LM324 Power Supply with variable voltage and current

[xavier60]

Assuming the op-amp isn't causing the loading, it would need a 1MΩ load to pull the voltage down that much.
The op-amp's input pins   actually should be sourcing a very small current.
What are you measuring with?

[xavier60]

The purpose of C15 is not clear. R21 isn't needed.

C15 is the blocking cap at the Vcc/Vss pins of the LM324.
Its Vss is wired at the left hand side of the shunt.

I see, C15 would be causing some unwanted feedback but there has been no indication of any trouble.

[mike_mike]

I measured using another multimeter and I got 3.61 V on the pin 6 and 3.65 V on the divider.
First time, I measured with a cheap M830BUZ from UNI-T and second time with a UT52 also from UNI-T.

I also found that the input impedance of the M830BUZ is 1M , as you said in the previous reply (it is specified in the multimeter datasheet).

I checked the CC led by connecting a load at about 19.18V output voltage and I reduced slowly the output current until the voltage started to go down. The led started to light right in the moment when the voltage started to go down. Is this procedure correct ?

[xavier60]

That's right. Keep in mind the reduced margin at higher CC settings where pin 5 doesn't drop as much when going from CV to CC mode.

[mike_mike]

What do you mean by "reduced margin" ?
Can you please be more detailed ?

[floobydust]

I dislike the LED circuit with no hysteresis and relying on T1's base voltage to be exactly the same either on or off, due to calibration of trimmer R7.
T1's base voltage depends largely on PSU output current, so CV at 1A going to CC at 0.9A is little change and I don't expect the LED to work properly.

[xavier60]

What do you mean by "reduced margin" ?
Can you please be more detailed ?

It's the extra change in voltage after the threshold is crossed. If you are worried, set pin 6 to 4.5V.
Pin 5 will always stay  close to 5.5V while the CC loop is not in control.

[mike_mike]

Ok, I added into the circuit a 10k trimmer between R6 and R8 (as it was in the original schematic) and I set the voltage on pin 6 at 4.5V.
I set the output voltage at 11.98-12.00V, and the current to about 1A, and I used a load with a transistor and a light bulb and I slowly increased the current using the pot from the load schematic (the attached schematic, I used a 10k pot). When the current reached 1.05A, the led illuminated and the output voltage started to decrease slowly as I was rotating the 10k pot.

In your opinion, the led notification circuit is Ok ?

[xavier60]

Although  It is more important to test at high current settings , it will be fine.

[mike_mike]

I checked the CC led at 3A current limit (this is the max current limit). The led started to illuminate right in the moment when the current consumed by the load was 2.99A and right in the moment when the power supply output voltage started to decrease.

If this test, and the test from the previous post are good, then the CC led will work correctly in all situations ?

[xavier60]

I checked the CC led at 3A current limit (this is the max current limit). The led started to illuminate right in the moment when the current consumed by the load was 2.99A and right in the moment when the power supply output voltage started to decrease.

If this test, and the test from the previous post are good, then the CC led will work correctly in all situations ?

So long as pin 5 drops well below pin 6 on the LED op-amp, which it should be now, it will be reliable.

[mike_mike]

I think that I have a new problem now.  I checked the power supply using 2k2 resistor for R30 instead of 3k3.

The tests are still valid if I change the R30 from 2k2 to 3k3 ?

[xavier60]

I think that I have a new problem now.  I checked the power supply using 2k2 resistor for R30 instead of 3k3.

The tests are still valid if I change the R30 from 2k2 to 3k3 ?

Changing R30 to 3.3K will increase the gm and decrease the voltage at the Base of Q1. I don't expect any problem.
You can check the stability of the CV loop with some load transient tests.

[mike_mike]

It's the extra change in voltage after the threshold is crossed. If you are worried, set pin 6 to 4.5V.
Pin 5 will always stay  close to 5.5V while the CC loop is not in control.

Pin 6 needs to be 4.5V with respect to the right or to the left GND of the shunt resistor ?

I think I don't clearly understood what is with those 2 GND's...

[xavier60]

It's the extra change in voltage after the threshold is crossed. If you are worried, set pin 6 to 4.5V.
Pin 5 will always stay  close to 5.5V while the CC loop is not in control.

Pin 6 needs to be 4.5V with respect to the right or to the left GND of the shunt resistor ?

I think I don't clearly understood what is with those 2 GND's...

We had decided that the right side of the shunt should be called ground.
Important elements of the regulator are referenced to this ground.
For example, the 12V regulator which is also the voltage reference, and also the voltage feedback divider.
The left side of the shunt develops a negative voltage with respect to  ground when the regulator is loaded.
Unless otherwise said , all measurements are taken with respect to  ground.

[mike_mike]

Hello, I want to continue testing, if necessary, this power supply schematic (reply #561 shcematic).
The lasts tests were the one with the resistive (R) load.
What are the next steps when testing this power supply ?

[xavier60]

Hello, I want to continue testing, if necessary, this power supply schematic (reply #561 shcematic).
The lasts tests were the one with the resistive (R) load.
What are the next steps when testing this power supply ?

I guess the way the voltage regulation responds to load transients could be tested. It will give a good indication of how stable it is.

[mike_mike]

I guess the way the voltage regulation responds to load transients could be tested. It will give a good indication of how stable it is.

Ok. I will study and try to test it.
What are the layout design guidelines for this power supply ?
I used for testing the attached layout. Is it any good ?

[xavier60]

C5 could be put back into service by moving its negative end to the same track that connects to the negative of  C2. This should avoid the ripple problem.

[mike_mike]

Ok, I will do that.
Are there any other necessary modifications or recommendations ?

LE: I added the C5 modification on the board.

[xavier60]

Ok, I will do that.
Are there any other necessary modifications or recommendations ?

It's difficult to spot all problems. The load transient tests might show problems if there are any. Include C5 for the tests.
I also suggest that you tack in the transistor and 2 resistors to test the low side fast limiting idea.

[Kleinstein]

For fast output regulation one may want an RC combination in parallel to R12 (from the LTspice schematics).
For high precision R12 should be directly from the output terminals. Similar the negative side should have separate current and voltage sense wires. At least the wires should be short.

The reverse diode at the output usually should be strong enough to carry the full load current - though ideally the requited current rating is more to the possible external supply (e.g. 2 nd LNG) that may be connected.

[mike_mike]

Ok, I introduced into the schematic, the modifications proposed by Xavier60, and I made a simulation in LTSpice. I eliminated the Fast CC that was near the power transistors and I used the solution on the R_Shunt.
I see a spike of about 40A on the shunt resistor, so there is something bad.
Please have a look ...

[xavier60]

I can't explain that result. It is a commonly used method for fast limiting and should work.
I suggest that the mod is applied to the actual power supply and carefully tested by increasing the switched overload current gradually.

[mike_mike]

Sorry about the mistake. I corrected.

I see that it is oscillating when the output current is set to max. But when the output current is set to 50% then it looks better.