LM324 Power Supply with variable voltage and current

[xavier60]

You could just leave yours as it is or do more experimenting if you have nothing else to do.
I'm a bit curious to know if it is still unstable with R5, 10K.

Should I leave the PSU as it is with R5=4k7 or R5=6k8 ? and C3=1nF  ?

I will try an experiment with R5=10k, for curiosity...

The response looks good with the 6.8K and 1nF.

[mike_mike]

I tried with LM258 instead of 358, at the PSU that is mounted into case.
Vout=27V, Rload=10R, LM258, C3=1nF (ceramic 2kv, blue color), R5=6k8
128_000
Also I tried with R5=10k and C3=1nF (ceramic 2kv - blue color)
Vout=27V, Rload=10R
128_001

[mike_mike]

I tested again, but this time the response looks good with C3=1nF and R5=4k7, LM258P.
Vout=28V, Rload=10R
1. 3x2N3055
130_000
2. 3xTIP35C
130_001

Please have a look and tell me what you think...

[mike_mike]

I tested with the exactly same PCB and transistors from the PSU mounted into the case.
I tried a few things to reduce the overshoot.
First I used C3=1nF and R5=4k7, LM358P, 3xTIP35C.
I tried the following things: changed the Q2 to 2N5551, changed the 1N5408 on the output of the PSU with another 1N5408, changed C2 and the 100nF on the output with some new 47uF and 100nF capacitors. I also changed C4 with a new capacitor, also 3n3.
I tested after each of the mentioned modifications.
Initially the response was: 131_000
After modifications it is: 131_005.
Vout = 28V, Rload = 10R.

LE: I got the best response after I made all final modifications that were changing the 47uF and 100nF output capacitors...

[mike_mike]

U1=LM358, Q2=2N5551, R5=4k7, C3=1nF, C2=47uF in parallel with 100nF ceramic disc (new capacitors).
Vout=27.2V, Imax=2.98A, Rload=10R, C3=1nF film capacitor
133_000
Vout=27.2V, Imax=2.98A, Rload=10R, C3=1nF 1kV ceramic capacitor
133_002
The PSU PCB is the same as in previous reply, the only difference is that the PCB and heatsink were moved to the nearby table for tests.
I think that the overshoot is probably influenced by some external factors...

[xavier60]

I tested with the exactly same PCB and transistors from the PSU mounted into the case.
I tried a few things to reduce the overshoot.
First I used C3=1nF and R5=4k7, LM358P, 3xTIP35C.
I tried the following things: changed the Q2 to 2N5551, changed the 1N5408 on the output of the PSU with another 1N5408, changed C2 and the 100nF on the output with some new 47uF and 100nF capacitors. I also changed C4 with a new capacitor, also 3n3.
I tested after each of the mentioned modifications.
Initially the response was: 131_000
After modifications it is: 131_005.
Vout = 28V, Rload = 10R.

LE: I got the best response after I made all final modifications that were changing the 47uF and 100nF output capacitors...

Do you mean that slight bump after recovery? I can make my PSU behave the same way by making C3 too large. I'm certain that it does not indicate a stability problem.
There is no point changing parts that have no affect on the CV loop like C4 and the output diode.
It seems like the replacement output capacitor has lower capacitance than the original.

[mike_mike]

Do you mean that slight bump after recovery?

I mean the bump marked with the red arrow...

[xavier60]

Do you mean that slight bump after recovery?

I mean the bump marked with the red arrow...

yes, you can just ignore it.

[mike_mike]

You can always add the tap change later. This is what I have in mind. U1A is configured as a Schmitt trigger, sensing the PSUs output voltage.
One section of S1 connects the PSUs Shutdown pin to ground while the other section disconnects the output of U1A from the input of U1B which stores the relay's state.

There are a few missing values for some resistors in the schematic that you presented, can you please add the values to the schematic ?
I'll add the values that I know:
K1 = 12Vdc/10A relay
D1=1N4007
U1=LM358
"In" is connected to the positive output of the PSU
"Shutdown"  - here I will connect the shutdown of the PSU
S1=is a DPDT switch
R3= 10k preset

[xavier60]

I have made some changes. Can you breadboard it first?

[mike_mike]

Yes, I will breadboard it.

[mike_mike]

I have tested the circuit. By connecting it into the PSU circuit and by rotating the 10k preset, I set the output voltage of about 14V as switch point...
If I set the output voltage to maximum (27.2V), and if I connect a overload to the PSU output, then the relay connects the 15V secondary and disconnects the 30V secondary of the transformer. When I disconnect the overload then the white LED from the PSU PCB flashes one time, before going into normal light mode.
Also, I checked the DPDT switch, and it switch off the output of the PSU.

[xavier60]

The bright flash of the white LED during recovery is expectected as it takes some time for the unreg to rise. R8 and C2 add some delay to the relay change over also.
 With no load connected, measure the  hysteresis by slowly adjusting the PSU's output voltage while taking note of the voltage difference that causes the relay to pull in and drop out.

[mike_mike]

When increasing the voltage, the relay goes ON when the voltage is 14.62V.
When decreasing the voltage, the relay goes OFF when the voltage is 13.49V.

[xavier60]

My calculation says that the  hysteresis should be 250mV. Is C3 a nonpolar type.

[mike_mike]

I checked the circuit on the breadboard, and it looks OK.
Yes, C3 is a nonpolar capacitor.

[xavier60]

Try removing C3 anyway,

[mike_mike]

Now I measured 14.7V when increasing the voltage, and 13.7V when decreasing the voltage.

[xavier60]

Check for oscillations.

[mike_mike]

I removed C3, and one of the terminals of R7 (100k) is now unconnected.
Also, my breadboard is a bit old, if I touch the components or wires with my finger, then the led from the tap changer is flashing...

[xavier60]

How well regulated and clean is the 12V? does it drop when the relay pulls in?

[mike_mike]

The 12V supply is from the LM324 PSU that I built previously in this topic.
If does not drop when the relay pulls in, but on my breadboard are imperfect contacts which I cannot fix. My breadboard is not good.
Probably, the one solution is to solder the components into a perfboard.

LE: the voltage at pins 8 and 4 of LM358 is 11.88V when the relay is OFF and 10.5V when the relay is ON...

[xavier60]

The 12V supply is from the LM324 PSU that I built previously in this topic.
If does not drop when the relay pulls in, but on my breadboard are imperfect contacts which I cannot fix. My breadboard is not good.
Probably, the one solution is to solder the components into a perfboard.

LE: the voltage at pins 8 and 4 of LM358 is 11.88V when the relay is OFF and 10.5V when the relay is ON...

That voltage drop would be the reason.
An LM7812 or LM3i7 on the final PCB should be stable enough.
Testing it  on perfboard will prove the design. Adding a 1uF capacitor from the trimpot's wiper to ground will help also.

[mike_mike]

I built the circuit on a piece of perfboard. It is working now, the is now about 200-250mV.
When increasing the voltage, the relay goes ON when the voltage is 13V and goes of when decreasing the voltage at 12.8V.
But, if I set the output voltage at 13V, and if I switch OFF the PSU from the Shutdown switch then when switching it ON again, the led goes off then on again and the relay also makes sound.
If I set the output voltage from the PSU at for example 16V or 8V, then this behavior of the led disappears.

The schematic is the one from reply #859.

[xavier60]

Try it with C3 removed,