0-30v power supply 0v adjustment and current limiting indicator LED not working?

Thread Starter

noel.ramakers

Joined Jun 9, 2020
16
Hello,
I needed two power supplies for electroplating. So I bought two kits from Ali. I bought them separately from two suppliers because I realized I needed second one after buying the first.
I build them both on the same day but they both have similar problems.
I use a 24v AC transformer and installed the supplied 5v1 zeners. After building them both I started to test them but I couldn't adjust the 0 volt as described in the manual. With the voltage and current pots on 0 ,I turned the trimpot RV1 all the way up and down on both boards and the output voltage doesn't change? The current limiting indicator LED is on all the time and I need that LED to work.
If I use my adjustable AV transformer on one board the current limiting indicator LED starts working on 22v on the other on 17V. The opamp's are fitted on a socket and I changed them around without changing the simptoms.
Can anybody point out what could be wrong? If more info is needed I will provide.
 

AlbertHall

Joined Jun 4, 2014
11,313
With the voltage and current pots on 0 ,I turned the trimpot RV1 all the way up and down on both boards and the output voltage doesn't change? The current limiting indicator LED is on all the time and I need that LED to work.
Try this again with the current limit pot turned up to maximum.
It sounds like the current limit is adjuastable down to zero and so when turned right down it will be in current limit and the voltahe controls will have no effect.
 

Thread Starter

noel.ramakers

Joined Jun 9, 2020
16
Try this again with the current limit pot turned up to maximum.
It sounds like the current limit is adjuastable down to zero and so when turned right down it will be in current limit and the voltahe controls will have no effect.
I tried it, with no luck. The voltage doesn't change. It stays at 0.00v


the circuit diagram :
0-30vdc-variable-power-supply-circuit.gif
 

Thread Starter

noel.ramakers

Joined Jun 9, 2020
16
With the negative meter lead on 0V output (marked 4 on circuit), measure pin 6 of each IC.
I tested with A on max and V on max. No load. 24.9v AC input.
U1 pin 6 = 9.8v
U2 pin 6 = 22.5v
U3 pin 6 = 25.7v
Max output voltage 21.7v
The same voltages with 20.6v AC input but the current limiting LED does its job?
 

AlbertHall

Joined Jun 4, 2014
11,313
Starting from there, turn the voltage pot down to zero. The output voltage should reduce to zero and the current limt light should stay off.
If that happens then try adjusting RV1 so the output is just zero.
 

Thread Starter

noel.ramakers

Joined Jun 9, 2020
16
Starting from there, turn the voltage pot down to zero. The output voltage should reduce to zero and the current limt light should stay off.
If that happens then try adjusting RV1 so the output is just zero.
When I turn the voltage pot down, the voltage reduces to zero but the limit light is on all the time. (only if I feed 20.6V ac it works as it should)
The voltage output is now -4.9mV RV1 doesn't change this at all.
 

AlbertHall

Joined Jun 4, 2014
11,313
From the voltage at the output of U3 the curretn limit light should be off so there is some problem with the circuit feeding that LED. We'll worry about that later.

Adjust the voltage pot to get a low voltage, say 0.5V. Now, does adjusting RV1 affect the output voltage, even a little bit?
 

Thread Starter

noel.ramakers

Joined Jun 9, 2020
16
I opened my "old" power supply. I build this a couple of years ago from a similar kit.
I tested with A on max and V on max. No load. 24.9v AC input.
U1 pin 6 = 10v
U2 pin 6 = 28.1v
U3 pin 6 = 28.2v
Max output voltage 26.9v
On this psu I can change the 0v up and down?
 
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