LM324 Power Supply with variable voltage and current

[mike_mike]

Not yet. I will try to do some tests.

[mike_mike]

I tried some tests with PSU Shorter. The load resistor was 0R, so I tested the PSU to short circuit response until now. Blue trace = pin 3 of LM555, yellow trace = output of PSU.
1. Vout=1.75V
a. Iout=2.6A 97_001, 97_002
b. Iout=1.3A 97_006, 97_007
c. Iout= almost minimum 97_008, 97_009, 97_010

2. Vout=15.14V
a. Iout=2.6A 97_013
b. Iout=1.3A 97_014
c. Iout= almost minimum 97_015

3. Vout=30V
a. Iout=2.6A 97_022, 97_023, 97_024
b. Iout=1.3A 97_019, 97_020, 97_21
c. Iout= almost minimum 97_016, 97_017, 97_018

Please have a look and tell me if the results are good ?

The next step should be testing with different loads, what resistor values should I use ?

[xavier60]

Test at 1 amp without going into CC mode.

[mike_mike]

It's oscillating at 1A load, current pot set to max output current, without going into CC mode.
1. 10V output, 10R load: 98_000, 98_001
2. 20V output 20R load: 98_002, 98_003

This mean that I should modify the value of C3 and C4 ?

[xavier60]

I need to see how it's laid out. Try decreasing R5 and increasing C3.

[mike_mike]

This is the layout that I made for this psu. I wanted to connect the 2 pots on the PCB by using 2 terminal blocks.
The PNP power transistor (Q2), 2x TIP35C, 1n5404, emitter resistors (R14,R15), the 100R resistor (R13) and R11 are on the heatsink.

[mike_mike]

I increased C3 to 220pF and I decreased R5 to 8k2.
1. Vout = 20V, Rload=20R: 99_000, 99_001
2. Vout=20V, Rload=10R: 99_002, 99_003
3. Vout=10V, Rload=10R: 99_004

[xavier60]

I would not have expected that small a change of R5 to make such an improvement. Are you certain that C3 was really a 150pF?
The ringing in bmp_99_001 could possibly be happening externally to the PSU. The probe should be connected at the PCB.
If the ringing is external, the amplitude will be greater as the probe is moved closer to the shorter.

[xavier60]

Anyway, I see that the response is 5uS, unnecessarily fast.
Make R5 4.7K and C3 470pF.

[mike_mike]

Yes, C3 was really 150pF.
Right on the output of the PSU: 100_000, 100_001
On the PSU Shorter: 100_002

[xavier60]

That proves that the ringing is external. Try those other values.
For example, my Agilent PSU has a 30uS response.

[mike_mike]

I tried with R5=4.7k and C3=470pF
Vout=20V, Rload = 20R

[xavier60]

Odd that it didnt change much and the ringing is large again?

[xavier60]

Show us mainly the voltage dip when load is turned on.

[mike_mike]

I tested again, it was my mistake, because I connected the ground clip to the wrong point.

[mike_mike]

Show us mainly the voltage dip when load is turned on.

[xavier60]

The 5uS/Div shot shows all that's important. It all looks fine, could be a bit slower.
Lastly, I would like to see how much the voltage steps at the Base of Q1 during the 1 amp step load.
It will tell me how much gain your output stage has.

[mike_mike]

Lastly, I would like to see how much the voltage steps at the Base of Q1 during the 1 amp step load.
It will tell me how much gain your output stage has.

Please find attached...

[xavier60]

The gm is 8, close to typical and doesn't explain why this build needed different compensation.
Need to see a few more built.

[mike_mike]

Does the circuit need more testing ?
I would like to know this because I want to build a case for the PSU...

[xavier60]

Does the circuit need more testing ?
I would like to know this because I want to build a case for the PSU...

You could do some more test at different load currents to be sure.

[mike_mike]

Can you please recommend some load resistors for the PSU shorter and the values for the output current and voltage in order to test the PSU at key points ?

[xavier60]

Can you please recommend some load resistors for the PSU shorter and the values for the output current and voltage in order to test the PSU at key points ?

5 and 10 ohms. You can adjust the voltage to get various currents.
Start at 100mA with your 20 ohm.

[mike_mike]

I tested with 5R, 10R and 20R as load, the current pot was set to maximum.
A few of the results:
Vout = 30V
Rload = 10R: 102_025 to 102_028
Vout = 12V
Rload = 20R: 102_12 to 102_014
Vout = 2V
Rload=5R: 102_007 to 102_011

Please have a look and tell me what you think.

Edit: I also measured the minimum output current, which is a few milliamps. When I rotate the current pot to minimum, the red led does not light up, until I connect a overload to the output of the PSU, for example a red led.

[xavier60]

The overshoot looks normal. Ill compare it to mine later.
The minimum current not going to zero could be due to opamp offset. If the opamp is socketed, try a few others.
If I had of anticipated this, the problem could have been covered up by a high value resistor from 5V to the inverting input to give it a definite negative offset.