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Thursday, December 17, 2015

_HowTo: Rotary Encoders & Raspberry Pi

After having found a simple and reliable solution for a rotary encoder using a PicAXE (see demistifying rotary encoders), I figured that I could easily port that solution to my Pi's.

Well, no! The Pi is so much faster that the solution did not port or translate, see this post for Details on how I developed one for the Pi.


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_HowTo: Demistifying Rotary Encoders

For a new project, I needed a way to reduce parts and complexity, so it was time to finally bite the bullit and start working on a microcontroller. My experience with embedded controllers dates back at least 35 years, which is why I had been putting the decision off for a long time. Things changed in that period, and I was not keen to dive in yet. After investigating the available solutions, I decided on the PicAXE family due to the very complete design environment, and the availability of a programming language other than C or C++.

The new project needed a large selection method for frequencies and voltages, and traditional rotary switches became expensive and complex. So I decided to use a rotary encoder together with an embedded controller. It also solved the problem of a complicated frontpanel, because I now could use a display driven by the controller.

While researching rotary encoders, I learned a lot about decoding them, and eventually decided on a method that is adequate for my application.

I wrote two posts on the PicAXE forum to explain this in more details, and here is the link:


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Monday, December 14, 2015

_HowTo: Using a single push-button to start/stop/powerdown the Raspberry Pi

A while back I did some work by another forum member to incorporate an interesting chip with Raspberry Pi's. It really lacks a "PC" like start/stop button, but this was left out most likely for cost reasons. There have been many designs made to solve this challenge.

Linear came up with a couple of chips that helps to solve this problem, and with the help of the Raspberry Pi foundation, an overlay was created to get a GPIO port that can signal the end of the Halt status.

Based on that work, I created a design that is well documented and rather easy to build. While I was at it, I came up with a couple more designs that uses this chip, the LTC2951-1, although there are several in this family. Unfortunately, these chips are hard to get, not in-expensive at about $5 each, and come in a tiny, very tiny SMD package. On top of that, MOSFET's are used to switch the power, and the right ones (with a low RDS-on) are also only available in SMD packages.

Eventually, I was able to come-up with yet another design that is even more simple, and only uses 4 resistors and 1 capacitor, in addition to a push-button.

Here is the link to the posts on the Raspberry Pi forum:


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Monday, October 26, 2015

_HowTo: Adding an Analog Output to the Pi (DAC)

Here is a post I wrote on the Raspberry Pi forum about adding an analog output to the Pi by using a DAC.

Here is that link :


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_HowTo: Adding an Analog Output to the Pi (PWM)

Here is a post that I put on the Raspberry Pi forum about using the Pulse Width Modulation feature to create a (static) output voltage. The result is more accurate than most would expect.

Here is that post :


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_HowTo: Adding an Analog Input to the Pi (ADC)

I wrote a post on the Raspberry Pi forums about adding an analog input to the Pi by using an ADC. The application I used to describe it was to measure the 5V supply to the Pi, which is still a major source of problems and confusion.

Here is the post :


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Tuesday, August 11, 2015

_HowTo: Controlling a temperature driven Fan with PWM

Because I need a fan for my power supply project, I started a little project to do that.
I don't like noise or hum, so I wanted to use a fan that would be off when not needed.

Typically, by using Pulse Width Modulation (PWM), rather than a voltage, you can precisely control the speed of the fan, and ramp it up when needed.

I was able to find some clever methods to implement this, and pieced together this design:

The building blocks are relatively simple. I use a DC-DC convertor, not a linear one, to bring the 24V DC to 5V, because a linear LM7805 would get too warm burning off the excess voltage.
I use a comparator with hysteresis to determine the starting point of the fan, based on the temperature reading (in ohms) of a thermistor that is mounted on the heat sink. I don't have the datasheet for the thermistor, but found that it gets from 10K at room temperature to about 5K when the heat sink is getting really hot.

I need 5V for the fan, because I have one that is only 10mm thick, and that is what I have room for.

The clever trick of this circuit lies in the fact that the Control Voltage (CV) input from a 555 timer is used to control the PWM.

The 555 is producing pulses, and the pulse width, and also a bit of the frequency is varied by applying a voltage to the CV input. The output of the 555 goes to a FET that drives the fan.

The whole thing works very well, although the low pulse frequency can be heard from the fan, so I needed to use C4 and C5 to remove that chirping sound.

There are two disadvantages of this design.
One is that you cannot regulate the full 100% of the pulse width. The minimum is OK at about 30%, it let's the fan spin very slowly, but the maximum is only about 70%.
The other disadvantage is that you cannot increase the frequency of the pulses above the hearing frequency of 20KHz, because then the effect of the thermistor on the PWM range is greatly reduced. 

In my application, that is not good enough, I need to be able to get to the maximum fan speed in order to keep things cool.

There are special fan/motor controllers that allow you to do that, so I have a couple of TC648VPA chips on order. Stay tuned.

The chips arrived and I made two different circuits. One for a 12V DC Fan I also ordered, and one for the 5V DC fan.  I used an Excel spreadsheet that is available on the MicroChip website to calculate the resistors (R1 and R2) to get the best starting and maximum fan speeds.

The TC648 works really well, and is a nice addition to my toolbox. Next step is to put the circuits on vero board and install them in the power supplies.

After playing and experimenting, I decided to make a few changes to the circuit. First, I implemented the VAS  pin. The explanation says to set the Auto-shutdown threshold with this pin, but it also sets the turn-on threshold. The nice thing is that the fan will be driven to full speed for a short period, making sure it turns on right away. This was a problem I noticed with the earlier circuit.

Second, I deleted the resistor that was in parallel to the NTC. I felt that with a 10K NTC, it didn't do much. I also experimented with C7, which sets the frequency to see if I could remove the audible noises at lower speeds. That only worked with a 10nF cap, but then the PWM spread is limited and therefore also the speed control. So I resorted to using the 1uF value.

BTW, after a lengthy search, I found the information about the NTC I was using, because I wanted to know the temperature curves. Unfortunately, the shop I got them from didn't mention the type or manufacturer. I have the TDK B57045K0103K000.

I also found a much more elaborate data sheet for the device here:

And here is the latest version of the schematic:

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_HowTo: RPi Start-stop button with shutdown and power-off features

I recently finished a design, based on an idea from sparkyPi, a raspberry Pi forum member, where he was using a rather special purpose chip to add start-stop functionality. He needed a special Device Tree feature to create a signal that the Pi sends out when the shutdown sequence has finished, and even got help from the designers to complete that. His attempts led me to find yet another method to do the same, but without some side effects that the other method introduces.

In the meantime, I ordered some of the chips (LTC2951-1 : Start-Stop Push Button Controller) he used, and I was able to built a working circuit that does not even need Device Tree features. Here is the link to the post:

In this post you'll also find the links to the other ones.


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Tuesday, August 4, 2015

Building a Dynamic DC Power Load (updated)

I finally got around to building my own DC Power Load.

My noise problems with the DC Power Supply, as described in another post, forced me to build this tool so I could measure the results.

There are many designs on the web if you care to take a look, but very few had a dynamic load feature, which is what I wanted.

I finally settled on one particular design and modified that through some trial and errors. Here is my starting point that I got from the work of Peter Oakes:

Here is my version:

Let’s go over the basic building blocks.

In order to have a stable setting of the load, independent of supply voltage changes, I decided to use a voltage reference chip. The voltage output of the chip was selected based on the maximum current I wanted to sink which is 5 Amps. By using a shunt of 0.1Ohm, 5 Amps translates into 500mV.
So I selected an LM 358 with 1.25V output. By using a resistor divider together with the current setting pot meter, I created a 500mV level, so the pot meter could supply 0..500mV, or 0..5Amps.
By just changing just one resistor, I can later change the load current to a maximum level of 12.5A.

The voltage over the shunt is fed to the negative input of U1, which will try to keep this input equal to the positive input that gets the desired reference voltage, by varying the output. The output of the Op Amp is connected to a MOSFET that can handle plenty of Volts, Amps and Watts.

To set the current sink, or the load, I used the output of the current setting pot meter and used a toggle switch to feed that to the Voltmeter of my display. The voltage from the pot meter in milli Volts corresponds directly to the current in Amps. (I first used a 10x op amp to convert the current to volts 1:1, but that’s overkill of you can multiply by 10 yourself)

As I learned with my power supply project, these Volt/Amp displays inject a lot of noise into the supply, so I used a series resistor, Zener and two capacitors to stop that from entering the rest of the circuit.

If S3 is in the Direct setting, the reference voltage of the pot meter is fed into the op amp. In the Dynamic setting I use a separate input coming from a function generator or equivalent, in order to pulse the load at certain frequencies. This allows you to measure the rise and fall times or other responses of the supply under test.

To eliminate possible oscillations and to add stability, C7, C3, C2 were added. I also created a possibility to add a capacitor parallel to R4, to better drive the MOSFET, but did not need it at the moment. A Sobel filter circuit was added at the output terminals for the same reason.
Unfortunately, the Volt/Amp display unit I have does not allow floating measurements, which is why I had to put the shunt into the ground loop. This will add a rather small (25mOhm shunt) current error.

I have another V/A display on order  that can be modified to make the current measurement floating, which also allows high-side current shunts, which are better anyway. Again Peter Oakes is the one that figured this out.
In any case that current shunt of the modified meter can be placed between the Drain of Q1 and the 0.1 Ohm shunt.

The Gate of the MOSFET is “clamped” to ground by a small resistor, just to make sure a problem does not create an unexpected short to the output. This resistor should be soldered right on the pins of the MOSFET and the shunt, to take care of bad or broken wiring.
By using the dynamic load, I was able to measure a rise and fall time of less than 50uSec, more than adequate for my applications.

One word of caution. 
The MOSFET is capable of 55V, 110A and 200W. With 30V and a couple of Amps, it does not even get warm on my rather small heat sink. The on resistance is only 8 milli Ohm, although we will never get that low because we don’t saturate the device. However, dealing with higher voltages and or higher currents increases the number of Watts rapidly. Remember that the two multiply!

Update : Aug 29, 2015

Over the last couple of weeks, several things have changed after the initial build. I received my new Volt/Amp meter, and modified that per the instructions by Peter Oakes. It now has a floating differential current sense circuit so I have put it in between the MOSFET's and the current sense shunt. I'm also very impressed with the accuracy of both meters. No more adjusting, and the decimal point and number of digits change like a "real" DMM, what it really is. On top of that, it has exactly the same accuracy as my DMM. Highly recommended!

I also installed a temperature controlled fan, it uses a nifty fan controller chip to do all that. Look at a separate post for details.

To get a better handle on the temperature at higher loads, I decided to use two MOSFET's in parallel. I was a little careless testing all this, thinking that this plus the fan would solve my heat problems. I was eager to start my tests of the two PSU's, which is why I built this tool in the first place. Well, I lost a MOSFET due to runaway thermal issues. Bummer!

Despite my search for information on how to put two power MOSFET's in parallel, all I could find (at the time), was the use of seperate gate resistors, to balance the two MOSFET's. Well, that didn't work very well. Due to the differences in the VGS for both devices, one took all the heat.

After swapping out the broken one, I was a little bit more careful, and noticed right away that there was a large difference in the thermal balance between the two. One was getting slightly warm, but the other too hot to touch. Apparently, the only real solution to the RDS(on) and therefore thermal balance issue is to build two circuits with separate op amps and sense resistors. I did not want to do that right away, and tried to see if I could get close enough. First I increased the value of the gate resistors, and also matched them to within 1 Ohm. That did not do anything to get a better balance. I then started to increase the value of the one one that got hot, but that did not produce the result I was after. In the end I used a 10K trimpot to set the VGS to a reduced level, and used my temperature probe to balance the two. I got to within a few degrees C after running the unit with a 1A load, such that the temperature was about 50 degrees C. It's not ideal, because I don't think the VGS delta has a linear relationship, but for now I'm happy.

Here is revision 3:

This design will most likely be further tuned while I'm going through the paces of testing and specifying my two power supplies, so stay tuned.

Update : August 30, 2015
Well, it didn't take too long for another refinement.

I kept mulling about the thermal balance challenge, and was not too happy about the comment I made earlier in the post above. While surfing for ideas for the parallel use of the power MOSFET's, I came across a comment that I took for face value, assuming the guy knew what he was talking about. I simply repeated his comment in the previous post that the "real" solution is to make two separate circuits. So to duplicate the op amp, FET and current sense resistor.

I kept thinking about that statement, and decided that it is misleading or even incorrect. Yes, you will get two circuits that together will share the load, but it does absolutely nothing to get a better load balance, or a better thermal balance. So, back to the drawing board.

Initially, I wanted to use two thermistors (or NTC's), thermally connected to each FET, and then use a comparator to decide if one was getting hotter than the other. The challenge was that I could not find an easy way to mount them on top of the MOSTFET's. Sure, I could have mounted them on the heatsink, but I would have lost some temperature sensitivity, with the heatsink acting as a dampener.

I then switched to using the junction temperature of transistors, but again could not find a good thermal connection to the FET's. I then went through my power transistor stack for ideas and found two excellent specimens. I happen to have quite some (very old, as in late 1970's) D40C5 darlington power transistors, that have a very flimsy heatsink. This turned out to be a perfect way to mount them with the same screw that also mounts the MOSFET, provided you also isolate the transistor tab, because it is the collector. I happen to have all the bits and bobs required, so I could continue.

BTW, the fact that I use darlington transistors, is only because I had them. You can use any TO22X kind of package, as long is you can thermally connect them to the MOSFET's.

In any case, whipping-up a small circuit to measure the delta temperature difference and drive a FET to drop the VGS on the gate of the FET that get's too hot was quite simple. By selecting a feedback resistor to create a small threshold, I managed to keep the temperatures within about 1 degree C. So whenever the temperature of Q2 id one degree C higher than that of Q1, the gate of Q2 is clamped and as soon as the temperatures are equal again, the gate is released again. Perfect!

Here is Revision 4:

Resistor R21 in the drain of the BS170 MOSFET is there to reduce the gate voltage, not to yank it to ground. With the resistor combination of R9, R14 and R21, the gate voltage drops from about 300mV,  just reducing the load and therefore temperature of Q2, while not affecting Q1, and therefore the output.

Just in case you are interested to use this idea in other applications, you need to be aware of two constraints. With this particular application, I'm not concerned about the self-heating of T1 and T2. I'm interested in the delta temperature difference, not the absolute difference. Also, the fact that the load will move from both MOSFET's to mostly one for shorter periods is of little concern for me. Both MOSFET's are individually capable of handling the full load.


After having used the DC load for about a year, I was a little annoyed about the fact that I could not obtain a zero load at the output, despite the trimmer on IC1 and using a small negative voltage for it. Although the potmeter (R6) was getting down to zero Volt, the output of IC1 remained at 3V3, resulting in a 0.0059 Volt over the shunt (R12).

I finally figured out a way to adjust the potmeter range at the lowest position to have an output of zero load. The trick was to apply a very small negative voltage at the lower part of R6, instead of connecting that to ground. I used a tap on the two diodes that created a small negative voltage for IC1, in an earlier attempt to obtain a zero Volt output, and fed that -0.7V to an added 10K trimmer. With R6 in the lowest position, the new trimmer is now adjusted such that the output is showing no load.

Here is the new schematic with that change:

June 13 2017
While going through the design of a new power supply (a new post will be coming), I was not satisfied with the dynamic switching of the load. The pulse that is coming in from the BNC input actually switches the load hard on and off, so the maximum load is used, and there is no way to limit that.

I also didn't really need a fully variable input to the load, a simple pulse would suffice. During the power supply design, I actually put a 555 based timer on a breadboard and fed that to the DC load external input. I also added a few parts to make the output variable. I liked the results, so I put everything on a little prototype board and added it to the internals of the DC load.

Here is the schematic of the Pulse Generator addition:

Those that follow my posts will notice that I no longer use Eagle. A few months ago, I switched to Diptrace. I finally ended up frustrated by the difficulty in Eagle of adding custom parts, so I started to look around. I dismissed all the free ones, including K-Cad. That is just too cumbersome, and at this moment only for experts I think. An electronic pen-pal advised me to try Diptrace, and I have been using that ever since. My new power supply, discussed in another post has been designed that way, and I also made my very first pcb layout with it.

Anyway, I made Diptrace resemble the Eagle schematics a bit, because those still look visually pleasing.

OK, back to the Pulse Generator. I used a 555 based circuit that produces a pulse train with approx. 180mSec on, and approx. 70 mSec off periods. By just changing C3, you can change the frequency. At first, I used the 12V supply to power this circuit, but later changed my mind about that. I didn't want pulses with a 12Vp-p output roaring through my box, when I do sensitive measurements.

I changed the single throw switch that selected between static and dynamic inputs, to a double throw switch. The other side now switches the pulses off, by shorting the charging cap C3 to ground, such that the 555 pulses are off when the static mode is selected. It keeps things more quiet in the box. At first I switched the power to the 555, but turning it on produced an unwanted output glitch causing a high load on the output until the 555 settles.

The DC load will be switched on during the on-pulse, and fully off during the off-pulse. Because of the 5V supply, the output of the 555 timer is about 5V p-p. That signal is used for a trigger out signal. That signal now goes to the same BNC that was previously used as a pulse input. This signal typically goes to a scope to trigger it. This trigger output is an important feature because if you test a power supply, as an example, you want to see the effects of the load regulation while you switch dynamic loads on the power supply output. If the load regulation is any good, it will be very hard to see any effects on the output of the supply, so knowing where to look, based on the trigger signal, really helps a lot.

A second divider (R5 and the potmeter R6) attenuate the signal, so at the wiper of the potmeter, I will have 500mVp-p at maximum, and 0Vp-p at the minimum.  A voltage level of 500mVp-p will turn the DC load fully on, which is the same as the static load adjustment potmeter does. I happened to have a special dual 10K potmeter, with separate controls (outer and inner) for each section. If you use other resistor values, you have to recalculate the dividers. Make sure that the maximum output on the wiper of the output setting (R6) potmeter is slightly above 500mV to be able to drive the load to the maximum.

The wiper of the output setting potmeter goes to the switch that selects the static or dynamic behavior of the DC load.

To create a minimum offset for the DC load in the pulsed mode, used to test Transient Response and Recovery Time Measurements for power supplies, the other half of the dual 10K potmeter is used to add a 0..100mV offset. This should be enough to create a "bias" current of up to 1Amp for the supply, and then drive it higher with the pulse. This allows you to see the "sagging" and negative pulse excursions of the supply output voltage better than letting the supply go to ground, or 0V when you don't add the offset.

If you now turn on the dynamic input, the pulsed output level (with or without out offset) can be set, and the Amp level will be displayed on the Amp panel meter display.

Because I had this special dual potmeter, all I had to drill was a single new hole in the front panel, and that just sits just above the static output level potmeter. The circuit is build on a small prototype board that is mounted on an extended post already mounting the main PCB.

Everything works really well, this simple DC Load is getting better and better, I'm happy.

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Tuesday, May 19, 2015

Building a Simple 1KHz Distortion Analysis Tool (upgraded)

For my audio projects, I wanted to have a method of simply measuring distortion without buying (too costly) or building a (too complex) full-blown analyzer.

I started out with a very low distortion 1KHz sine wave generator and a Twin-T filter, and put them in the same enclosure.

This is the resulting instrument.

The low distortion sine wave generator is to feed the DUT, and at the output, use a filter to supress the sine wave again. What is left should be the distortion created by the DUT and some additional noise.

The components

As it turns out, designing a very low distortion since wave generator is not that easy. Luckily I found a unit that is excellent for a reasonable price. It is called the Mickevich (the designer) Ultra Low Distortion  (<0.00001%) 1 KHz Sine Wave Generator.

What was left was to design a precise filter, the required power supply and the box to put it all in to.

In addition, I needed an external sound card interface that is used as a digitizing front-end for the PC-based FFT analyzer.

I described this project on a diyaudio forum where much more information can be found about the oscillator, and the filter I used : 

When I picked-up my electronics hobby again after I retired (early), this was one of the very first projects. I wanted this tool because at the time, I was designing and building headphone amplifiers to be used in a plane. 

In 2022, I decided to describe the unit here in more detail, so I'm not depending on the diyaudio Forum. I also decided it was toime to design a PCB and use better components for the filter, as they were recommended on the Forum.

The Ultra Low Distortion Oscillator

In order to make good measurments you need to have a very low distortion sine wave oscillator. I found the Victor Mickevics design and purchased one for 1 KHz.

Victor uses a novel FET-based AGC design with a rather simple circuit to get astounding results.

The results for this little board are stunningly good. Here it is measured with non-professional equipment and that shows only 0.0003% THD and 0.0024% THD+N. The actual specifications are even better with second harmonic at -136 dB (0.15 ppm) and third harmonics so low it cannot be measured (< -140 dB).

Unfortunately, it seems Victor is no longer active and selling them.

The Twin-T Notch Filter

The Twin-T notch filter is based on a simplified design from Dick Moore (richiem) here are the details
I tailored his design for just a single 1 KHz operation.

Here are the schematics that I put together for the combined instrument:

The power supply

The sine wave oscillator needs about 35VDC or a little higher, to create a +/- 15V.
The Twin-T also needs +/- 15V, but I didn't want to use the supply already on the oscillator. I did not want to disturb it.

This supply feeds the 1KHz oscillator with about 34VDC and the Twin-T notch filter with a balanced +/- 15V supply.

As I typically do, I use a separate AC transformer outside of the box, to avoid mains hum etc. WHen I was winter birding in Texas, I used a Rainbird 24VAC transformer, alas that was only for 115V.

Back in Europe, I use a 12-0-12VAC transformer for several projects, so I used the 24VAC to feed the power supply.

Because Victor's oscillator needs 35VDC, I could use a full bridge rectifier and an LM317 and keep things cool, rather than using a doubler and deal with 70VDC as input to the LM317. They will get hot, causing temperature related drifts. I also did not want to "steal" the +/- 15V coming from Victor's oscillator to feed the notch filter, so I added it's own supply. To create just enough headroom for the first LM317, I adjusted the output going to the oscillator to 34V, instead of 35V.

The second LM317 is used to supply 30VDC, that is split into +/- 15V.

Twin-T Notch filter

Here is the original filter circuit:

I used two (cheap) 1K 10-T pots for the filter, and shunted them with 1K. This will still leave them linear enough. Even with this shunt, the adjustment is still a little too coarse for my liking. 
I use R11 and R15 (rather than 10T trim pots) to set the filter notch about in the middle of the 10T filter pots.

To get the filter as accurate as possible, I purchased 8 good 10nF caps, and sorted them by value to get the optimum balance of the filter. The measured values are in the schematic. At the time I used a cheap Arduino based capacitance meter, which turned out to be less precise. 


Because I also wanted a nice enclosure, and not very expensive, I used a plastic one that only set me back $27.  I needed to shield the inside and did that the easy way by using copper foil (look for guitar hum materials) and made two boxes out of some circuit board material. So far this seems to be adequate.

Pictures of the instrument

After I was done and started to test things, I added an additional toggle switch to the instrument with an input attenuator.


Based on inputs on the Audio Forum, I learned that I can improve the notch filter some more by using better filter capacitors, the special audio film kind, and by using a better Opamp for IC1 (an AD797 or an LME49990).  I now have the capacitors and the AD797 in stock.

Since then, I also purchased a higher precision capacitance meter, the Juntex LC-200A. A very good meter for the price. I can do a much better job matching the values. I should update the filter with the better tuned filter capacitors, and change the OpAmp.


So in 2022, I decided to pick-up where I left and upgrade the Twin-T Notch filter. Now that I'm a lot more experienced with PCB design, I created a new schematic and a layout. 

Here is the updated schematic:

Basically the same circuit, just some different resistor values and the Opamp changes.

I used the Eagle to KiCad import system, but what a mess that creates when you want to make changes and add layouts/components etc. I may end up redoing the schematic from scratch to get it clean, but for now, I have a layout. Because I'm starting with better capacitors for the filter, I re-calculated the resistor values accordingly. They now follow the theoretical values. 

The Opamps will go in gold plated DIL-8 sockets so I can easily replace them.

Here is the completed unit after the upgrade.

Note that there is no more room for the PCB-made box.

After powering it on and letting it warm up, I had to tweak some resistor values (R4, R5, R8, R9 and maybe R13) again to get the trimmer and the two potmeters roughly in the middle of the range to have an even spread either way of the sweet spot. The values are in the schematic above. With these new values, the trimmer has a good range, and the two pots are tuned to be in the middle of the range. 
Adjusting them with these changes works great, I'm happy.

The PCB can be ordered from your favourite supplier. 
The Gerbers are on the Github :


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_HowTo: Triple function button to reboot/halt/reset/restart RPi

When I got started with my Raspberry Pi, I quickly ran into situations where I needed to reset, restart or halt the Pi. I designed the following circuit and added some code to let me do that. It is quite simple but effective. Since then, other methods have been presented by others, but maybe this will be helpfull as well.

Here is the post on the forum :


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Monday, May 18, 2015

_HowTo: Failsafe(r) use of GPIO pin for critical applications

In an effort to avoid run-away issues with my HVAC system, located on another continend, I wanted to make sure that a crash of the Raspberry Pi that I used as a controller, did not cause a desaster. Also, I needed to prevent Python crashes from keeping the furnace or the air conditioning system running unchecked.

Here is a link to the post that went through the discovery process and with the help of some smart contributors, eventually led to a failsafe design in hardware and software. This resulting hardware and code has been running for about 18 months to my satisfaction.

The post was published on the forum :


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_HowTo: Simple UPS for RPi

Quite some time ago I designed this simple but effective battery operated emergency supply. It is used to prevent short brown-outs and also allows the Pi to shutdown without corrupting the SD card.

here is the form post from :


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_HowTo: Detailed description of the popular DHT22 temp & humidity sensor

I wrote this post a while back when I wanted to use this sensor for my thermostat application built around a Raspberry Pi.

Here is the post on the forum :


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Tuning a 0..30V DC 0..3A PSU DIY kit

Before you read on, you should know that after building two versions of the DIY kit, I was not very impressed about the stability, noise, but above all, the complete lack of any form of protection. Think about that before you connect something valuable to the supply.

Subsequently, I designed a more modern power supply that you can follow here:

After building two of them as well, and learning a lot and have a lot of fun experimenting and building, I still decided to purchase a lab quality professional power supply.  That should hopefully tell you a few things. 

There are many things you can learn about the kit in this post, and also select possible improvements, but please understand that I can't really help you anymore. It's been too long ago.

Due to a move, I recently sold my large lab PSU, and needed a substitute. I wanted to be a little more flexible, and did not want a huge and heavy supply on my bench anymore.

Searching the web, I came across an inexpensive DIY kit that implemented a very popular design for a power supply. I seldom if ever need more than 1A, so I used the kit to tune it to my liking, and also added the latest modifications for the original design.

I added an LCD display and one addition to the original design is a current setting mechanism, using the display, so you can set the current limiting or constant current mode before connecting the DUT.

I have built two supplies and can connect them in parallel to get more current, or in series to get a true dual +0..30V *zero* -0..30V supply or a 0..60V supply. One is designed for 3A and one for 1A max.

After some fiddling, I also designed a simple dual tracking system when the two supplies are used in series, so one supply controls the other.

Tuning a 0..30V 0..3A PSU kit

The very popular PSU schematic for a lab power supply that can supply 0..30V and 0..3A ( has created a lot of interest.
So much so, that several Chinese suppliers have created a kit with just about all parts including a PCB for a very attractive price. I paid $12.64 and that is with free shipping. On top of that, it took less than 2 weeks for the kit to arrive.

I purchased something this one:

Note that this link may no longer work over time, just search for “AC 24V 0..30V 0..3A DIY kit” and you shall find…

If you buy the parts from a usual on-line supplier, you probably spend more on the shipping cost alone. So where’s the catch?

The kit is based on the original article and has some issues that need to be addressed. There are several postings on the electronics-lab forum that go into great details about the original design:

However, if you adhere to a few simple requirements, you can use this kit without too many changes and create a fine supply for your bench that will probably be adequate for 90+% of your power supply needs.

Here is the constraint: If you stay below a maximum current of 1.5A, the kit will work perfectly with a couple of changes in the components and we will discuss that here. The kit can also be extended with more functionality and that is also discussed here.

To start off, several components supplied with the kit (the list is at the very end of this post) are different from the original design, so let’s go over those:

D7 and D8 are 1N4733A 5V1 zener diodes, and they require 49mA for a bias. This deviates from the original design that has low current 5V6 zener diodes with a lower bias. Q3 is a 2SD9015 and Q1 is a 2DS9014. Q2 is a 2SD882 and Q4 is a 2SD1047. Q4 is much easier to mount on a heat sink, compared to a 2N3055. 

Most other parts are following the original design, however, a few resistors supplied with the kit have a wattage that is too low. R2 of 82R should be 1W, R3 of 220R should also be 1W. The supplied 0.25 Watt resistors will get too hot. R1, which is 2K2 1W will also get pretty hot, so mount that a little above the PCB, or replace it with a 2Watt resistor. R7 should also be mounted a little above the PCB.

An extra part that is not in the original design is an LM7824, to create a 24V DC supply for a fan. If you are like me, you will have a lot of 12V fans, because that is the voltage used in PC’s. In any case, I switched the LM7824 to an LM7812, because I drive a few additional LED’s with it, and also supply a Volt/Ampere display with it. If I decide to change the unit for a higher current, I may need a fan, and I have several 12V DC fans in my stash. If you decide to keep the LM7824, double the resistor value that go to the LED’s. The meter can handle the 24V. (see below) You can mount the LM78XX on the PCB, but I didn’t. It gets pretty warm, so it went on the heat sink.

If you are going to limit the maximum current to 1 or 1.5 Amp, there is no need to go fancy on the transformer, and you can use a more or less standard transformer with 24VAC and 1 or 1.5A current.
The supplied TL081 op amps’s have a deficiency that we will want to avoid, so we will not use them.
The kit comes without a schematic, parts list or PCB layout, although the stenciling on the PCB shows the values, but not the part numbers.

Let’s go over the changes I made to the original design. Here is the original schematic:

The parts list supplied with the kit is listed at the end of this post, with the changes and additions.
From that supplied parts list, we will not use D7 and D8 is a 1N4733A 5V1 zener needing a 59mA bias. We will replace this type with a BZX55C5V6 or BZX79C5V6 zener, both requiring only 5mA bias current. U1 will set the reference voltage to twice the zener voltage so 11.2V. With the required 5mA bias for D8, R4 should be 1K, not 4K7. 

Because we need to limit the maximum current to either 1 or 1.5A, R18 needs to be recalculated. This resistor had the wrong value (56K) in the original design anyway.
Here is a simplified diagram to help with the calculation, just in case you want to use another maximum current version:

Let’s see where the original calculation for R18 went wrong, and resulted in a maximum current that would literally blow a fuse, or more.
To calculate R18 for a maximum current of 3A:
Vref = 2 x D8 of 5V6 = 11.2V
Voltage over R7 of 0.47R at 3A is = R7 * Imax = 1.41V
At max current setting of P2, the top is 0R and the bottom is 10K
P2+R17 = 10K + 33R = 10033Ohm
For the equivalent circuit:
R18 = P2+R17 * (Vref+VR7 - VR7) / VR7
R18 = 10033 * (12.61 – 1.41) / 1.41 = 79K694
The original value was 56K, but that would mean a maximum current of :
VR7 = 56000 / (56000 + 10033) * 12.61 = 1,916V / 0.47R = 4A! Oops…

The following values are calculated for R18 with the new low current 5V6 zener diode for D8:

R18 = 72.5K @ 3.0A
R18 = 169 K @ 1.5A
R18 = 259 K @ 1.0A

If you want to be precise, you can still use the original R18 value of 56K, but add a trimmer of 200K or 250K in series. This trimmer can be mounted on P2, so you don’t have to mess with the PCB.

So what else was wrong with the original design, (if!) we keep to the 1.5A max. Well, the original design used Op Amps that had a flaw.

Several more changes are related to their replacement. Because we will not use the TL072, we can drop Q1, R13 and R14. They were needed to remove a glitch from the output that was caused by the TL072. The circuit around Q1 was designed such that as soon as the negative 5V6 supply collapses, when the mains is switched off, it would immediately turn Q2 off, and therefore also the output. With Q1 in place, it would protect the Device Under Test or DUT from voltages higher than what you set the output to. That can be deadly for the DUT.

Unfortunately, the circuit around Q2 is still not perfect. There were still situations by which a glitch was introduced at the output when the main supply is switched-on or switched-off. 
Let me show you:

Switching on: The top trace (A) is the output of the PSU at 25V and with a 500mA load. The bottom trace is the negative supply. The negative supply goes down from 0V in the rhythm of the main frequency, in my case 50Hz, until the D7 zener kicks in. The base of Q1 is set to 0V by R13 and R14, but this setting is upset with the supply “swinging” into place, turning Q1 on and off. Depending on the point in time when you flip the switch in relation to the main frequency, you will see this behavior. If you try it 10 times, you may see this effect once or twice.

So what happens when you switch the supply off at the mains level?

Bottom trace is the output to the DUT. So there is another glitch that can happen. Not always, but it can happen.
So, although the circuit around Q1 did a good job as intended, it removed large spikes above the output voltage setting, it was not perfect.

By replacing the three TL072’s with the TLE2141, we can eliminate the Q1 circuit all together. Furthermore, with the new op amps, the negative supply can be reduced from -5V6 to about -1.3V. That’s why we will not need D7.

We’re not done with the negative supply yet. In the Current Limit (CL) mode, for all practical purposes, the supply actually switches to a Constant Current (CC) mode. U3 does not switch from rail to rail, but switches to about +3V. This is enough to turn the CL LED on, but there is still a voltage at the output. You can now slowly turn P2 counter clockwise, and you’ll see the voltage at the output drop, while the current stays the same. This is the Constant Current mode. So in the CL/CC mode, the output from U3 switches from the positive supply of 26V to about +3V and then slowly goes to the level of the negative supply, at which point the output at the terminals is removed completely. 

Unfortunately, this is not really a great CC mode, if you look at the voltage output supply:

There are two sources for the 1.7 V p-p “noise” riding on the output of the supply. One is mains hum, the result of the rather crude way the negative supply is concocted. The higher frequency noise is the result of the closed-loop activity between U3, U2 and the output stage. U3 and U2 are in a constant battle to keep the output high (U2) and at the same time, U3 is limiting the output to stay within the current limit. There is little we can do about that without doing a major redesign, but we can at least remove most of the mains ripple.

We do that by replacing R3 with an LM337 voltage regulator (U6), and we set the output level at -1.3V with two additional resistors, R25 and R26. We’ll also add a small filter capacitor, C14 of about 22uF/10V.

If you have a habit of supplying your DUT with power by turning the PSU on and off, even with the above changes, you may still introduce a glitch in the voltage at the output terminals. I have experimented with a few possible solutions, but gave up because I could not find a simple solution to fix this. 

Here the mains is switched off while we’re looking at the output voltage. The slope is depending on the current that is pulled from the supply, so that curve may be steeper, but it’s still not very pretty.

Here the main power is applied while the output has been set for 3.3V, the most critical voltage level for devices under test. Notice the large spike that significantly exceeds the maximum voltage that has been set.

So to still allow a clean turn on and turn off to the DUT, I added a double throw switch into the mix. One part of the switch connects the anode of D9 to ground, because this will remove the power from the output. To show myself that there is no power on the output, the other half of the switch turns on a red LED. The LED is connected between the 12V and via a 4K7 resistor to the switch, which connects it to ground. Simple and effective.

I also wanted to have a Voltage and Current display, so I purchased one of these:

These are below $10 on Amazon or eBay. The small red and black wires on the right provide the power to the logic of the unit, and that can be anywhere between 3.5 and 30V DC. I connected them to the LM7812. Note that these displays should really be galvanically seperated from the supply to avoid noise injection. The alternative is to do some serious filtering in the supply voltage chain to avoid that noise.

These displays are capable of handling a car battery or big motor currents (up to 10A with the internal shunt), and therefore the current and voltage sensing wires are very thick. I replaced them with different wiring. In any case, the red wire is connected to the output of the PSU, and is the voltage sensing input. This device has an internal shunt resistor, and that is connected between the yellow and black wire. To make it easy, I connected the black wire to the minus output of the PSU (4) and that makes the yellow wire the “new” minus output. The shunt will make a tiny difference because it sits outside of the feedback loop, but the error is extremely low, because the shunt is extremely low in value as well.

On the back of the unit are two tiny trim pots you can use to adjust the voltage and current.
To set the voltage of the PSU precise, I used a 10-turn pot meter.  Look on eBay, they are not that expensive ( $6.59 with free shipping) if you forgo the super accuracy. (

There are two more additions that I made. One is to add an LED to show that the unit has main power. That green LED is connected between the 12V and through a 4K7 resistor to ground.
The final addition is another 3300uF/50V capacitor (C12) parallel to C1, to give more stability to the raw supply and to reduce ripple at higher currents.

I used a large heat sink, and mounted the LM7812, Q2 and Q4 on it. There is plenty of room to add another output transistor parallel to Q4, if I decide to increase the current.

With this heat sink, I will not need a fan with the current staying below 1.5A. 

From left to right: Q4, Q3 and the LM7812.
Q4 and Q3 are isolated, the LM heatsink is ground, so does not need it.

To create the front panel, I printed a design on photo paper, used double sided tape to fix it to the metal front panel and cut out the holes.

Here is where the 24V AC comes in. I can use different size transformers, and use them for several applications this way.

I did not use the supplied 10K pot meter for the current setting, because it did not come with a nut. It needs an M7 nut I didn’t have, so I used another 10K pot meter I had in my stash.
The supplied pot meters with the kit use an M7 nut, so I ordered a washer/nut set here : (

After I finished all the modifications and started to experiment with the supply, I saw the need to add a way to show the current limit setting, so I have now added a little circuit to the supply so I can set the Constant Current/Current Limit.

Because I already have a voltmeter, the easiest method was to use that to show the current setting. However, showing the value on the current meter display with the unit I use is tricky.

To show the current setting on the voltmeter, all we really need  is a convertor that translates the current limit setting to a voltage.
To show the relation of 1A = 1V, with R7 at 0.47R, we need a multiplication factor of 1/0.47 = 2.127.
By using an additional op amp (U5), we will make this circuit independent of the maximum current of the PSU.

If you look at the schematic, the circuit around U4 implements that function.

RV2 can be adjusted by setting P2 to the maximum value of the current, say 1A. You can measure the voltage at the wiper of P2 with a DMM and set P2 to read 1.00V on the DMM. If you implemented R18 in combination with a trimmer, adjust that trimmer first to show 1.00V with P2 at maximum. Push the CC set button and adjust RV2 to have the voltmeter of the PSU show 1.00V as well.
Here is the final schematic:

Here is the original parts list as supplied with the kit, but with my changes and additions listed as well:

R1 = 2K2 1W                                                                        Replaced with a 2W version
R2 = 82R                                                                               Replaced with a 2W version
R3 = 220R                                                                             Not needed (replaced with an LM337)
R4 = 4K7                                                                               Value changed to 1K
R5, R6, R13, R20, R21 = 10K                                               R13 not needed
R7 = 0.47R 5W
R8, R11 = 27K
R9, R19 = 2K2
R10 = 270K                                                                          Value changed to 1K
R12, R18 = 56K                                                                    R18 see text
R14 = 1K5                                                                             Not needed
R15, R16 = 1K
R17 = 33R                                                                             Value changed to 68R
R22 = 3K9                                                                             Value changed to 1K5
RV1 = 100K 10turn trimmer                                                 replaced by a 5K 10 turn trimmer
P1, P2 = 10K linear                                                               P1 replaced with a 10 turn potmeter
C1 = 3300uF / 50V
C2, C3 47uF / 50V
C4 = 100nF
C5 = 220nF
C6 = 100pF
C7 = 10 uF / 50V
C8 = 330pF
C9 = 100pF
D1, D2, D3, D4 = 1N5408
D5, D6, D9, D10 = 1N4148
D7, D8 = 1N4733A 5V1 zener                                          D8 = BCX55C5V6, D7 not needed
D11 = 1N4004
Q1 = 2SD9014
Q2 = 2SD882
Q3 = 2SD9015
Q4 = 2SD1047                                                                     Not needed
U1, U2, U3 = TL081                                                           Replaced by 3x TLE2141
U4 = LM7824                                                                      Replaced by a LM7812
D12 = red LED
Sockets for U1, 2, 3, input and output connectors, sockets and wire harnesses for P1 and P2, heat sink for Q2

Additional parts:
R23, R27 = 4K7
R24 = 1K
R25 = 240R
R26 = 10R
RV2 = 2K

RV3 = 200K or 250K (optional, see text)
U5 = TLE 2141
U6 = LM337
C 11 = 47uF/25V
C12 = 3300uF/50V
C13 = 22uF/10V
D13 = 10V 1W
D14 = Green LED
D15 = Red LED
Volt/Ampere panel meter
S1 double throw switch
S2 single throw push button

Modifying the PCB to the latest version of the supply

In the above text, I have given an overview of the changes to the components supplied with the kit, to make it work a little better. 

First of all, we need to implement the supply changes to the opamps (through D13), and so a few traces need to be cut on the PCB. This will allow us to also switch to the TLE2142 opamps.

The photo below will show you what traces to cut (in blue) on the component side of the PCB:

1.   The connection of the unregulated supply to the emitter of Q3
2.   The connection of the unregulated supply to R19
3.   The  unregulated supply connection to U3 pin 7

To install the new 10V zener diode D13, you need to remove some of the lacquer on the positive supply trace, as indicated on the photo.

The cathode of D13 is then soldered on this spot, and the anode goes to the emitter of Q3 and also to the disconnected end of R19.
See this photo for a closer look:

The original zener D7 is not installed but C14 will be mounted in this location.

The LM337 will be mounted in place of R3, and I just figured out a way to make the connections to the ADJ pin and R25 and R26 to connections that are near. Make sure the (metal) body of the 337 does not connect to anything, it carries a voltage. Use heat shrink tube if needed. With only about 10mA current, it will not get warm at all.

Turn to the reverse side of the PCB, and look at this photo:

The new C10 is mounted on the reverse side of the PCB.
R10 is mounted on the back to make it easier to connect to the negative supply.
The pin 7 of U3 is connected with a wire to the anode of D13.

The following values of components from the kit are now changed:

   R10 (from 270K to 1K),
   R17 (from 33R to 68R),
   R22 (from 3K9 to 1K5),
   RV1 (from 100K to 5K) and
   U1, 2 and 3 (from the TL081 to the TLE2141)

Despite what others have posted, I had to connect the minus supply of U2 to the negative supply, not to ground. The reason was that I could not get the output to go to 0 Volt with P1. It did go to 0V with the current limiter. With a negative supply of only -1.2V, it still does not go to 0V, but +25mV is close enough. (RV1 at 5K and R10 of 1K allowed the output to be adjusted from +43mV to + 25mV)

It has been stated that R15 and D10 have no purpose, but if you connect U2 to the negative supply, R15 and D10 remove any negative voltage from the output of U2 to the base of Q2.

Finally, if you only use the supply to about 1A, you can use a 220K value for R18 and you do not need to add RV3.  If you use a 24V AC transformer, you probably don’t need to limit the maximum output to a precise 30V, and if so, you don’t need to install RV3 and R11 stays at 27K.

So with these changes and a few more parts, the kit can be modified and the total price will still be very attractive.

Latest update. August 4 2015

I was still not very happy with the CC mode of operation. Even with the above mentioned modifications, there is still too much noise and a mains ripple on the output during the CC/CL mode.

As it turned out, a lot of this noise comes from the Volt/Amp display I'm using. The switching regulator that is used on this display injects a lot of noise back into the supply. I also was still not satisfied with the ripple on the reduced supply (by D10) for U3, U5 and Q3, and connecting the display to this supply made it all worse.

So, to tackle these problems, I went back to using the LM7824 that was part of the kit, and used that instead of D10, the 10V zener that was used to create the supply to U3, U5 and Q3.

To counter the noise injection from the display, I now used D10 to reduce the raw supply and used that to power the display unit.

While on my quest to reduce noise, I also moved the display current shunt from the output terminal, to outside of the current feedback loop. This reduced some more noise, but also made the current setting more precise. (because the shunt was inside the feedback loop, the voltage over the shunt at higher currents created an error. Small because the shunt seems to be only 25 mOhm, but still)

In order to put the shunt there, you need to cut a PCB trace from the raw ground supply to R7 and connect the current meter shunt output at the supply side of R7. Make sure R21 and R17 are not measuring the current shunt of the meter, but only R7. The current meter shunt input goes directly to the connections of the anodes of D3 and D4 and the negative connections of C1 and C2.

To eliminate a possible ground loop, the ground supply lead for the display is no longer used. The ground for the display unit is coming from the shunt connection to the raw supply ground.

To eliminate large currents on the PCB as much as possible, I connected the collectors of Q4 and Q3 directly to the point where the cathodes of D1 and D2, and the filter capacitors C1 and C2 come together.

I also installed the "optional" trimmers to set the maximum output voltage (RV2) and maximum output current (RV3). It is important to set the maximum current limit, because the granularity of P2, a normal pot meter, is greatly increased allowing you to set the current level more precise.

C16 is used to eliminate some more noise.

Because the LED's D14 and D15 are now connected to the 24V rails, their current limit resistors (R27 and R23) need to double in value.

Lastly, the output capacitor C7 was enlarged from 10uF to 470uF. That seems a lot, but professional supplies actually use a lot more.

Here is the final schematic with the latest revisions:

The rise time of the supply is now about 5mSec and the fall time is just over 2 mSec at maximum voltage and current, measured with a dynamic electronic load, capable of 50uSec transients.

With all these modifications, the output noise is now 18 mV p-p across the voltage and current spectrum, and, more importantly, stays at that level in the CC/CL mode. To qualify that, the noise floor of my scope with the probe tip grounded is 12 mV p-p, and with the supply switched off, the noise floor is just below 16mV p-p. With a positive mind, you could deduct that the output now only adds 2 mV p-p noise. Mission accomplished!

One future mod I'll do is to add a parallel output transistor to Q4. My typical applications are low voltage, and this is the largest burden for the pass transistor, because it has to bleed-off the excess voltage. I'll rearrange the LM7824 on the heat sink to make room for the second 2SD1047. I'll use .22R emitter resistors (because I have them already) to pair them up.

And yet another update: Aug 14

Not only did I indeed install a parallel series transistor (2SD1047), I also modified one of my two supplies such that it could handle more current.

I'll continue to use one which is fed by a 24V 1.5A transformer, but that maximum output is limited with a current in excess of about 25V, when the regulation starts to falter because the raw voltage starts to collapse.

So, I needed a transformer with a higher voltage rating and a higher current rating to pull this off. Unfortunatly, the most common transformers are 15-0-15 or 30V at 3A or more, and that will produce a raw voltage that is too high for the choosen op amps. The TLE2141 can handle up to 44V, but 30V AC already translates into 30 * 1,414 = 42V. Without a load, even with the bridge diode voltage drops, that is still too much. More so, since two op amps are also fed with a negative 1.3 V supply. A 14-0-14 supply would be ideal, but I could not find one.

With the higher currents, you also need a fan to cool things, so that was added as well. See a separate post on a solution that I built. At a later date I'll include that circuit into the main schematic.

The transformer I ended up buying is a 15-0-15V AC at 3,3A. With 3,3Aac, I should be able to get a solid 2Adc, plenty for my purposes. I also changed the 4 diodes that were used in the full bridge configuration and selected a bridge with 600V 10A that can be mounted on a cooling fin. A bit overkill, but it was for the same price as an 8A version. You need some overkill because of the in-rush currents to the main filter caps. The two 3300uF filter caps are inadequate for these currents, so I installed two 10,000uF at 63V ones. I used a separate enclosure to put this all in, and use 4mm banana posts and jacks to connect the raw supply to the PSU. If you do that, remember to also feed an AC signal to the PSU because that is used to create the negative 1.3V rail. The enclosure is completed with a main switch, a main fuse and a power indicator. I also feed the AC 15-0-15 taps to banana jacks on the front panel, so I can use that for other purposes.

While running some more tests, I decided to put the ampere meter shunt back at the output. There was too much of an error in the measurement, because it included the currents of the actual supply itself.

The changed schematic for the new supply is as follows:

 You'll notice that I departed from using the original way of showing all connections with wires. I now grouped the functionality so it's hopefully easier to understand.

Because the op amps are limited by their 44V rail-2-rail supply, I went back to using an LM317 to create a nice and steady 33V. This is just enough headroom to regulate the output to 30V. I used this supply to feed all op amps now, and that also required resistor value changes for the LED bias resistors. It also means that the supply modification with D10 needed to be undone on the PCB.

You'll notice that the bridge rectifier diodes are gone, and so are the filter caps and the bleed resistor. They all moved to the raw supply enclosure. I actually doubled the value of the bleeding resistor by putting two 2K2 2W resistors in series, because I found it was getting too hot with the additional voltage. I also changed D13, the Zener diode feeding the V/A display, to a more beefy 1W version, that I only had in a 22V version. I paid special attention to getting the main raw connections ( they are now a bit thicker in the schematic) to the required parts, and avoided going through the PCB as much as possible. C7, the 10uF on the output terminals is an anomaly, I just left it on the PCB, but is has little use compared to C10 which is mounted directly on the output terminals.

Other than that, there were no major changes, and the supply works really, really well. I now only need to install the fan controller but I wanted to play with the fan starting point a little more so it's quiet with small loads but kick in when needed.

Update Aug-28-2015:
I finally was able to find a simple but effective method to "tie" my two supplies together and create a tracking +30 0 -30V supply, or a +60V supply.

The principle is easy, if you connect the 0V output of one supply to the +0..30V output of the second supply, you actually can create a +30 0 -30V supply, or a 0..60V supply. You need to adjust both voltage potmeters to set the values, but if you want to measure a circuit with a variable voltage, you need a tracking mechanism. This can also be called a master/slave combination.

The trick is to make the voltage setting of one supply depending on the setting of the other supply. I experimented with various ways, but finally settled on the following circuit.
Let me explain.
The slave supply must be modified as follows. The connection of the wiper of the voltage setting potmeter (P1)  must be disconnected, and fed to a switch. The switch connects back to the old wiper connection as you can see in the schematic. The other side of the switch goes to a voltage divider that sits between the positive output of the master supply and a resistor combination connected to the 0V of the slave supply.
To connect the two supplies together, the 0V of the master gets connected with a lead to the + output of the slave, and this becomes the new 0V. The above schematic should make that clear. If you want a 0..60V supply, the + is the + of the master, and the 0V is the 0V output of the slave.
The modification for the master is even easier. You need to add one resistor (R40) to the + output, and feed the other side to a connector such that it can be fed to the slave. As you can see on one of the pictures of my supplies in the beginning of this post, I originally used a 3-pole DIN connector to feed the 24V AC to the PSU. I have now switched to banana jacks, and have used the DIN connectors to tie the two together.

The trimpot R41 needs to be set such that the voltage setting on the master is the same as the voltage output on the slave. The signal going to the switch will be close to the reference voltage of 11V2.

I found that the best tracking accuracy can be obtained if both supplies are set to 30V in the +/- mode as in the schematic. You can then flip the switch to the Tracking mode, and you adjust R41 until the slave also reads 30V. You will notice that the tracking is pretty accurate (about 1%) until you go below 4-5V, it then gets increasingly out of sync to a few 100 mV at 1V. This must be due to the linearity difference in the gain of both the U2 op amps. All the other methods I tried were to eliminate this, but I did not succeed. On the other hand, this accuracy is good enough for me.

I have also added R43 as a security measure, to make sure the slave supply will not have an (undefined) output if the link between the sense resistor in the master is not connected to the slave or when the switch is moved from one position to the next.

You should also know that you need to set both current limits independently for both supplies, but if the master goes into current limit or constant current mode, the slave will follow suit, regardless of it's setting.


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