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: http://www.paulvdiyblogs.net/2017/07/my-new-power-supply.html

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.
Enjoy!
paulv

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 (http://www.electronics-lab.com/projects/power/001/index.html) 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:

http://www.electronics-lab.com/forum/index.php?board=2.0

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

Or

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. (http://www.ebay.com/itm/10K-Ohm-3590S-2-103L-With-Turn-Counting-Dial-Rotary-Potentiometer-Pot-10-Turn-/231499582066?pt=LH_DefaultDomain_0&hash=item35e6734a72)

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 : (http://www.aliexpress.com/snapshot/6615095977.html?orderId=66887397125609)

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

PCB

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.



Enjoy!

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