Linear lab power supply

[ArthurDent]

The good SIL socket strips that are .1 inch on center have an insert that effectively grabs the component lead and makes a very good contact. They are made so you can easily snip off the number you need.

[JuanGg]

Sorry for not posting in a while. Exams...

I sometimes solder in IC socket pins to where I expect to have to change component values later.
I get them from chopping up machined pin IC sockets. Or they can be bought in SIL form.

I tried a couple values and ended up using 320 K, mainly because I didn't have an easy (no more than two resistors) way to make 400 K.

The good SIL socket strips that are .1 inch on center have an insert that effectively grabs the component lead and makes a very good contact. They are made so you can easily snip off the number you need.

I have used those before. They are really nice, although a bit more expensive than the standard ones (which I have a lot of) that you have to take one pin off to cut them to length.

I have worked a bit more on the code, and updated the calibration constants so it is reasonably accurate in setting and measuring voltage and current. Everything works as expected and I find the interface easy to use. Constant current mode is indicated by all decimal points showing up on the current display, I saw this on a comercial power supply on this forum somewhere. See first two images.

So here is the problem:
While I was working on the Constant Current mode detection, which is done by a 1/2 voltage divider on the CC op-amp's output, i noticed the decimal points flashing. Attached is a scope screenshot of the output of said divider going to the microcontroller. That is with no load at all and max current set to 100 mA. The "oscillation" goes away when I set the max current to around 300 mA and then starts again going down to 30 mA or so. It seems to work fine drawing some current trough a resistor, but it sometimes oscilates again. I would expect this right on the edge of current limiting, but not drawing 0-30 mA with constant current set to 100 mA.

I went and measured the current sense amplifier's output, same conditions as first measurement, which is the second scope screenshot. Any ideas why this happens?

    Juan.

[Kleinstein]

It looks like there could be a problem with the current control loop under some conditions.
For a first test one could short out the transistor Q5, so that the control would be more conventional for a start.
I know the extra anti wind-up can be a good idea, but it could also cause some problems. So it would be good to know if the regulation work without.  From simulations I know that the anti-windup part can be difficult.

[xavier60]

I'm able to recreate the same symptom here also by changing R8 to 1MΩ.
R8 maintains a consistent voltage on C3 so that there will be a  consistent controlled current overshoot when a sudden overload occurs.
It also causes an unwanted feedback path. This is why I chose the extreme value of 10MΩ.
The anti-windup circuit is stable for me with a 2.7MΩ for R8. Or R8 can be removed for now.

[JuanGg]

I went to the local electronics shop and got the only 10 M resistor they had. I fitted it in and the problem goes away. I'll try with 5 or 3 M for the second channel, which I am still building.
    Juan

[xavier60]

I went to the local electronics shop and got the only 10 M resistor they had. I fitted it in and the problem goes away. I'll try with 5 or 3 M for the second channel, which I am still building.
    Juan

Except for one small problem that I overlooked, I don't expect any show stoppers from here on.
There is some voltage overshoot when transitioning from CC to CV. This is caused by windup of the CV loop while in CC mode.
Because the amount of overshoot is related to the current setting and output capacitor size, the overshoot will be minimal with your implementation of the design.
A resistor in series with a capacitor placed across the top divider resistor prevents the overshoot.

[xavier60]

The addition of D8 prevents windup of the CV loop while the power supply is operating in CC mode.
It limits the charging up of  C3 while the CV op-amp's output is pegged at close to full rail voltage.

[JuanGg]

The addition of D8 prevents windup of the CV loop while the power supply is operating in CC mode.
It limits the charging up of  C3 while the CV op-amp's output is pegged at close to full rail voltage.

I'll add that diode just in case.

I have assembled the second board and got the opto-isolated serial communication working. I also tested and wired the thermistors that measure heatsink temperature and tested the fans. So far everything works as expected. I have 3d printed some PCB mounts and I'll assemble the thing shortly. Then I'll have to calibrate both channels and bend the aluminum sheet top cover.
    Juan

[xavier60]

I have since changed D8 to a silicon signal  diode.
I have been testing here with the regulator now powered from  rectified and filtered transformer secondaries.
There is a small amount output ripple while in  CC mode.
I'm blaming this on the Early Effect that causes BJTs to not act as perfect constant current sources. I other words, current increases with increasing Collector to Emitter  voltage.
Your shunt is ten times higher in value. I'm expecting this to greatly reduce the current ripple.

[xavier60]

I omitted one of the transistors in the output path to reduce the regulators dropout voltage, expecting this to cause problems.
With a few tweaks, it's working well. The dropout is now 1 volt at 5 amps.
The big surprise is the linearity improvement  of the output stage. The waveform is from the Base of Q2 with the output loaded with a 5 amp peak triangle wave. I purposely allowed the dwell at 0 amps.
R14 provides some feed forward which greatly reduces the ripple current in CC mode.

[JuanGg]

I'll test my PSU on CC mode. If it doesn't look too bad, I may leave it as-is.

I have most of the mechanicals done. Those heatsinks were a pain to drill through without a drillpress. I used some thermal paste i got of ebay.
I don't think fans on this thing will ever need to come on.
PCBs are held in place nice and solid via 3d printed brackets.

   Juan

[xavier60]

The recent changes had caused the regulator to produce uncontrolled output whenever the 8V rail was below 3 volts because the output of the TLC072 goes Hi-Z at this low supply voltage. A pull-down resistor on the output of the CV op-amp solves this.

The 2nd schematic is a simplified version. I have omitted the anti-windup circuitry for the CC op-amp.
Sudden and large overload current causes D5 to conduct via the CC op-amp's feedback path. This short circuits the op-amp's negative feedback path and also provides a current path to ground to rapidly discharge the feedback capacitor, C4.
It's not as fast as the original anti-windup circuit, but fast enough at about 5µs.

 Update:

I found a problem with the simple version. If the rise time for overload current is very fast, the non-inverting input of the CC op-amp is driven too far negative causing a messy transition from CV to CC mode.
I could have added another diode.
Instead, I have now connected the op-amp's GND pin to the other side of the shunt.
The extra supply fluctuation to the TLC072 doesn't unsettle it at all, even before adding the RC filtering.
I have updated both schematics.

[xavier60]

With the front panel done,  I can put it to practical use. I still need to build a board for fan and relay control.
When I placed it amongst my other test equipment, I noticed that it causes the image on my DSO's CRT screen to wobble.
I guess that there is too much magnetic leakage from the transformer.  Ill try replacing the transformer with a toroidal type.

Extra: My Agilent U8002A PSU is even closer to the DSO and only causes image wobble when loaded. I figure that there is very little magnetic leakage from its toroidal transformer, only from its 900µH DC choke.

I have chosen this transformer. https://au.mouser.com/datasheet/2/410/media-845752.pdf

Knobs,
https://www.aliexpress.com/item/KAISH-10x-Guitar-Amp-Effect-Pedal-Knobs-1-4-Davies-1900H-Style-Knob-Set-Screw-Various/32829147338.html?spm=a2g0s.9042311.0.0.118f4c4dt6ZdMN

[JuanGg]

With the front panel done,  I can put it to practical use. I still need to build a board for fan and relay control.
When I placed it amongst my other test equipment, I noticed that it causes the image on my DSO's CRT screen to wobble.
I guess that there is too much magnetic leakage from the transformer.  Ill try replacing the transformer with a toroidal type.

Extra: My Agilent U8002A PSU is even closer to the DSO and only causes image wobble when loaded. I figure that there is very little magnetic leakage from its toroidal transformer, only from its 900µH DC choke.

I have chosen this transformer. https://au.mouser.com/datasheet/2/410/media-845752.pdf


Knobs,
https://www.aliexpress.com/item/KAISH-10x-Guitar-Amp-Effect-Pedal-Knobs-1-4-Davies-1900H-Style-Knob-Set-Screw-Various/32829147338.html?spm=a2g0s.9042311.0.0.118f4c4dt6ZdMN

That looks really nice, I am looking forward to see it finished.

Here is how mine is doing:
I have cut and folded the aluminum metal cover, which was relatively easy as it's pretty thin. The software side still needs some work, but I am on it (doing it the analog way has its advantages).
I left it running for an hour or so on a 20 power resistor at 10 V. The heatsink gets to about 25 ºC and then the fan kicks in and cools it down in a matter of seconds. See photos attached.

[xavier60]

Yours is looking neat also. I especially like the plastic work. I have a more tedious method of making a cover. I will be screwing together 4 lengths of extruded angle aluminum.
I have made a PCB for the simple version that mainly omits the anti-windup for the CC op-amp. The combination of the diode from the op-amp's inverting input to ground and moving of the op-amp's GND pin to the negative side of the shunt seems to work well enough for fast current limiting.
I have included the option for a TL431 shunt regulated 5V reference. And also the constant current preload.

The anti-windup idea is still useful for larger designs like my 30V 20A bench supply. Its micro-controller controls the charge on the CC feedback capacitor from a lookup table so that there can be the proper amount of allowed current over shoot for all CC settings.

[JuanGg]

The anti-windup idea is still useful for larger designs like my 30V 20A bench supply. Its micro-controller controls the charge on the CC feedback capacitor from a lookup table so that there can be the proper amount of allowed current over shoot for all CC settings.     

That's interesting.

I have made a front panel overlay, colors should have matched my electronic load (I copied the color code in inkscape) but seems that the copy shop goofed up.
I also added rubber feet and found a place for it in my bench.
    Juan

[xavier60]

Very nice, well done!  What's next I wonder?

The toroidal transformer fitted in well and doesn't cause image wobble on my DSO's screen when positioned close to it.
I have properly fitted the PCB of the simplified version for more practical testing. I have set the compensation to give 3µs load transient response.
Also fitted a noisier fan. https://au.element14.com/sanyo-denki/109r0612g402/axial-fan-60mm-12vdc-27-5cfm-39dba/dp/2768778

[xavier60]

My bench supply project developed a nasty fault. As the voltage setting was increased from minimum, the output voltage would stay at zero then jump to the full regulated voltage of 33V at the 10% setting.
The cause was because of a break in the resistance wire in the 10 turn Pot. The wiper voltage would jump from zero to full reference voltage as it passed over the break.
I don't think that this is common problem.  There is no easy way to protect against it.

[iMo]

I do not know how is your pot wired, but a resistor (large value) from wiper to GND may help. When the wiper is loosing contact the resistor pulls the input of the opamp to GND.
PS: my favorite hint with PSUs

[xavier60]

I do not know how is your pot wired, but a resistor (large value) from wiper to GND may help. When the wiper is loosing contact the resistor pulls the input of the opamp to GND.
PS: my favorite hint with PSUs

I often gave that advise also since I had a batch of 10 turn Pots all develop open wiper contact. There are actually 3 wiping contacts in series in the construction of  helical 10 turn Pots. https://partnership.bourns.com/bu/bu_prec.shtml
A few drops of PAO oil seems to have permanently fixed them.
With the break in the resistance element, the Pot acted more like a switch, selecting either 0V or the full 5V reference for the CV reference input.
I have thought of two ways to protect against this problem that can be used in combination.
One is to monitor current flow through the resistive element.
The other is to monitor for sudden increases in wiper voltage. I'm going with this one for now.

[xavier60]

To the control board I have added a dual comparator which monitors for sudden increase of wiper voltage for the CV and CC Pots via RC differentiators.
The comparators' output pins are wire ORed together and are able to immediately shut down the main regulator's output.
The control board's PIC16F88 micro-controller also gets interrupted and latches off the main regulator.
I have setup the time constant and threshold so that I can purposely cause a false trip by turning the Pot shafts very quickly with knobs removed.
With the knobs fitted, it takes many attempts to cause a false trip.

[panoss]

Just ordered 10 of them (for the same price as 5 ?) and a couple spare components. About 1 € each board including shipping. Really cheap. Let's see how it goes.
    Juan

[sorry for the off topic but] Which is this pcb service that charges so low?
(I suppose it's not Allpcb or JLCPcb because they charge shipping, not for that cheap)

[info]

It could be jlcpcb, because he said each pcb costs $1 and he got 10, which corresponds to the $2 per 10 boards and $8 for the cheapest shipping method. Just saying

[JuanGg]

Sorry for not posting in a while. I've been busy...

Very nice, well done!  What's next I wonder?

The toroidal transformer fitted in well and doesn't cause image wobble on my DSO's screen when positioned close to it.
I have properly fitted the PCB of the simplified version for more practical testing. I have set the compensation to give 3µs load transient response.
Also fitted a noisier fan. https://au.element14.com/sanyo-denki/109r0612g402/axial-fan-60mm-12vdc-27-5cfm-39dba/dp/2768778

Next comes a bit of documentation. I'll tidy things up a bit and make all final files available on an online repository.
That transformer looks supperb. Good to see it doesn't cause any problems.

To the control board I have added a dual comparator which monitors for sudden increase of wiper voltage for the CV and CC Pots via RC differentiators.
The comparators' output pins are wire ORed together and are able to immediately shut down the main regulator's output.
The control board's PIC16F88 micro-controller also gets interrupted and latches off the main regulator.
I have setup the time constant and threshold so that I can purposely cause a false trip by turning the Pot shafts very quickly with knobs removed.
With the knobs fitted, it takes many attempts to cause a false trip.

Interesting problem and solution. Luckily, not having analog pots eliminates that concern in my design .
What I found is that if the op-amps get loose on their sockets, I get 30 V out. The microcontroller notices that and outputs an error message. It only happened once after moving the supply around a bit while making the case, so it shouldn't be a problem.
    Juan

[JuanGg]

It could be jlcpcb, because he said each pcb costs $1 and he got 10, which corresponds to the $2 per 10 boards and $8 for the cheapest shipping method. Just saying

I did precisely that, plus buying a few spare components to get the most out of shipping.
    Juan