LM324 Power Supply with variable voltage and current

[mike_mike]

I will use some multi turn pots from SR Passives, which I hope are not really that bad ..
I verified one of the pots and I found that it is working good. When I start rotating it, the current is 0mA then - as rotating - the current goes to 2mA.

In one of the previous posts you said that I can remove the 1M resistor (R10) that is connected from the CC pot's wiper to GND. It will be Ok if I will install it back into the circuit ? I read that this resistor was suggested in the topic were you built your power supply.

[mike_mike]

I verified with a 5K multi turn pot (10 turns) for U7, and I found that works better than using a single turn potentiometer.

I would like to ask if it is better to add the 1M resistor from pot's wiper to GND ?

[xavier60]

I verified with a 5K multi turn pot (10 turns) for U7, and I found that works better than using a single turn potentiometer.

I would like to ask if it is better to add the 1M resistor from pot's wiper to GND ?

There is no need for the bleed resistors if it can be seen from the schematic that the wiper voltage will go down if the wiper looses contact with the Pot's resistive element.

[mike_mike]

One more question: I made again the tests for checking if 7812CV oscillates, and I found that if I connect the probe between the input of 7812 and GND of 7812, I got a strange wave form. I also added a green led in series with a 1k resiston on the output of the 7812CV Please have a look at the attached screenshots (Vout = 26.6V, Ilim=3A):
No load: 255-257.jpg
2.88A load: 268, 269.jpg
Also I am getting with 2.88A load on the input of 7812CV the following screenshots: 270-272.jpg

Is the wave form from the attached screenshots normal ? If no, what should I do ?

[iMo]

It is 100Hz 200mVpp with load, coming from the bridge rectifier. It is normal, imho.

[mike_mike]

Ok, but in the first screenshots (255-257 and 268-269) I have got a waveform that is randomly moving on the screen sometimes it looks like 255.jpg, sometimes like 256.jpg, sometimes like 257.jpg and sometimes it has another form, etc etc. I would like to ask if that waveform is normal ?

[xavier60]

There might be fluctuations of the mains voltage. It would be a very small amount and difficult to detect while looking at the whole waveform.

[mike_mike]

I made some more measurement on the input of the power supply (on the filter capacitors) and I found that the fluctuations are also present there.
Vout=26.6V, Ilim=3A, Iload=2.88A.
1. No load, with the GND clip connected to the GND of 7812: 285,287.jpg
2. With load, with the GND clip connected to the GND of 7812: 292,293.jpg
3. No load, with the GND clip connected to the GND of filter capacitors: 294,296.jpg
4. With load, with the GND clip connected to the GND of filter capacitors: 298, 299.jpg

Please have a look at the screenshots and tell me what you think.

[xavier60]

It's small mains fluctuations, likely normal.

[mike_mike]

I think I worked like a nothing in this project. I found that I have about 38Vdc on the input of the 7812 in my project 
I didn't knew why, then I checked the 12V zener diode, and I found that it does not have a voltage drop of 12V. It have about 1-2V voltage drop. This diode is installed correctly... but the input voltage for 7812 was a little bit higher that recommended drop voltage in the datasheet...
But, the output voltage of the 7812 is 11.88V. So the output voltage is ok ...

Are all my tests and measurements pointless ?

[xavier60]

Are all my tests and measurements pointless ?

The 7812 didn't fail, so there is no need to suspect that a problem has been caused.
The zener has been damaged by a current surge.
Try a resistor instead of the zener.

[iMo]

Afaik there is a resistor in series with the zener in my schematics above..

[mike_mike]

I tried with a 470R resistor instead of the 5R resistor in series with the 12V zener, and I got the following results:
On the input of the 7812, with no load 29.8V, on the output of 7812: 11.89V
On the input of the 7812, with 2.88A load at Vout=26.6V: 25.5V, on the output of the 7812: 11.89V.

@imo: that resistor was installed in the circuit when I found the problem. Also, it was installed during the tests.

Also, I tried with 820R resistor but I was not satisfied by the voltage on the 220uF capacitor, because was too low (about 20V without load).

The voltage measured on the 470R resistor is about 11-12V, during short circuit on the output (max current).

Are there any other things that needs testing ?

[xavier60]

Is the faulty zener still in circuit?

[mike_mike]

No, the faulty zener was removed from the circuit.

Also, I removed the 5R resistor (the one which was in series with the zener), and I used a 470 R resistor.

[xavier60]

Im having trouble accounting for 24mA of current draw.

[mike_mike]

I know that about 10mA are drawn by the green led that I installed at the output of the 7812...

[xavier60]

I know that about 10mA are drawn by the green led that I installed at the output of the 7812...

That would just about explain it. All should be well.

[mike_mike]

Ok, then I will use the 470R resistor.

Will the power supply work correctly with the 470R resistor modification ?

[xavier60]

Ok, then I will use the 470R resistor.

Will the power supply work correctly with the 470R resistor modification ?

I don't know what problems the mod will cause, likely none. There would now be less ripple at the 7812's input.

[mike_mike]

I made again some of the tests, and I found that in the E of Q1, the waveform looks different in comparison with what we had initially.
Please have a look at the screenshots and tell me what you think.

The rest of the screenshots seems like they look like the previous screenshots.

[xavier60]

That's expected for when the power supply unloads. The large dip is due to the CV loop recovering from a small overshoot.
It is a very well behaved response.

[mike_mike]

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.

I measured again with the latest modifications and I have got:
V (D9) = 5.56V and V (R25) = 1.59V.
Regarding the R25 voltage, it is still normal 1.59V instead of 1.35V ?

[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.

I measured again with the latest modifications and I have got:
V (D9) = 5.56V and V (R25) = 1.59V.
Regarding the R25 voltage, it is still normal 1.59V instead of 1.35V ?

The voltage across R25 does not need to be any particular value. It will vary with output load and the temperature of the PNP driver.
With only the residual output load, the drive current through Q1 is mainly the B-E drop of the PNP driver across the 1K resistor plus a small amount of Base current, about 50µA.
Start by measuring the B-E voltage of the PNP driver.

[mike_mike]

I forgot to specify that the mentioned R25 voltage was measured with 100 mA load at 15V output voltage.

LE: BE voltage of pnp driver is 0.56V. So I got 0.56V / 1000R = 0.00056A