I need maximum 25V at output. I tried some time ago with a 24Vac transformer but when I set the output voltage at 25V and I connected a constant 3A load (like a resistor), then on the scope the output of the power supply was not a perfect horizontal line, it looked somehow strange (I deleted the screenshot with the scope screen from my pc)... the output which should be a horizontal line was a horizontal line in combination with the ripple on the filter capacitor.

Did you have enough smoothing capacitance?
Ripple ≈ It/C

 

hi,
A 100uF cap on the output is useless for 25V at 3Amp, I would not use less than 1000uF.
E

 

I tried some time ago with a 24Vac transformer

What was the transformer's current or VA rating?

 

Did you have enough smoothing capacitance?
Ripple ≈ It/C

It was 4700uF/63V on the input.

hi,
A 100uF cap on the output is useless for 25V at 3Amp, I would not use less than 1000uF.
E

It was 100uF in my circuit.

What was the transformer's current or VA rating?

It was 100VA.

 

hi mike
Use at least 1000uF/Amp for the smoothing cap, this uses a 25Vrms transformer.

E

 

I will add a darlington transistor circuit instead of 2N3055 (Q1) because the current draw from LM723 will be too high if using only 2N3055.

 

If you are running out of headroom then a Sziklai pair would get you a Volt closer to the power supply.

 

hi mike,
Ref the 2N3055, see image. Worst case hFE is 20 at Ic of 4Amp.

Also, the LM723 max output base drive current is 150mA
So I would expect 20 * 0.15A = 3Amp as the minimum possible C?E current.

Give it a try.
E

 

This is the last schematic.

 

hi mike,
If you want others to run your asc file they need the full-wave bridge model.
E

 

Please find attached.
It looks like is a small amount of oscillation on the 0.15R resistor.

 

hi,
Have you measured the Load current pulse in the output load!.
E

 

I can see that the current through the load (R12) is very high but through the R21 (0.15R) it is about 36A, which means it is 12A through each power transistor.

 

hi,
I was under the impression that you are designing a 28Vdc @3Amps power supply.
The high load current spike is due to dumping the 28V charge on the C2 1000uF, when switching load On

The 0.15R emitter resistor should limit the Iout to 4A max, after the spike.

Have you revised the specification.?

E

 

Have you revised the specification.?

No, I am designing a 25Vdc @3Amps power supply. This was the initial specification.

 

Hello @ericgibbs , I found a schematic which is tested and it is functional. I tried to do a test with the oscilloscope, it does not oscillate at this moment but the output voltage is going down with about 200mV when connecting a load, even if the current limit is set at about 4A and I connect a 3A load. The output voltage was set at 29Vdc.
I tested using different potentiometers, different LM723, different driver transistor and it looks like the problem persist even if I replaced the mentioned components with new ones.

 

hi mike,
Please post the LTS asc file for that circuit in post #36.?
E

 

hi mike.
I have drawn out the circuit.
E
Note your pin723 pinout locations are slightly different from mine, move to suit.
E
[/ATTACH]

 

Please find attached my simulation.
I have seen on the scope that the output voltage does not return to the set value. While the load is ON, the voltage is about 200mV under the set value.

 

hi mike,
You keep changing the circuit and component types.

E