LM324 Power Supply with variable voltage and current

[xavier60]

I tried another pcb and the same waveform appeared on the oscilloscope .

With mine, there is no overshoot at the output of the CV opamp. It seems that your compensation is no longer suitable for some reason.
What are the current values for your CV compensation RC?

[mike_mike]

R5=4k7 and C3=1nF.

[mike_mike]

I made a test and I found that only the green led is making problems, in constant voltage mode and by using the electronic load with a resistor of 7.8R. If I connect it, then that strange response appeares, but if I leave the green led disconnected then the response look normal. I checked this using 2 pcb's, and the same problem appeared.

[xavier60]

R5=4k7 and C3=1nF.

That should already be making it slow enough. You could try going a bit smaller with the resistor   and larger with the capacitor to see what it does.

[mike_mike]

I tried with 3.9K and then I changed the capacitor to a new one (also 1nF) but the waveform looks the same.

How can I calculate of How can I check if it is safe for the 358 if I use R30=R31=2k2 ?

[xavier60]

This could be a waste of time, try decreasing R7 from 10K to 4.7K

[mike_mike]

With R7=4k7.

[xavier60]

Is that with the LED connected also?

[mike_mike]

Yes both led's are connected. I used a 3mm green led and a 3mm red led.

[xavier60]

Well, that's a nice response then. Hope it stays that way.
To calculate the maximum opamp current, we first need to have some idea of the lowest possible output voltage, say 1V.
 LED current is  (7V - LED voltage) divided by R30 or R31. Add 1.5mA for R7.

[mike_mike]

Ok, the voltage on the led is 2.25V.
(7-2.25)/10k=0.475mA
The total maximum current = 1.5mA (the current throug R7)+0.475 (current through led)=1.945mA.

What tests should I do after modifying the R7 value ?

[xavier60]

Ok, the voltage on the led is 2.25V.
(7-2.25)/10k=0.475mA
The total maximum current = 1.5mA (the current throug R7)+0.475 (current through led)=1.945mA.

What tests should I do after modifying the R7 value ?

You could change R5 back to 4.7K and check the response again.

[mike_mike]

The R5 was already 4k7 in my previous post and screenshot.
The below screenshot is with 5mm regular red led and 5mm regular green led.


The 5mm regular led are not sufficiently bright and I will probably use the 3mm led's which are brighter.

[mike_mike]

Do I need to do any other test, using different load resistance, to check the stability ?

[xavier60]

Do I need to do any other test, using different load resistance, to check the stability ?

You could do that.

[mike_mike]

1. Vout= 32.8V, Rload=6.7R: 000.jpg
2. Vout=32.8V, Rl=10R: 001.jpg
3. Vout=15.2V, Rl=10R: 002, Rl=8.45R: 003, Rl=5R: 004
4. Vout=2V, Rl=5R: 005, Rl=0.66R, Rl=006

Please have a look and tell me what you think.

[xavier60]

That all looks normal, no signs of instability.
In future, no need to post all of the traces if they look good.

When a high load is suddenly applied causing the output of the CV opamp to immediately step up by a volt or so, some current briefly flows through the compensation RC, causing a brief current decrease through D1 so increasing its dynamic resistance. The increase in total compensation resistance would have been the cause of the brief instability near the start of the load transient pulse.
Decreasing R7 maintains higher current through D1.

[mike_mike]

This is with 0.5 ohm load and 2.5V output, it is still normal ?  I am asking because of the voltage drop, which is 2.5V, which is equal to the output voltage.
The maximum output current set to 5.18A.

[xavier60]

I did a quick test on mine and got a 500mV dip. My lead lengths are longer than they should be for the test.
Your build turned out to have a much slower response than mine, I don't think it fully explains your result.
It didn't drop that much in previous tests.
You could take a look at the opamp outputs to see what's happening.
Dont use AC coupling on your DSO when it's not needed.

[mike_mike]

I did the test again, this time I connected the oscilloscope probe (yellow trace) to the pins of the 47uF capacitor, C2.
244_000 = output of the PSU
244_001 = pin 7 of LM358
244_002 = pin 1 of LM358

[xavier60]

Now that's close to what I would expect.
It also shows that in practice PSUs don't need to be super fast when the response can be degraded so much by typical lead lengths used on the bench.

[mike_mike]

Please have a look at the attached screenshots.
That oscillation has conection with the moment when the load is changing from off to on ? It is normal ?
I tested at Vout=2.5V, Rload=10R.

[xavier60]

It looks like ringing in the external wiring.
What is odd, why does the voltage on CH2 drop before on CH1?

[mike_mike]

CH2 is the waveform on the output of NE555, I think that probably it is normal because CH2 needs to drop first then CH1...

[xavier60]

So it's the output transition of the 555 ringing the capacitance of the MOSFET and wiring inductance. Is there more ringing at the Drain of the MOSFET?