LM324 Power Supply with variable voltage and current

[xavier60]

No, it will be fine.

[mike_mike]

What are the consequences if I replace R34 and R37 with a wire in the circuit ?
I saw that R37 limits the current through the LM317, while the R34 limits the current through the preload...

[xavier60]

What are the consequences if I replace R34 and R37 with a wire in the circuit ?
I saw that R37 limits the current through the LM317, while the R34 limits the current through the preload...

R34 isn't needed with a permanent reverse polarity protection diode across the output terminals.
R37 is in case the LM317 ever breaks down, very rare. So R37 can be bridged out or something else put there that will act like a fuse.

[mike_mike]

I just built a new PCB with a few modifications, for example I replaced the connectors for the potentiometers with another type of connectors and I modified the position of a few other components.
It is sufficiently to test the PSU, by checking the response at 1A load ?

[xavier60]

Yes, that will be fine.

[mike_mike]

I powered on the circuit and after that, the white and the green led's started flickering, and the PSU does not work normally.
I checked the PCB twice but it seems ok to me. Checked with another LM358, but the same thing happens...

[xavier60]

Check that the 8V and 5V are stable with your DSO. Check that the CS resistor is connecting the 2 grounds together.
Show the overlay.

ALSO: disable CC mode by removing D2.

[mike_mike]

I finally found the problem: it was the connection between the potentiometers and the PCB (I used new connectors, but they are not good, so finally I soldered the wires to the PCB).
Modifications in comparation to my previous build:
1. Used 12V zener (I run out of 10V zener)
2. C3 is also 1nF, but it is not ceramic disk, now it is MLCC
3. 3n3 capacitor, initially it was ceramic disk, now it is film capacitor (it has green color)
4. I used new led's
5. I used LM258 from TI, to check if the PSU works good with it
6. I used new pots (both for current and for voltage)
7. All other components are new, except the LM317 and filter capacitor
8. I used 1x 4700uF/63V in parallel with 3x 2200uF capacitors and a 6A rectifier
9. I used TIP35C as power transistors, and BD244CG

Test:
1. Vout=10V, I=2.89A, Rload=10R
107_000, 107_001
2. Vout=20V, I=2.89A, Rload=20R
107_002, 107_003

Output of LM317: 107_004
Output of Tl431: 107_005

[xavier60]

That all looks normal. There is no need to show traces like the 2nd one. It's just a disturbance caused by the 555.

[mike_mike]

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.

The problem is that with the LM258P, the minimum output current is about 20mA.
If I need the minimum current to be about 0mA, or very close to 0mA, should I connect a resistor like you said ?

Edit: I  checked with a 10M resistor, and now the current starts from about 0.25mA (which is good) and it is starting to increase after 0.5 turns of the potentiometer... (I use a multiturn 10k potentiometer for current).

[xavier60]

What kind of 10 turn pot are you using? I had a lot of trouble with Bourns  knock-offs.
After some use, the resistance wire would break on the CV pot causing the power supply output to be either 0v or full voltage.
I have had no trouble after changing to these over a year ago, http://www.bochen-cn.com/wxd3-13.html

[mike_mike]

This is the pot that I am using for current: POT2218P-10K, from SR Passives.
And for the one for voltage (also 10K), please see the attached datasheet.

Edit: connecting the resistor, as you said in one of the previously replies, between +5V and pin 2 of 358 (or 258), it is a bad ideea ? Or it can lead to problems ?

[xavier60]

Edit: connecting the resistor, as you said in one of the previously replies, between +5V and pin 2 of 358 (or 258), it is a bad ideea ? Or it can lead to problems ?

No problem, It just applies a very small amount of offset to the opamp's input.

[mike_mike]

Ok, then I will connect a 10M resistor between the +5V rail and pin 2 of Op Amp.
What should I do next ? does this modification require new tests, for example, testing the response at 1A load or the response at short circuit at the output of the PSU ?

[xavier60]

Ok, then I will connect a 10M resistor between the +5V rail and pin 2 of Op Amp.
What should I do next ? does this modification require new tests, for example, testing the response at 1A load or the response at short circuit at the output of the PSU ?

It will cause no change in performance. You can check the fast current limiting like with your previous design.

[mike_mike]

I connected the ground clip of the scope at negative output of the PSU, the probe at the other end of the 50m resistor and the other probe at the output of 555.
1. Vout = 15V, I=2.89A, Rload = 0R
108_000, 108_001
2. Vout = 32V, I=2.89A, Rload = 0R
108_002, 108_003

It looks like the current through the 2x TIP35C is about a maximum of 25A...

[xavier60]

Ill have to test mine again to see what it is with the faster opamp.
I'm certain that yours is safe because the spike is so brief at about 10uS even though the data sheet doesn't show a load line for 10uS.
https://www.onsemi.com/pdf/datasheet/tip35a-d.pdf

[mike_mike]

... and this is with 2x2N3055 and BD244CG
Vout=32V, I=2.89A, Rload=0R

It looks like this time, the spike is a little bit smaller in amplitude than when using TIP35C...

It is still safe, even for 2N3055 ?
When you check if it is safe or not, do you look at the SOA graphic in datasheet ?

[xavier60]

I'd call it safe if they are 2 of these 2N3055's, http://www.farnell.com/datasheets/20519.pdf
The SOA graph includes a 50uS load line.

Testing with the CC set to 1A, mine makes an 800mV triangle shaped pulse across the CS resistor, 16A. The pulse is 1.2uS wide at its base.
With the CC set to 5A, it peaks at 20A.

[mike_mike]

Testing with the CC set to 1A, mine makes an 800mV triangle shaped pulse across the CS resistor, 16A. The pulse is 1.2uS wide at its base.
With the CC set to 5A, it peaks at 20A.

I think this is because you used another type of Op Amp ?

I think that I will mount the circuit into a case and I will put the PSU to work, together with the transformer, heatsink and a digital ICL7107 voltmeter...

[xavier60]

Testing with the CC set to 1A, mine makes an 800mV triangle shaped pulse across the CS resistor, 16A. The pulse is 1.2uS wide at its base.
With the CC set to 5A, it peaks at 20A.

I think this is because you used another type of Op Amp ?

I think that I will mount the circuit into a case and I will put the PSU to work, together with the transformer, heatsink and a digital ICL7107 voltmeter...

Mainly due to faster opamp and faster output transistors.
How will you power the digital voltmeter?

[mike_mike]

I will power the digital voltmeter using a individual 9Vac/2VA transformer, and a individual +/-5V power supply.
Please find attached the schematic of the voltmeter. The displays that I used have different part numbers, but they are also common anode.

[xavier60]

That's a good way to power it. Trying to use the 8V control rail would cause problems.

[mike_mike]

What if I use more than 2 power transistors (2N3055 or TIP35C) to reduce the dissipation per transistor ?
Is there any thing that needs modifications beside adding 1 or 2 more power transistors, if possible ?

[xavier60]

What if I use more than 2 power transistors (2N3055 or TIP35C) to reduce the dissipation per transistor ?
Is there any thing that needs modifications beside adding 1 or 2 more power transistors, if possible ?

Adding more power transistors might slow it down and affect the stability. Can you do a quick test first?