cheapo chineese 0-30v 3amp psu issues.

Thread Starter

m3vuv1

Joined Dec 11, 2018
14
For what its worth if i lift one leg of R11 and one leg of C9,it still behaves the same,voltage adjusts to a max of about 9v and the current limit still works.
 

panic mode

Joined Oct 10, 2011
1,782
Do you understand what those parts do in this circuit? You should not just go around and disconnect things without at least trying to predict circuit behaviour.

C9 is also in the feedback loop filtering any high freq noise. Should not have this disconnected.

R11 is part of feedback that sets output voltage (R11/R12 is a voltage divider). if R11 is open, U2 would work as voltage follower so PSU output would match potential at wiper of P1.
If disconnecting R11 does not make difference, R11 is either defective or you got there wrong values for some of components - either R11 is too large or R12 is too low or there are shorts (solder bridge etc.).

you should show your construction method. There may be things that are obviously wrong or at least help narrow down possible issues.
Also show parts list. Finally measure voltages at key points, and post the values.
 

Thread Starter

m3vuv1

Joined Dec 11, 2018
14
Do you understand what those parts do in this circuit? You should not just go around and disconnect things without at least trying to predict circuit behaviour.

C9 is also in the feedback loop filtering any high freq noise. Should not have this disconnected.

R11 is part of feedback that sets output voltage (R11/R12 is a voltage divider). if R11 is open, U2 would work as voltage follower so PSU output would match potential at wiper of P1.
If disconnecting R11 does not make difference, R11 is either defective or you got there wrong values for some of components - either R11 is too large or R12 is too low or there are shorts (solder bridge etc.).,looks like ive got it sorted,pins 2+3 of u2 were the same voltage in cc mode but went haywire in cv mode,did some checking and found d9 leaky,replaced that and all seems good now apart from i cant get it to zero,3mv is the lowest i get it with rv1,i did swap c3 from a 47uf to a 100uf to try and cure the spike,dont know if thats the cause yet,i'll swap it back in the morning and see as its 1am here now.

you should show your construction method. There may be things that are obviously wrong or at least help narrow down possible issues.
Also show parts list. Finally measure voltages at key points, and post the values.
 

panic mode

Joined Oct 10, 2011
1,782
you should not put your own text into quote, makes it next to impossible to read. your content should be before or after quote.

looks like ive got it sorted,pins 2+3 of u2 were the same voltage in cc mode but went haywire in cv mode,did some checking and found d9 leaky,replaced that and all seems good now apart from i cant get it to zero,3mv is the lowest i get it with rv1,i did swap c3 from a 47uf to a 100uf to try and cure the spike,dont know if thats the cause yet,i'll swap it back in the morning and see as its 1am here now.
3mV is pretty darn close to "zero" as far as things go for a PSU outputs. that is margin for ripple, noise... that is 1/10000 of the PSU range.
 
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Thread Starter

m3vuv1

Joined Dec 11, 2018
14
you should not put your own text into quote, makes it next to impossible to read. your content should be before or after quote.



3mV is pretty darn close to "zero" as far as things go for a PSU outputs. that is margin for ripple, noise... that is 1/10000 of the PSU range.
i have 3 of these from an old gas analyzer,would they be a better replacement for the op amps?,see attached data sheet
 

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Audioguru

Joined Dec 20, 2007
11,251
The circuit is a Chinese copy of an old Greek kit that had many parts overloaded. It uses TL081 opamps that have the Opamp Phase Inversion problem where its output goes as high as it can when its input becomes within a few volts from its negative supply (when the power is turned on or off). Q1 detects that the negative supply drops first and clamps the output. But the value of the main filter capacitor C1 is much too small do allow it to work properly when there is a load. The voltage divider resistors feeding the base of Q1 might be wrong in the Chinese copy.
 
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