Author Topic: LM324 Power Supply with variable voltage and current  (Read 9263 times)

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Offline imo

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Re: LM324 Power Supply with variable voltage and current
« Reply #400 on: May 16, 2019, 03:03:55 am »
@MM: to your simulation above: doublecheck what is the ESR of the 47uF capacitor. I think you still using 0.1ohm there.

I would highly suggest NOT TO USE the built in esr, but instead add a special ESR resistor into the schematics.
You can see the ESR value directly in the schematics then, and we may refer to a particular ESR resistor.

Enclosed the SIMULATION ONLY .asc V17 - use it for the simulation.
« Last Edit: May 16, 2019, 03:10:37 am by imo »
 

Online Kleinstein

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Re: LM324 Power Supply with variable voltage and current
« Reply #401 on: May 16, 2019, 03:05:06 am »
The test still missing and repeatedly asked for if loading the supply to less than current limiting. So this is the more classical load test. E.g. CC limit to 3 A and a test current of some 10 mA (min load) - 1 A  (e.g. 20 Ohms at 20 V) -  10 mA.

The other point would be a short load spike, so that the fast current limit engages, but not yet the slow current limit. So maybe some 5 A for some 20 µs or so.
 

Offline mike_mike

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Re: LM324 Power Supply with variable voltage and current
« Reply #402 on: May 16, 2019, 03:19:02 am »
I added a 20R resistor as load, and I set the output voltage at 20V.
SlowCC is 3A. Min load is 820R (this is the resistor that I had).
If the GND was aligned to the grid, then nothing was shown on the scope, only the blue trace.
So I was needed to reduce the V/div.
C1=220p, C5=2n2.

Please have a look at the attached screenshots and tell me what you think.
« Last Edit: May 16, 2019, 03:35:13 am by mike_mike »
 

Offline imo

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Re: LM324 Power Supply with variable voltage and current
« Reply #403 on: May 16, 2019, 03:23:03 am »
Sim 16mA/1A C1=470pF C5=2n2 Islow=3A Vout=21V

Sim 16mA/1A C1=220pF C5=2n2 Islow=3A Vout=21V
« Last Edit: May 16, 2019, 03:27:56 am by imo »
 

Offline mike_mike

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Re: LM324 Power Supply with variable voltage and current
« Reply #404 on: May 16, 2019, 03:52:42 am »
I see that the oscillation have smaller peak to peak values for 220pF...

What should I test next ?
 

Offline imo

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Re: LM324 Power Supply with variable voltage and current
« Reply #405 on: May 16, 2019, 03:58:50 am »
Look a the CV opamp's output with above test.
« Last Edit: May 16, 2019, 04:02:12 am by imo »
 

Offline mike_mike

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Re: LM324 Power Supply with variable voltage and current
« Reply #406 on: May 16, 2019, 04:35:55 am »
The output of the CV op amp and the GND on the power supply output GND
Vout=21V, C1=220pF, C5=2n2, SlowCC=3A, Rload max=20R, Rload min=1.2K
0973-0976
Vout=21V, C1=470pF, C5=2n2, SlowCC=3A, Rload max=20R, Rload min=1.2K
0977-0979

If I use higher V/div, then I see only the blue trace on the screen.
I used those load resistors, because those values are the most close to the specified load currents.
Please have a look at the attached screenshots and tell me what you think.

Later Edit: The last screenshot show the pin 3 output of the 555, at PSU Shorter. I used 220k/4k7/330nF. Please also have a look at this and tell me if it is good.
« Last Edit: May 16, 2019, 05:34:52 am by mike_mike »
 

Offline mike_mike

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Re: LM324 Power Supply with variable voltage and current
« Reply #407 on: May 16, 2019, 06:56:41 am »
I used a new configuration for the PSU Shorter.
I used 220k/220k/100nF.
Vout=20.9V, SlowCC=max (3A), Rload=20R, Rloadmin=1.2k
C1=470pF, C5=2n2.
0985-0989
0990-0992
These were measured on the output of the CV opamp.

Please have a look at the attached screenshots and tell me what you think. Does they look good or not ?
« Last Edit: May 16, 2019, 07:01:48 am by mike_mike »
 

Online xavier60

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Re: LM324 Power Supply with variable voltage and current
« Reply #408 on: May 16, 2019, 07:46:08 am »
The activity at the CV op-amp is expected and correct. The negative going pulse at the end is the loop's response to the slight output overshoot.
The shape of the output voltage dip pulse is correct for the load transient tests. Just the same as mine, http://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg2405490/#msg2405490
I can see that the gm is 3 but this is ok because it's offset by your voltage divider having about 3 times more gain than mine.
Expect larger overshoots if a short is removed while the fast limiting is active.
Any high frequency ringing or hash that occurs at current switch off is likely due to parsitics.
« Last Edit: May 16, 2019, 08:06:47 am by xavier60 »
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Offline floobydust

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Re: LM324 Power Supply with variable voltage and current
« Reply #409 on: May 16, 2019, 10:26:01 am »
DS0989, DS0991 shows 500usec dip, what is taking so long.
 

Online xavier60

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Re: LM324 Power Supply with variable voltage and current
« Reply #410 on: May 16, 2019, 10:33:11 am »
DS0989, DS0991 shows 500usec dip, what is taking so long.
There isn't much load current to discharge the output capacitance during the overshoot.
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Offline floobydust

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Re: LM324 Power Supply with variable voltage and current
« Reply #411 on: May 16, 2019, 10:36:41 am »
With a shorted output in (slow) CC mode, the CV op-amp sits there saturated and can take a long time to recover + the slew time. It's seeing i.e. 7.4V differential (21V setpoint and 0V out). I wondered if we would see that delay, or if SPICE models stay accurate. I limit the differential input voltage on CV op-amp (internal comp cap/integrator windup) to speed up recovery, using a diode.
 
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Online xavier60

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Re: LM324 Power Supply with variable voltage and current
« Reply #412 on: May 16, 2019, 10:52:07 am »
With Kleinstein's suggestion, the compensation feedback for the CV op-amp is now taken from the diode OR-ing node.
As far as I can figure, there is no CV windup while the slow CC loop is in control.
The overshoot is caused by the reaction time of the op-amp and output transistors.
« Last Edit: May 16, 2019, 12:59:51 pm by xavier60 »
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Online xavier60

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Re: LM324 Power Supply with variable voltage and current
« Reply #413 on: May 16, 2019, 11:01:05 am »
Adding back the correct values for C4 and R4 wont prevent overshoot for all circumstances.
There will be a serious CV windup problem while the fast limiting is active.
« Last Edit: May 16, 2019, 12:17:55 pm by xavier60 »
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Offline mike_mike

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Re: LM324 Power Supply with variable voltage and current
« Reply #414 on: May 16, 2019, 02:53:41 pm »
I read and tried to understand all your messages.
Can you please let me know what should I test next ?

There will be a serious CV windup problem while the fast limiting is active.
Should be implemented a solution for this problem ?

...and I will double check the power transistor circuit.
« Last Edit: May 16, 2019, 03:54:00 pm by mike_mike »
 

Online xavier60

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Re: LM324 Power Supply with variable voltage and current
« Reply #415 on: May 16, 2019, 04:28:17 pm »
Try a pulsed short circuit test with a short duration like 100µs. Monitor the output for overshoot while changing the CC setting.
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Online Kleinstein

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Re: LM324 Power Supply with variable voltage and current
« Reply #416 on: May 16, 2019, 05:32:15 pm »
I read and tried to understand all your messages.
Can you please let me know what should I test next ?

There will be a serious CV windup problem while the fast limiting is active.
Should be implemented a solution for this problem ?

...and I will double check the power transistor circuit.

The first point would be to look if the overshoot with only the fast current limit active is actually that bad.
Preventing that kind of overshoot could however get difficult, as it is hard to sense when the fast current limit will engage. My suggestion would than be a different fast current limit at the low side. A transistor could pulling down the base of Q1 if the shunt voltage exceeds 0.6 V or so.
 

Online xavier60

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Re: LM324 Power Supply with variable voltage and current
« Reply #417 on: May 16, 2019, 05:40:14 pm »
Something like this. I don't expect it to work too smoothly.
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Offline mike_mike

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Re: LM324 Power Supply with variable voltage and current
« Reply #418 on: May 16, 2019, 05:56:58 pm »
Try a pulsed short circuit test with a short duration like 100µs. Monitor the output for overshoot while changing the CC setting.
So I will need to adjust the PSU shorter to output a pulse of about 100us and then to look at the output of the power supply at different slowcc settings ?

And I will need to remove the blue led and the resistor in series with it ?
« Last Edit: May 16, 2019, 06:03:04 pm by mike_mike »
 

Online xavier60

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Re: LM324 Power Supply with variable voltage and current
« Reply #419 on: May 16, 2019, 06:28:40 pm »
Try a pulsed short circuit test with a short duration like 100µs. Monitor the output for overshoot while changing the CC setting.
So I will need to adjust the PSU shorter to output a pulse of about 100us and then to look at the output of the power supply at different slowcc settings ?

And I will need to remove the blue led and the resistor in series with it ?
Don't modify anything yet except the shorter. We want to see how bad the problem might be first.
The idea is to apply shorts of short duration that don't allow enough time to transition from fast to slow CC control.
Adjust the CC setting to find the worst voltage overshoots at the output.
No need to show us everything, just the worst of what you might find.
« Last Edit: May 16, 2019, 08:09:37 pm by xavier60 »
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Offline mike_mike

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Re: LM324 Power Supply with variable voltage and current
« Reply #420 on: May 16, 2019, 09:00:11 pm »
I think that there is a problem with the Fast CC because I am getting only 1.09V on the RShunt, so this is about 4.95A, but it should be about 8A ... Please see reply #398
I am missing something ?
I don't know what should I do ...

« Last Edit: May 16, 2019, 09:02:38 pm by mike_mike »
 

Online xavier60

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Re: LM324 Power Supply with variable voltage and current
« Reply #421 on: May 16, 2019, 09:06:45 pm »
The Base of Q1 might not be going high enough. Bridge out the blue LED.
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Offline mike_mike

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Re: LM324 Power Supply with variable voltage and current
« Reply #422 on: May 16, 2019, 09:14:07 pm »
I replaced the blue led with a short circuit, and now the current through the Rshunt is about 8A.

But if I bridge out the blue led, then when the power is cut off, the output of the supply will go to max voltage ? (I saw a short pulse on the screen of the oscilloscope when I plugged out the power supply from the mains).
« Last Edit: May 16, 2019, 09:16:29 pm by mike_mike »
 

Online xavier60

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Re: LM324 Power Supply with variable voltage and current
« Reply #423 on: May 16, 2019, 09:16:33 pm »
I replaced the blue led with a short circuit, and now the current through the Rshunt is about 8A.

But if I bridge out the blue led, then when the power is cut off, the output of the supply will go to max voltage ?
I have previously explained why I think that the UVLO should still work, best is to test it.
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Online xavier60

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Re: LM324 Power Supply with variable voltage and current
« Reply #424 on: May 16, 2019, 09:22:53 pm »
If you have another smaller power supply use it to power the 12V rail. Then see what happens as the voltage of the 12V rail is decreased.
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