LM324 Power Supply with variable voltage and current

[xavier60]

Or you could trigger from the Gate drive of the MOSFET either by connecting CH2 to it or using the external trigger input.

[mike_mike]

I used the method described in reply #599.
Please find attached the screenshots.
Vout=26.6V, I=2.6A, Rl=10R (I don't have a resistor which draw exactly 2A).

[xavier60]

That's a very good response.  Reasonably fast with no sign of instability.
The fast current limiting spike is higher than I would prefer but it's safe enough.
Reducing it would involve reducing the amount of voltage the CC op-amp has to slew down by to take control. I have no immediate suggestions.
If you want to check something else, look at the settling time for the slow CC with a mild overload.
Although not at all critical, I like to see it have a time constant of about 100µs.

[xavier60]

I do have a suggestion.
You could experiment with raising the Q point of Q1's Base voltage by using a higher voltage zener, removing the LED and R16, and increase R1 to reduce unnecessary loading of the op-amps. Try 10K then check that the Base of Q1 can be pulled up to close to 12V when needed to be.
The idea is to reduce the voltage that the CC op-amp's output has to slew down by to take control of Q1.

[mike_mike]

Should I test using a 5v1 or a 6v8 Zener diode instead of 3v3 ?
Or the voltage of the Zener diode needs to be higher than my suggestion ?

[xavier60]

Start with the 5.1V,

[mike_mike]

I made the modifications into the circuit using the 5.1V Zener diode.
I made a quick test, the voltage is variable, the current also. I used 2 x 12V/20W light bulbs in series as load.

The next test is to see on the oscilloscope the output of the CC op amp while driving a overload into the PSU Shorter ?

Can the power supply remain at the latest schematic (the one from post #582, with D3=FR107 and with the added overload led notification) ? If yes, then should I start to test for oscillation and adjust the values for the 2n2 and 470p capacitors ?

[xavier60]

I made the modifications into the circuit using the 5.1V Zener diode.
I made a quick test, the voltage is variable, the current also. I used 2 x 12V/20W light bulbs in series as load.

The next test is to see on the oscilloscope the output of the CC op amp while driving a overload into the PSU Shorter ?

Yes, do that and see if the current spike has been reduced.
Also look at the Base of Q1 to see how high the voltage gets before the CC op-amp takes control.

Can the power supply remain at the latest schematic (the one from post #582, with D3=FR107 and with the added overload led notification) ? If yes, then should I start to test for oscillation and adjust the values for the 2n2 and 470p capacitors ?

Yes, but there should be no need to change the compensation except for increasing R9 later to increase the settling time of the CC loop.

[rstofer]

What should I do to solve this problem ?
Sorry for bothering you and others with such problems but I searched the internet from top to bottom and I did not found a good power supply schematic. This year I built about 5 power supply each with different schematic and all of them had problems...

That's because none of them work very well.  They all have warts!

If there was a known good PSU, truly capable of delivering high voltage and high current and also capable of delivering less than 1V at 5A (this is the worst case heating), it would be in a sticky at the top of the forum.  PCBs and kits of parts would be offered everywhere.

Every time somebody wanted a PSU, we could just point them at the sticky.  Instead, we continue to hash over the same schematics, with the same failings, endlessly.  There is no rainbow, no pot of gold, only rain!

On a brighter note, you can use a fast acting fuse to protect the transistors.

[mike_mike]

With Vout=26.6V, Iout=3A, PSU Shorter = 220k/4k7/100n.
The current spike has been reduced to 2.61V: 0058.jpg
The base of Q1: 0062-0063.jpg , with the ground probe to right side of R_Shunt.

With Vout=26.6V, Iout=3A, PSU Shorter = 220k/220k/330n.
The base of Q1: 64, 65, 66.jpg

LE: I updated the reply.

[xavier60]

That's about 12 amps. It might seem like a lot but because the duration is so short, it's safe enough already.
You could experiment more.
A quick way of getting the operating voltage up at the Base of Q1 is to change to 1K the resistor across B-E of the PNP driver transistor.
Also by trying the 6.2V zener.
We don't want to go too high with the operating voltage of Q1's Base because we still want to leave enough drive reserve for the output.
And also the overload detect circuit might be affected.

[xavier60]

The Q1 Base waveform looks odd, could be a measurement problem. Take the measurement at the Emitter next time.

[mike_mike]

Ok, I will take the measurement in the Emitter of Q1. I will also change the B-E resistor of the PNP transistor to 1k.
I do not have any 6.2V Zener diode, only 6.8V or 5.6V.
Should I use 5.6V or 6.8V Zener ?

[iMo]

The "shorting" pulse shall be longer than the expected response.. Otherwise you will see a lower amplitude of the peak current because the short stopped earlier than the "max peak"..
Thus the short should be, say, 200us, and its period, say, 20ms - not to smoke the transistors (1:100 duty)..

[xavier60]

Ok, I will take the measurement in the Emitter of Q1. I will also change the B-E resistor of the PNP transistor to 1k.
I do not have any 6.2V Zener diode, only 6.8V or 5.6V.
Should I use 5.6V or 6.8V Zener ?

First I need a better idea of the range of voltage change at the Base of Q1 as the power supply load changes from light load to the full 3 amp load.

[mike_mike]

The "shorting" pulse shall be longer than the expected response.. Otherwise you will see a lower amplitude of the peak current because the short stopped earlier than the "max peak"..
Thus the short should be, say, 200us, and its period, say, 20ms - not to smoke the transistors (1:100 duty)..

I am trying to find the correct values for R1, R2 and C for the PSU Shorter, but I don't find something to result the 200us short and period of 20ms. It would be good if you can help. I used the calculator from:
https://www.allaboutcircuits.com/tools/555-timer-astable-circuit/

@xavier60: I will return with the results, after modifying the PSU Shorter if the modification will make the results more suggestive.

[xavier60]

I haven't been paying attention to the shorter. You roughly want an on duration in the order of milliseconds and tens of milliseconds or more for off.

[mike_mike]

Are you meaning that the Mosfet from the PSU Shorter needs to be On for about a few miliseconds to tens of miliseconds ?

[xavier60]

Are you meaning that the Mosfet from the PSU Shorter needs to be On for about a few miliseconds to tens of miliseconds ?

ON for a few milliseconds to give time for the CC loop to settle. Then OFF for some long time like 100ms or more so that the power transistors don't get too hot.

[iMo]

fyi

[mike_mike]

Using the resistors and capacitor recommended by imo:
R_Shunt: 86-88.jpg with short circuit at PSU_Shorter.
E of Q1 with respect to the GND of the right side of the R_Shunt:90
B of Q1 with respect to the GND of the right side of the R_Shunt:92
E of Q1 with respect to the GND of the left side of the R_Shunt:95
B of Q1 with respect to the GND of the left side of the R_Shunt:93-94

Vout=26.6V, Iout=3A

[xavier60]

"B of Q1 with respect to the GND of the left side of the R_Shunt:93-94 "
You must mean the right hand side?
Try the 5.6V zener.

[mike_mike]

With 5.6V Zener:
On the R_Shunt: 99-101.jpg
with the ground clip of the probe to the left hand of the R_ Shunt
B of Q1: 102
E of Q1: 103
with the ground clip of the probe to the right hand of the R_ Shunt
B of Q1: 104
E of Q1: 105

@xavier60 It was on the left hand size. Please also have a look at the attached screenshots.

[xavier60]

11.4 amps. You could leave it at that. Keep in mind that the peak is very narrow, not flattened out as with the VB-E limiter.
Just do some more tests at other voltage and current settings. Also add some resistance in series with the Shorter, 1 or 2 ohms.

[xavier60]

I would like to see some voltage measurements with a light load, about 100mA.
The zener voltage.
The voltage across R25.
With the 1K across B-E of the PNP driver, R25 should have 1.35V across it.

Extra: Use the right side of the shunt for all measurements. It took me a while to realize that Q1's Base waveform was with respect to the left side of the shunt. This was showing the Base and shunt waveforms added together, not useful.