LM324 Power Supply with variable voltage and current

[xavier60]

Yes.

[xavier60]

If you cant get good results, you could go back to a previous idea of shorting CV reference to ground. Last time this was tried, it caused problems with the tap changer because the voltage rose too slowly when the short was removed. Reducing C9 should fix this. The tap changer could also be given more delay.

[mike_mike]

Ok, I have reviewed the PSU schematic, but I don't want to modify it because already it works good, excepting the tap changer circuit.

[xavier60]

The tap changer's delay can be increased to ignore the overshoot. First by increasing R8 to 220K, then reducing R11 if more delay is wanted.
The delay can be measured by the same test in Reply #1243.

I connected a relay to the Shutdown on my PSU. Although the effects of contact bounce can be seen on the output voltage, there's no over-shoot. I guess the reason would be due to component and/or construction differences.

[xavier60]

I'm able to get over-shoot by replacing the TLC072 with an LM358.
A 100nF and 10-ohm snubber across the relay contacts fixed the over-shoot on mine.
Part of what's happening when your mains is turned off is the 12V goes down slowly causing the 2 small relays to click over at different times.

[mike_mike]

A 100nF and 10-ohm snubber across the relay contacts fixed the over-shoot on mine.

Do you mean the big relay NC contacts ?

[xavier60]

A 100nF and 10-ohm snubber across the relay contacts fixed the over-shoot on mine.

Do you mean the big relay NC contacts ?

No, the NC contact of the Shutdown relay. I temporarily connected a small relay to my PSU for testing this problem.
My PSU is normally shut down by a BC548 driven by the microcontroller,

[mike_mike]

Please find attached the schematic that I used and the waveform from the DSO.

[xavier60]

I dont think the contact bounce of that relay can be easily fixed. You can try a larger capacitor, but it will cause other problems.
My relay is HK19F-DC-12V-SHG

[mike_mike]

Ok, could I leave it as it is ?

[xavier60]

Ok, could I leave it as it is ?

Yes, I think it's the only option.

[mike_mike]

I managed to find a DPDT relay, with 12Vdc coil voltage (RT424012).
Please find attached the screenshot after using the new relay.

Edit: I added a new screenshot.

[xavier60]

That relay looks the same as the type I tested. Did you need to use an RC snubber across the Shutdown contacts?

I see there is some over-shoot on your 2nd test.

[mike_mike]

Did you need to use an RC snubber across the Shutdown contacts?

Yes I used 10 ohm resistor in series with 100nF capacitor.

[xavier60]

If it doesn't get any worse than that, it will be fine. OTH, you can add extra capacitance to yours because your PSU has the 4.7K pull-up resistor.
Mine is still 10K.

[mike_mike]

OTH, you can add extra capacitance to yours because your PSU has the 4.7K pull-up resistor.

Do you mean the R7 resistor from the attached schematic ?

[xavier60]

Yes. the higher current will be charging up the snubber capacitor more quickly every time the contacts bounce open. You will see this happening with a faster time-base.

[mike_mike]

Ok, but after adding the snubber circuit, and after modifying the tap changer circuit, does the PSU needs to be again tested for stability ?

[xavier60]

The snubber might cause a one-time slow load transient response, I don't expect any sustained instability due to D4.

[mike_mike]

I made a quick stability test, for my "peace".
Vout=31V, Rload=6.5R
Blue trace = output pin of LM555, PSU tester
Yellow trace = output of the PSU.

[iMo]

@mike_mike: I've knocked my "PSU Tester" together finally.. All junkbox parts except the pcb

[mike_mike]

@iMo I like your PSU Tester, it looks very nice, especially the arrangement of the components on the pcb.

[mike_mike]

@xavier60 , I tested again the LM358 psu, using 31V output and 6 ohms load. I used the 5A version of the PSU and I found that the response looks normal, but the white led is flashing a little bit while doing the tests (probably at the PSU tester frequency). Is this normal behaviour ?
I tried to increase the Rload to 6.5 ohms, but the white led is still flashing.

The white led is flashing very fast but it is hard to see.

[xavier60]

Q1's output drive current returns to ground through the LED. The drive current changes with output loading.
I think the LED could be replaced with a TL431.

[mike_mike]

Is it possible to add to this PSU schematic 2 led's, one red and one green. The red one to turn on when constant current is working and the green one to torn on when constant voltage is working.
I think that I need to add them into the outputs of the CC and CV OpAmps.
The schematic was tested in this topic a few years ago,but is seems to be ok for be to use instead of LM723 and TL082 one, which is under test now.
Also, as a modification, it is good to replace the LM 7812 by LM317 + 2 resistors ?
And R22 replace by 9.1K instead of 12K. This is to have the output voltage to about 30V.

EDIT: I updated the schematic.