cheapo chineese 0-30v 3amp psu issues.

Thread Starter

m3vuv1

Joined Dec 11, 2018
14
Hi all ive just built one of these,it works but ive noticed if its in current limit mode and i switch it off i get a fairly big voltage spike at the output,when its not in cc mode but cv its fine,is this normal?,if so is there a fix except bin the thing?,cheers Paul m3vuv.p.s its one of the hyland things.
 

Thread Starter

m3vuv1

Joined Dec 11, 2018
14
Hi all ive just built one of these,it works but ive noticed if its in current limit mode and i switch it off i get a fairly big voltage spike at the output,when its not in cc mode but cv its fine,is this normal?,if so is there a fix except bin the thing?,cheers Paul m3vuv.p.s its one of the hyland things.
heres the schematic
 

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crutschow

Joined Mar 14, 2008
23,157
It would seem to be somewhere in the current-limit circuit, which I think is R7, Q1, and associated circuitry, but I can't figure out how it's supposed to work. :confused:
Is there an adjustment for the CC mode current?
If so, which pot?
 

Thread Starter

m3vuv1

Joined Dec 11, 2018
14
It would seem to be somewhere in the current-limit circuit, which I think is R7, Q1, and associated circuitry, but I can't figure out how it's supposed to work. :confused:
Is there an adjustment for the CC mode current?
If so, which pot?
p2 is the current adj pot,r7 is the current sense resistor.
 

crutschow

Joined Mar 14, 2008
23,157
Don't see anything offhand that would cause a spike in the CC current when the voltage is shut down.
How high and long is the spike?
Does the CC LED, D12, go off when this spike occurs?
 

Thread Starter

m3vuv1

Joined Dec 11, 2018
14
Don't see anything offhand that would cause a spike in the CC current when the voltage is shut down.
How high and long is the spike?
Does the CC LED, D12, go off when this spike occurs?
i havent measured the spike on my scope yet but it makes a 5w 12v bulb flash bright so i would guess about 12v or so,also the led goes out intantly on turn off but the bulb ive got on the output flashes at the same time the cc led goes out,its like the cap c1 is dumpping its energy into the output at turn off,only does it in cc mode tho.
 

AlbertHall

Joined Jun 4, 2014
8,187
U3 compares the voltage across R7 to a fraction of the reference voltage from U1 and this is the current limit circuit.
Q1 seems to be checking the positive and negative rails and if the negative rail is too positive, or the positive rail is too negative, Q1 pulls down the base of Q2 reducing, or shutting down, the output. I don't know when this circuit may be triggered - maybe at switch on or switch off?
 

panic mode

Joined Oct 10, 2011
1,775
i think Q1 disables output until negative rail is charged since OpAmps would not be able to function near ground rail
 

crutschow

Joined Mar 14, 2008
23,157
The current-limit opamp U3 needs the negative rail voltage for proper operation.
If the negative rail is dropping out before the positive rail, that could cause U3 to turn off, allowing current above the set limit to flow.

If you have a scope you can monitor the positive and negative rails to see if that's a problem.

If so, a larger value for C3 would keep the Negative Rail from decaying so rapidly and possibly eliminate the voltage spike.
 

Thread Starter

m3vuv1

Joined Dec 11, 2018
14
Have you got a model number or even better a schematic? It would help!
Don't see anything offhand that would cause a spike in the CC current when the voltage is shut down.
How high and long is the spike?
Does the CC LED, D12, go off when this spike occurs?
thought i had it sorted,removed q1 and found the resistance between base and collector was 1.82 ohm,replaced it with a bc 548 but its just the same,btw the spike happens at the same time the cc led goes out.
 

panic mode

Joined Oct 10, 2011
1,775
what are the rail voltages? do they swing much during transition? how do you enter cc mode - by turning P2 or by adjusting load? does P2 have a burn spot?
what is construction method? are connections solid and wires short?
 

Thread Starter

m3vuv1

Joined Dec 11, 2018
14
what are the rail voltages? do they swing much during transition? how do you enter cc mode - by turning P2 or by adjusting load? does P2 have a burn spot?
what is construction method? are connections solid and wires short?
well i changed q2 and no more spike,issue then was the most it will output is just over 9 volts under load or with no load,suspected q4 so changed it for a 3055,still the same low output but no spikes,also changed q1,as you turn the voltage up it sort of flikers just before max like a voltage limit is kicking in,its on a pcb short wires +ve rail holds at 27v under load 28v with load and the neg rail at 5.1 dead on,any ideas where to look now?,ive swapped all the op amps but no change!.
 

AlbertHall

Joined Jun 4, 2014
8,187
Is the limited output voltage a new fault or was it doing that before you changed transistors?
Have you replaced the transistors with the same type (it sounds like Q4 wasn't a '3055 originally - what was the original transistor)?
 

Thread Starter

m3vuv1

Joined Dec 11, 2018
14
Is the limited output voltage a new fault or was it doing that before you changed transistors?
Have you replaced the transistors with the same type (it sounds like Q4 wasn't a '3055 originally - what was the original transistor)?
no the op would go to 29v before i swapped q2 the original was a 2sd882,its hfe/beta was 437,the replacement was sommat i had in my junkbox but only had a hfe of 49,i dont know if thats the issue,i would of thought the controll elements would of compensated for that?
 

AlbertHall

Joined Jun 4, 2014
8,187
no the op would go to 29v before i swapped q2 the original was a 2sd882,its hfe/beta was 437,the replacement was sommat i had in my junkbox but only had a hfe of 49,i dont know if thats the issue,i would of thought the controll elements would of compensated for that?
Replacing Q2 and/or Q4 with lower gain devices may well reduce the available output. You should replace with the original types or at least good equivalents.
 

Thread Starter

m3vuv1

Joined Dec 11, 2018
14
Is the limited output voltage a new fault or was it doing that before you changed transistors?
Have you replaced the transistors with the same type (it sounds like Q4 wasn't a '3055 originally - what was the original transistor)?
q4 originaly was a 2sd1047
 
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