can draw just about 3A before the voltage starts to drop.

This is what is in the T2 12-0-12VAC 2A transformer box:

 And here is my T3 15-0-15 3.3A transformer box:




There is really nothing special about them, although I use a not so well known trick to get the most out of the relatively small capacitors in the T2 box. The little series resistors actually separate the two electrolytes. The first will take the full charge and discharge rectification smoothing cycle, and the second one is more of an effective filter due to those 0.6 Ohm resistors. The 9-0-9 AC transformer is feeding the auxiliary power section explained above. It is switched in parallel to the main transformer, and is connected to a 3 pin DIN chassis part. The main three outlets are going to 4mm banana sockets.

The 0V or AC transformer center tap is also brought out (as 12 or 15VAC), and that is used, together with the negative DC output, to create a separate and isolated 8V DC supply for the Volt and Amp panel meter.


Panel Meter & Fan Supply
The power for the Panel Meter and the Fan is coming from this power section. In an optimal situation, both of these supplies should be separate and isolated from the main supply. Normally, this is done by added separate winding's on the main transformer. That's not so easy to do, and getting a transformer like that is close to impossible.

I need 12V to run the cooling fan, and I need anywhere from say 5..25V DC to power the panel meter.
The reason to keep these supplies separate from the main one is because they introduce a lot of noise. The fan with PWM activity and the display with switching noise.. You don't want that superimposed on the auxiliary and reference supply. Besides that, that aux. supply is floating on top of the main supply so you really don't want to mess with that.

Initially, I wanted to try the following method.
I wanted to use the 0V AV centertap of the transformer together with the negative DC main power output to create a lower value DC supply. Using the "raw" supply for that purpose is possible, but the voltage is most likely too high for normal regulators, and you generate a lot of extra heat to bring that voltage down. The extra rectification and filtering will help to isolate the two supplies somewhat.

So, this supply is rectified and goes to a 12V linear TO220 type voltage regulator. It will be mounted on the main heatsink. As the fan controller, I use a chip that I learned to use a while earlier, and although it is a little strange to figure out (for me), I still like it a lot. The TC648B is a dedicated controller for fans, and you can set the starting point, minimum RPM, maximum RPM based on an NTC input and a cut-off point for the fan as well. The resulting PWM is rather clean and all you need is a small MOSFET or transistor to drive the fan. I have configured it such that normally the fan is off, but will kick into life at about 30-35 degrees. I especially selected this rather low temperature, because it will take a while before the heatsink can be cooled down, especially when it is heating up quickly, and you don't really hear it in the beginning anyway.

The fan supply needs to be isolated from the main supply due to the induced noise coming from the fan and the PWM to drive it. 

The panel meter is a combined volt and ampere meter is fed by the unregulated 12V supply. I may need to add a small capacitor at the panel meter to further decouple the noise coming from it. The panel meter I selected is a type I really like, because it acts more like a DMM with switching decimals. It can display 0-33.000Volt and from 0 to 9.999mA and then switches over from 1.000 to 3.000 Amp. Perfect for the job.
In the first version, that was the basis for my PCB layout, I used this schematic:


I actually build a prototype for this section as well, and it seemed to work. I did not spend a lot of time on it, because it seemed so simple...
So I went ahead, and created a PCB layout and ordered the boards from OSHparc.

While waiting for the boards to arrive, I actually started to spend a little more time with the complete setup, and started to profile and measure everything. It was than that I noticed a problem, a BIG problem when I was measuring noise levels when the supply was in the lowest output settings. There was way too much noise  seeping into the supply.  So I had to redesign it, while the PCB is already in fabrication. Bummer!

While trying to keep the noise down, I actually saw that the panel meter I used was generating a lot less noise than previous panel meters I used before. I figured that if there was too much noise in the final product, I could still add some filtering to the supply afterwards, but not on the PCB anymore.

The biggest problem that I noticed was with the noise introduced by the PWM driving the fan. So I switched my attention to isolating that from the main supply. In these attempt, I used the DC-DC isolator method to electrically isolate the fan supply. Because the fan is rated above the single DC-DC isolator, I used two in parallel.

So my version 2 looked like this:


So I again started to measure things and found that even this setup produced too much noise. I new that the DC-DC isolators produced a lot of noise on the outside, but that was isolated, so no problem, I thought. I've used these devices before, but in other applications, so I thought I was safe.

I was wrong, it turned out that the DC-DC isolators generate a lot of switching noise on the input side, and that was something unexpected, and made these devices useless in this application.

Bummer! Another reset was needed.
The fan itself generates a lot of high frequency noise that no matter what I tried, I couldn't get it to a level as low as I wanted it. 

In my version 3, I have now resorted to the easy way out, using a separate 12VDC switched wallwart. The wallwart itself is a few feet away from my sensitive equipment, and where it comes in to the supply, I'll make sure it is quiet.

So here is the latest version (3) of this section.

Note that I added a Schottkey diode to protect the supply from input polarity reversals and I used a capacitor to filter out noise. Both are mounted on the DC chassis part.

After I got the PCB, I needed to isolate the fan controller section by some trace cuts and I also needed some cuts and wires to isolate and create the new panel meter supply. The LM340 on the heatsink was no longer needed, so I put a 78L08 TO92 regulator on the connector pins intended for the leads going to the LM340. 

After having populated the PCB and putting everything in the enclosure, I spend some time tuning the fan controller circuit, and I am now happy with the results. I dropped the slow start feature of the fan, because that uses PWM to slowly ramp the speed up. I don't wanted to avoid that noise. I figured that if the heatsink starts to warm up, it's better to make it run full blast, because of the inertia that the heat sink has towards the real temperature of the series transistor(s). 

So now, the fan kicks in at full blast, and terminates when no longer needed. The fan will kick-in at about 30 degrees C. This is all determined by the resistors setting the VAS pin (that sets the termination threshold) and the resistors setting the Vin pin that sets the starting point.

The panel meter supply looks like a bit of an overkill, but that meter will be on the front panel, close to the volt and current setting potmeters, so I want it to be as clean as possible, with no mains rectification hum present. These components are on the main PCB already anyway, so I may as well use them.



Series Transistor and Output Section

There are actually two versions for the two supplies that I will build. One is intended to be used with the T2 box, with limited output current at maximum output voltage, and one for the T3 box with the full 30V 3A.

This is the T2 version with only one series transistor:



And this is the T3 version with two series transistors in parallel to stay within the SOE by sharing the current, and spreading the heat.



Voltage and Current Control Section

Here is the main control section for the voltage and current control, and the output switch on/off circuit.


Note that I used two resistors in series to set the maximum voltage (R35 and R36) and current (R19 and R24) for this supply. This allows you to use various combinations. If only one resistor is needed, use a 0 Ohm one or a solder blob for the other. The current resistor selections in the diagram are for a 10K potmeter. If you use another one, I recommend a value somewhere between 1K and 50K, you need to recalculate the values. Initially, I used a good single turn potmeter, but after trying to set the current at milli-Amp settings, I switched to a 10-turn version.

My suggestion is to find the point where the maximum voltage, in my case 30V starts to drop when you apply more and more current. This will be the maximum current level the supply can deliver at 30V, and then select the resistor value for R19 and R24, so that the CC LED just comes on at the maximum value. 

Make sure that you first properly adjusted the reference voltage after a sufficient warm-up period before you try to figure out what the maximum current at the maximum voltage is. In my case, using my DC load, I could get just over 1.45A at 30V from my T1 box so I selected 120K + 56K resistors for R19 and R24 to get to 176K.

The value of the voltage setting potmeter is not critical and also not influencing the maximum output voltage. You can use whatever you have available but I suggest to use the same range as for the current potmeter.
Make sure you have normal red LED's (I use 3mm ones), not high efficiency ones, nor ones that use a large current.
Protection diodes D16 and D17 are not really needed. I put them in, and left them in, while I was testing. They protect the OpAmp input from stupid mistakes, but if everything is wired correctly, you don't need them. (their added capacity makes the supply a little faster without them)

Room for C21 was added for possible frequency compensation, but was not needed. Just don't install one. I also found that it did not really matter a whole lot if I used other OpAmps. I would stick to those that have a FET input, but otherwise you can try different ones. I used sockets for all the OpAmps, so I could play with various versions. Better is to solder U8 and U7 directly.


Thermal Protection



I changed the red LED to a higher intensity blue one, to better show the thermal cut-out event.

I used one of the many LM741 OpAmps I still have in my stash dating back to the 1970's. This is one of the few OpAmps that can be used as a comparator without introducing unwanted side-effects. If you want to know more about this, use Google, otherwise, use a real comparator instead.


Turn Off Protection



All the parts, except for those in the transformer box, and in the "On Output Terminals" and "On Heatsink" are on the main PCB. I finished a layout, using through-hole technology parts and SMD. All the resistors and capacitors, as well as the 1N4148 diodes are SMD, the rest is THT, simply because I have a lot of them. Plus, it makes it easier to replace them when needed. The Opamps are DIL versions and are socketed, also for easy replacement.

Here are some pictures:

This is the T2 transformer box.




This is my T3 box with the 15-0-15VAC 3.3A transformer.



Some of you will remember that in the early PC days, we had parallel interfaces to connect PC's to printers. As there was only one parallel printer interface, you could use a switch box to connect several printers to one PC. This enclosure is from such a box, all there was in it was one PCB with a rotary switch to connect two printers to one PC.


This is the main controller section with the bias supply, the volt and current controllers and the thermal switch.
I used good quality sockets to be able to select and switch the compensation networks.



This is the auxiliary supply with the 5V reference section.




Here is the 4.096V reference part, still on a bread board, which is how I typically start out and them transfer the circuit to a soldered protoboard.





This is the section that watches the raw auxiliary supply (the 13V tap) and connects to the bias totem pole to
quickly turn the output off and prevent glitches.







And here is the whole mess on my desk connected together. In the forefront is the Volt/Amp panel meter.




In Part 4 I'll show the final Power Supply and some measurements.
http://www.paulvdiyblogs.net/2017/08/my-new-power-supply-project-part-4.html

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