LM324 Power Supply with variable voltage and current

[mike_mike]

I tested it, (with 7812) and when I plugged out the PSU from the mains, there appeared a short pulse on the oscilloscope screen, which did not happened when the led was there.

What should I do ?
It is already about 3 weeks since I am working at this PSU, and I still do not find a good working version... and I am very disappointed. 

[xavier60]

That is not a good test. Can you do a proper test?

[mike_mike]

Yes.
I used a LM317 1.25 to 25V power supply. I set the LM317 to 12V and I reduced gradually the voltage to 1.25V. The scope was set to 250uS/div. The trace gone from 28.5V (max output) to GND on the screen, without suddenly going up.

[iMo]

SIM: with the zener D9=3V3, R7=2k2 and RED LED in the UVLO you get

R30=3k3  I_FastCC=8.1A
R30=4k7  I_FastCC=8.3A

[xavier60]

Yes.
I used a LM317 1.25 to 25V power supply. I set the LM317 to 12V and I reduced gradually the voltage to 1.25V. The scope was set to 250uS/div. The trace gone from 28.5V (max output) to GND on the screen, without suddenly going up.

So the UVLO works.
You have not included your age in your profile?  Are you young and still learning?  Will this power supply have a particular application? Knowing this might allow us to better anticipate other possible problems.

More: The regulator should have shut down at about 6V or higher?

[iMo]

It is already about 3 weeks since I am working at this PSU, and I still do not find a good working version... and I am very disappointed. 

The more you know the more unhappy and in doubt.
Not having eevblog handy you would simply build a PSU based on some schematics from the web, and use it happily 

[mike_mike]

New test: D9=3V3 zener, R7=2k2, red led in the UVLO and R30=3k3.
It seems to work now.

@xavier: Yes, I am still learning and I am young. I am a enthusiast at electronics. I like to build circuits. The application of the power supply will be to power different equipment, starting from 1W led and PCB mini drills to digital micro controllers such as Arduino.

I did not measured the output voltage with the oscilloscope, but at some output voltage, the trace gone suddenly to the ground.

[xavier60]

New test: D9=3V3 zener, R7=2k2, red led in the UVLO and R30=3k3.
It seems to work now.

Are you meaning that the UVLO didn't work with just the blue LED bridged out?

[mike_mike]

@xavier60 I mean that the fast CC is now correctly working after using the configuration suggested by @imo.
With the blue led, at some point when the voltage dropped below 12V, the output voltage of the power supply suddenly descend to 0V.

@imo I did not expect such problem from this power supply, this is why I am i doubt... But, if  you or others can and will help me, then I will try to make the power supply work correctly.

[xavier60]

@xavier60 I mean that the fast CC is now correctly working after using the configuration suggested by @imo.
With the blue led, at some point when the voltage dropped below 12V, the output voltage of the power supply suddenly descend to 0V.

I'm puzzled by this. No doubt that the fast CC is working properly. Was imo's mod necessary?

[mike_mike]

Yes. Imo's mod was necessary, in my opinion.
Regarding the OVLO, when the  power voltage of LM324 is going down, the output voltage of the power supply is also going down.
But when the power voltage of LM324 is going up, then the output voltage of the power supply starts to go up only when the power voltage of LM324 is about 6-7V.
This happens with imo's mod.
I was wrong in the previous affirmation. I apologize.

[mike_mike]

Please let me know what else should I test, in order to make the power supply to work correctly.

[iMo]

There is a so called RED LED test
First wire an 1k/1W resistor in parallel to your output capacitors (like a permanent load).
You set the output voltage to the maximum and the SlowCC to minimum, say 20mA (doublecheck what current you get at minimum CC setting).
Edit: Switch PSU off.
Then take an RED LED (could be smoked, be warned) and connect it to the PSU output (anode to OUT+, cathode to OUT- such it lits).
Switch the PSU a few times (ie 10x) on/off. Wait a few seconds between the switchings.
When the LED survives your PSU is perfect..

[mike_mike]

I set the output to 27.5V, the SlowCC to 18mA (measured with the multimeter set to mA directly on the output of the power supply) and I smoked instantly 2 red leds. When I connected the led, it was smoked instantly. There was no "real smoke", only one led lit up for a fraction of a second then it was off (it died), and the second led did not lit up at all. There also was a 1k/5W resistor on the output.
I think that this is because of the output filter capacitors which are discharging into the led (2x47uF). Am I right ?

[iMo]

Try to set the 18mA, and switch the PSU off. Wait a bit till the caps discharge. Then wire the LED in. Then switch the power on. Wait a bit and switch it off. Whether the LED survives that.

[Cliff Matthews]

My ancient Micronta used to pass that test, but I added a DVM with 10uF MLCC right behind the new binding posts.. Doh! 

[Kleinstein]

The slow and fine CC limit will wind up in the short time, when still in under-voltage look out.  So while turning on the CC OP will go all the way towards the upper limit, while the voltage is not sufficient to turn Q1 on. This is not that unusual, it just resembles the normal operation. It just take a considerable time for the slow current limit to activate.  In this time only the fast limit (at some 8 A) is active and this is little too much for a small LED, even if only for some 10 µs.

The delayed action of the fast current limit is one reason why the fast current limit is kind of needed.

Quite a few commercial lab supplies likely have a similar limitation.  I would consider this not a bug but more like a feature, giving higher priority to voltage regulation, so that small spikes in the current would not cause trouble the voltage regulation.

[mike_mike]

Try to set the 18mA, and switch the PSU off. Wait a bit till the caps discharge. Then wire the LED in. Then switch the power on. Wait a bit and switch it off. Whether the LED survives that.

Yes, the LED survived. I used an red LED.

[iMo]

 

[iMo]

@Kleinstein - I've done a sim with the lower side shunt fast cc and it looks it helps in simulation. It would need to be tested in HW, however.
When working in HW the FastCC 8A peak should disappear.

The below reduces the max 3A SlowCC to 2.89A or less based on the R27 value. That should not be a problem. The 8A fastCC peak is eliminated.
 
You may also try with the 2 resistors like in Xavier's schematics above.

[mike_mike]

You may try with the 2 resistors like in Xavier's schematics above.

Do you mean as it is in the reply #417 ?

Later Edit: it looks like there are some high current pulses if I use the Xavier's schematic.

[mike_mike]

Does anybody have some advices regarding this PSU ?
Can this PSU be used as it is, with no problems ?

[Kleinstein]

I would consider the PSU useful. The regulation and behavior on turn on seems to be about at the level of the cheap Chinese supplies, or a little better. With an LM324/LM358 the precision of the current regulation may not be very good (depends on the luck one has with the OP - not all LM324 have poor offset drift, but they can).

Some check on the heat sink temperature may be a good idea. Without tap switching there can be quite some heat.

[mike_mike]

I am trying to select the proper value for C1, in order for the power supply to not oscillate.
I found that at Vout=26V, SlowCC=0.22A and Rload=10R, the voltage on the shunt looks like the attached screenshot. I can't figure if the blue and thick line is a oscillation or it is only noise.
Please have a look at the attached screenshot and tell me what you think.

[Kleinstein]

From the scope trace it is hard to tell if there is some oscillation. Even with a better scaling it could be tricky to see.

The output of the OP for the CV mode loop could be a little more sensitive, though not much.