Author Topic: LM324 Power Supply with variable voltage and current (Read 169633 times)

mike_mike · « **Reply #475 on:** May 31, 2019, 06:20:21 am »

If I use 3A diode, then the power supply can start to oscillate, because of the diode ?

xavier60 · « **Reply #476 on:** May 31, 2019, 06:39:17 am »

Quote from: mike_mike on May 31, 2019, 06:20:21 am

If I use 3A diode, then the power supply can start to oscillate, because of the diode ?

Why would you want to think that? All sorts of things are expected to be connected to a power supply's output without causing instability.
It's not going to care much about a few PicoFarads of junction capacitance.

mike_mike · « **Reply #477 on:** May 31, 2019, 03:15:15 pm »

So can I also use a P600K or a 6A6 diode on the output ?

Kleinstein · « **Reply #478 on:** May 31, 2019, 04:06:34 pm »

There is no real problem with a larger diode at the output - the first point would be leakage of the diode than could make the current reading to be off a few µA.
If in doubt the backwards diode should be stronger as it should come up with the current of a possible external 2 nd supply.
If charging batteries, there could be a fuse that would bow if there is too much reverse current.

mike_mike · « **Reply #479 on:** June 01, 2019, 03:27:06 pm »

Can someone please explain why I am getting this jittering signal when measuring the output of the power supply with Vout=2V, Rload=10R and the output current almost down to zero ?
Video:
https://streamable.com/b8ium

It is a problem with the power supply or a problem with the oscilloscope ?

LE: This jittery signal appears only at this output voltage of the power supply. If I set the power supply to a higher voltage, for example 26V or 14V then the jitter does not appear.

iMo · « **Reply #480 on:** June 01, 2019, 03:53:07 pm »

You shorting the output, right?
It looks ok to me, the small jitter may come from 230V mains hum induced in the wiring.
You may also look at the bridge voltage (your 35V) with the second probe how clean it is.

mike_mike · « **Reply #481 on:** June 01, 2019, 04:05:00 pm »

Yes, I am shorting the output.
I made a second video, with the bridge voltage.
I did not managed to see both signals on the screen because one signal was triggered and the other was moving fast...

Video: https://streamable.com/sijci

Kleinstein · « **Reply #482 on:** June 01, 2019, 04:15:37 pm »

The jittery horizontal moving looks like a trigger problem, so like a poor setting at the scope.
The vertical movement could be something like mains hum or maybe a poor contact somewhere.

xavier60 · « **Reply #483 on:** June 01, 2019, 11:13:22 pm »

The output stage will pass some ripple current due to the Early Effect of BJTs.
It is difficult to confirm this as the reason.

floobydust · « **Reply #484 on:** June 02, 2019, 01:09:09 am »

mike_mike, if you are getting high ripple at load, check your wiring and PCB layout.
Wiring or traces in the wrong spot, you can end up amplifying ripple or adding instability at higher load currents.
Some PCB traces/wiring will develop an unexpected voltage drop across them at load.

The filter capacitor (-) should run right to the sense resistor and then the output (-) banana jack.
The CC op-amp input (circuitry) should connect right at the sense resistor - not up or downstream of it.
The potentiometer returns, and voltage sense divider are also critical. A pic of the PCB layout helps.

You can also line trigger the scope to view mains ripple if you are hunting for that.

mike_mike · « **Reply #485 on:** June 02, 2019, 04:45:29 am »

Quote from: floobydust on June 02, 2019, 01:09:09 am

A pic of the PCB layout helps.

The ripple from the last video is at the input of the power supply (on the bridge rectifier) and not at the output.
Please find attached the power supply layout.
There is a very, very big problem. I already ordered in China 5 pieces of pcb, so modifying the layout needs new pcb's, and at the second order I will need to pay the entire shipping taxes which are not cheap.
Modifying the layout also needs new tests ?
And I was trying to do my best while designing the pcb, and I was trying to design it using as example the pcb from the other power supply in this topic.

iMo · « **Reply #486 on:** June 02, 2019, 07:07:31 am »

Nice board!
There is still place for drilling 2 mounting holes when necessary.

floobydust · « **Reply #487 on:** June 03, 2019, 03:57:21 am »

You've already built the PSU with this exact same PCB layout? Just a little confused what circuit you are actually running, and getting scope traces from.

The first video is 2Vpp ripple at 35Hz ? Into A 10R load?

mike_mike · « **Reply #488 on:** June 03, 2019, 05:18:37 am »

Quote from: floobydust on June 03, 2019, 03:57:21 am

You've already built the PSU with this exact same PCB layout?

Yes.

Quote from: floobydust on June 03, 2019, 03:57:21 am

The first video is 2Vpp ripple at 35Hz ? Into A 10R load?

In that video the slowCC is active (the output current is set to almost 0A), this is the reason i'm getting 2Vpp ripple...

iMo · « **Reply #489 on:** June 03, 2019, 05:48:30 am »

That is not 2V "ripple" in the first video. It is Vout=2V shorted periodically (35Hz) via 10ohm resistor while SlowCC is at minimum.
Signal looks ok.

floobydust · « **Reply #490 on:** June 03, 2019, 05:58:59 am »

It looks like a 35Hz oscillator to me. That's not mains "ripple" which is 100/120Hz.
"... measuring the output of the power supply with Vout=2V, Rload=10R and the output current almost down to zero"
"the slowCC is active (the output current is set to almost 0A), this is the reason i'm getting 2Vpp ripple..."
"Yes, I am shorting the output."

I give up trying to follow this thread. Where is the 2Vpp? Please say where you are measuring the scope trace. I hope I'm wrong and it's the short-circuit pulser.

I would expect things to go unstable with the current-limit setpoint set very low. It is unrealistic to dial in a mA or two without hitting the noise floor of the design. Usually there is a small resistor from R25 to 0V to limit the lowest setting.

You've poured the cement, ordering PC boards. If you're still testing after that, hope there are no surprises.
The design looks good. I would've used less gain but that's another argument that other people did not agree with.

iMo · « **Reply #491 on:** June 03, 2019, 06:22:31 am »

Quote from: floobydust on June 03, 2019, 05:58:59 am

..
You've poured the cement, ordering PC boards. If you're still testing after that, hope there are no surprises.
The design looks good. I would've used less gain but that's another argument that other people did not agree with.

@floobydust: this is the "Beginners" section.
A statement like "I would have used less gain" does not help much here. You had the chance to provide a concrete advise how to decrease the gain (and how to measure the gain) during the design phase.

mike_mike · « **Reply #492 on:** June 03, 2019, 07:47:04 am »

Quote from: floobydust on June 03, 2019, 05:58:59 am

I would expect things to go unstable with the current-limit setpoint set very low. It is unrealistic to dial in a mA or two without hitting the noise floor of the design. Usually there is a small resistor from R25 to 0V to limit the lowest setting.

Can you please explain this ?

xavier60 · « **Reply #493 on:** June 03, 2019, 07:51:22 am »

I think that the wobble in the trace from Reply #479 is the 100Hz input ripple modulating the output capacitor charge rate every time the PSU unloads.
The next logical test is to see how much output ripple there is with continuous CC into the 10Ω resistor.

floobydust · « **Reply #494 on:** June 03, 2019, 10:16:52 am »

Quote from: imo on June 03, 2019, 06:22:31 am

@floobydust: this is the "Beginners" section.
A statement like "I would have used less gain" does not help much here. You had the chance to provide a concrete advise how to decrease the gain (and how to measure the gain) during the design phase.

It was back at the Bode plot in post #352. There's no mention of where the AC source is injecting. If I had the .asc and models I could participate. I saw the 106dB and compare with LM317 just over 60dB yet it achieves 0.1% load regulation.

With several chefs in the kitchen, I just let it go to see how compensation and stability works out, instead of interrupting the thread and ruffling feathers, going into technical discussion on feedback - when OP just wants a working circuit.

I think for beginners it's best to have something tolerant, very stable despite variation in parts and construction.
You get a circuit stable in SPICE at one operating point, but some details are missing, such as real ripple and the actual transistors can have a huge spread for hFE, fT and even AVOL for the LM324. It's a big deal for the Sziklai pair+driver and open-loop gain, and dominant-pole compensation.

It looks like everyone is saying mission accomplished with PCB's in fab. If issues come up we'll have to back up. I thought the bridge voltage is very noisy.

mike_mike · « **Reply #495 on:** June 03, 2019, 11:02:37 am »

Quote from: xavier60 on June 03, 2019, 07:51:22 am

The next logical test is to see how much output ripple there is with continuous CC into the 10Ω resistor.

Please find attached the screenshots from the output of the power supply, with Vout=2.05V, I=0.17A and RLoad=10R/5W.

If I rotate the current potentiometer, down to 0A, then the ripple disappears. Same thing happens when I set the current limit higher.

xavier60 · « **Reply #496 on:** June 03, 2019, 11:29:04 am »

Is that output ripple more than what is on the input? As with my project, the output ripple disappears in CV mode also but I have only a fraction of that ripple in CC mode.
The cause of the thickening of the trace needs to be found. Is it still there with the mains turned off?

mike_mike · « **Reply #497 on:** June 03, 2019, 11:46:08 am »

With the probe on the output of the power supply, with the mains turned off: 502.jpg
Ripple on the input capacitors, with the 10R load at Vout=2.05V: 503.jpg

xavier60 · « **Reply #498 on:** June 03, 2019, 12:10:06 pm »

The trace thickening looks the same with the mains off, so dont worry about it for mow.
The CC ripple is more than can be blamed on Early Effect. Try removing C5 for now. I dont have any more ideas for now.

mike_mike · « **Reply #499 on:** June 03, 2019, 03:46:52 pm »

1. This behavior of the power supply can be harmful to the power supply or to the load, or it is something that also happens in other power supplies ?
2. Regarding the screenshot with the mains turned off, it was took with the cable disconnected from the wall socket, and after a few minutes of inactivity of the power supply. Is that the correct measurement ?
3. Regarding the attached schematic, of the power supply that has been discussed earlier in this topic, if I will use a 30 Vac/ 5A transformer, what are the modifications necessary in the schematic ? I need the same output voltage and current (0-30V and 0-3A). The only thing that modifies is the transformer. If the current project will be bad, then I will use this power supply, but only if it can be used with 30 Vac transformer. I want to use the 30Vac transformer, because, using the 24Vac transfomer, when the output voltage is about 25V and the load is about 3A, then the ripple appears on the output of the power supply. But if I go with the output voltage down to 24V, then the ripple disappears.


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Author Topic: LM324 Power Supply with variable voltage and current (Read 169633 times)

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