How this linear PSU circuit works, and why it doesn't

[bogdan2014]

I've built this linear lab PSU circuit based on a project found on the internet. I admit it was pretty much copying and adapting sections of it to fit my needs, without actually understanding how it's supposed to work, so consequently I'm stuck with troubleshooting it. I suck at linear circuits so I'm asking for help.
The schematic is posted below. Ignore the tracking pre-regulator part as it's bypassed for now. What's basically hapenning is that I only get about 5V out regardless of the action of the two potentiometers (current and voltage). I suspect there's something wrong with Q3/Q4 circuit, since that should work on its own, maybe R9/ R10 values are too high?

[xavier60]

Assuming that the output is unloaded, for any non-zero setting of RV2, pin 7 of U2b should be high. Q2 should be turned on hard with next to no Collector voltage and providing a solid ground path for Q1's Emitter. Confirm this.

With VR1 set half way, what voltages are on pins 1,2 and 3 of U2 as well as its supply pins? And check the output voltage again.

The voltage at each end of R10 might be useful.

[mikerj]

TL072 is not a suitable device for this circuit, it suffers from phase reversal when it's limited common mode input range is exceeded which is almost certainly the case in this circuit.

Measure the voltages on the input and output pins of both op-amps and report back.

[ggchab]

To always sink some current ? But useless when the output is close to 0V
Some years ago, mjlorton has made some videos on a power supply using a similar circuit. But I don't he finished the project.

[bogdan2014]

Assuming that the output is unloaded, for any non-zero setting of RV2, pin 7 of U2b should be high. Q2 should be turned on hard with next to no Collector voltage and providing a solid ground path for Q1's Emitter. Confirm this.

Confirmed.

With VR1 set half way, what voltages are on pins 1,2 and 3 of U2 as well as its supply pins? And check the output voltage again.

With RV1 half way, 2.5V at pin 3, 0.5V at pin 2, 10.7V at pin 1. Supply voltage is around 12V. And pin 1 of Q1 is at almost 0. Output voltage is 3.1V (with 12V input).

TL072 is not a suitable device for this circuit, it suffers from phase reversal when it's limited common mode input range is exceeded which is almost certainly the case in this circuit.

It's actually an LM358 there, I've used the TL072 for the footprint. I also have some other op-amps I can try.

WTF is that LM334Z (U6) doing from V+ out to GND?

That's a constant current sink, it gives a minimum load to the circuit. It's currently not populated on the PCB.

[xavier60]

Check the voltage across R10 and all other voltages in the output stage. Measure the voltage across Base and emitter of Q3.

[bogdan2014]

I think I found the culprit, Q3. Replaced it and it seems to be working, I get the full output and the voltage adjustment works. I'll will test the rest of the circuit too.

[xavier60]

I'm curious about the stability. Check for oscillations at the op-amp's outputs if possible.

[mikerj]

The ground side of the voltage feedback divider appears to be on the wrong side of the current sense resistor, so it won't compensate for the voltage drop across it.

[bogdan2014]

Yes, that is true. Will fix that.

[xavier60]

Actually just about everything should be grounded to the top of the CS resistor except for the negative of the unregulated supply and the op-amp's negative supply pin. There will then be the problem of ripple current through R1 will also flow through the CS resistor.
Is there a link to the original design?

Edit: Forget all of that for now, it will mess up the current regulation. Test it as is for now.

[xavier60]

This design is similar to yours, https://www.eevblog.com/forum/projects/simple-diy-linear-psu/msg2951262/#msg2951262
It can be used as a guide to how everything should be grounded with respect to the CS resistor. The ripple in the reference bias current can be solved with a constant current source.
The design I have linked wont work properly with a pre-regulator.

[bogdan2014]

https://www.mjlorton.com/wp-content/uploads/2015/08/Bench-PSU-v25-Schematic.pdf
This is the original design

[xavier60]

https://www.mjlorton.com/wp-content/uploads/2015/08/Bench-PSU-v25-Schematic.pdf
This is the original design

I see a current source for the reference zener, another LM334 could be used in your design.
The way the grounding has been done, the way I see it, the load regulation will change with the CV Pot setting. The output voltage would increase with load for some part of the setting range.
Test your design for stability first. For C5 to be effective, put a 10K resistor in series with the signal path from the CS resistor.

[Miti]

Shouldn’t there be a diode at the output of U3?

[bogdan2014]

Shouldn’t there be a diode at the output of U3?

Damn, good catch
Well, good thing I don't plan to use the pre-regulator anyway.

Test your design for stability first. For C5 to be effective, put a 10K resistor in series with the signal path from the CS resistor.

Yes, there's place on the PCB for that resistor, but I had a jumper in place.
I poked around with the oscilloscope and found some oscillations from pin 1 of the op-amp all the way through to the output. It's actually audible.

[xavier60]

Shouldn’t there be a diode at the output of U3?

Damn, good catch
Well, good thing I don't plan to use the pre-regulator anyway.

Test your design for stability first. For C5 to be effective, put a 10K resistor in series with the signal path from the CS resistor.

Yes, there's place on the PCB for that resistor, but I had a jumper in place.
I poked around with the oscilloscope and found some oscillations from pin 1 of the op-amp all the way through to the output. It's actually audible.

Q1 and Q2 add too much gain to the CV and CC loops. I have not heard of anyone being able to properly stabilize that type of design.
It sounds like you have already made a PCB?
It might be possible to adapt the CV loop from the design I have been using but not the CC loop.
 

[bogdan2014]

Yes, I already have the PCB manufactured. I don't think it's worth adapting it, so I either find a way to reduce the oscillations or re-do the whole project.
Maybe lowering the gain somehow?

[bogdan2014]

I re-did the project based on your PSU. Looks better and simpler. I'll go ahead and do a PCB.

[xavier60]

If a TLC072 op-amp is used, the addition of R7A will solve the power down voltage spike problem.

[bogdan2014]

I'm back at this project and I'd like to post the schematic before sending the project to the PCB manufacturer. If there's anyone still watching this topic, please check and report if anything is wrong. Thank you

[xavier60]

I'm back at this project and I'd like to post the schematic before sending the project to the PCB manufacturer. If there's anyone still watching this topic, please check and report if anything is wrong. Thank you

There are some serious mistakes.
Make the current sensing the same as this,
 https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3089254/#msg3089254
Notice that Im using a white LED in Q1's Emitter circuit. But stick with your Zener because of the 12V control rail.

 Copy the TL431 circuitry from this,
 https://www.eevblog.com/forum/projects/linear-lab-power-supply/?action=dlattach;attach=751023

It's good that you are using an independent supply for the control rail.

[eblc1388]

As someone has already commented.

Edit: I've overlook a mistake. Even with the changed current sensing connection, it will not work as the voltage drop across the sensing resistor will  be negative w.r.t. ground. This requires the Opamp comparator be wired as inverted amp for the circuit to work.

[bogdan2014]

Can I ask why there's a voltage divider following the adjustment pots and why the CC pot divider is reference to the return, not the 0V? The values seem quite high.

[xavier60]

Can I ask why there's a voltage divider following the adjustment pots and why the CC pot divider is reference to the return, not the 0V? The values seem quite high.

If you mean the divider after the CV pot in the simplified version? It's because the 8V control rail is being used as the reference.
I don't quite understand the 2nd question. That isn't really a divider after the CC pot, they are more so summing resistors.
Make R6, R7, R2, R3 all 10K. C6 and R17 aren't needed. 2mA is enough for the TL431.
U4 should return to GNDA.
The CV compensation should come from the ORing node.

EDIT:
R4 actually is too high because you are using a higher value CS resistor. What max current are you aiming for?