Hi all,

I have bought 2 of these power supplies. Now the problem is the negative voltage can be only trimmed down to -6.9v but i would need -5v.
Anyone of of you have an idea how i can mod the TL431 circuit at the LM337?
Its working in the positive circuit with the LM317.

Thanks

 

What value is W2?

 

Try R3=15K & R4=100K.

 

Try R3=15K & R4=100K.

That might mean the output voltage won't go high enough - unspecified by TS.

 

What value is W2?

W2 is 104 / 100k

 

That might mean the output voltage won't go high enough - unspecified by TS.

not enough high is not that terrible.... i only need to adjust the -5v.

Try R3=15K & R4=100K.

Ok, thanks

 

W2 is 104 / 100k

Hmm... With those values it should go to -4.6V so if it only manages -6.9V then there may be something wrong with something.

 

Hmm... With those values it should go to -4.6V so if it only manages -6.9V then there may be something wrong with something.

I have 2 of them and it happens with both... and the positive voltage is working. So i think its about the circuit.

 

Have you tried changing R3 & R4. They are shown in the wrong position on the schematic.

 

Have you tried changing R3 & R4. They are shown in the wrong position on the schematic.

Ok, i changed:
R3=15K & R4=100K

Now i can only trim down to -10.82V

 

They are shown in the wrong position on the schematic.

They are in the same position for both positive and negative supplies.

Check the actual values of R3 and R4 on the board.

 

They are in the same position for both positive and negative supplies.

Check the actual values of R3 and R4 on the board.

I had checked them when i was changing the 2 resistors. They are both correct shown in the schematic.

 

They are in the same position for both positive and negative supplies.

Check the actual values of R3 and R4 on the board.

R4 -10.8V / -3.7V
R3 -1.22V / 0V

 

R4 -10.8V / -3.7V
R3 -1.22V / 0V

I just noticed that the two resistors we are talking about are R3 and R41 (R4 is 220Ω across the LM337)

Set W2 for minimum voltage then measure the voltage between ground and:
The junction of R3 and W2
The junction of W2 and R41
The output voltage

 

I just noticed that the two resistors we are talking about are R3 and R41 (R4 is 220Ω across the LM337)

Ahhh... wait... you mean i should change R41 with 100k and not R4 ?

I have now changed R4 form 220R to 100k and R3 from 100k to 15k....

 

R4 should definitely be 220Ω.
You can try making R41 100k and R3 15K and see what output you get.

 

R4 should definitely be 220Ω.
You can try making R41 100k and R3 15K and see what output you get.

Yes, it can be now trimmed to -4v Great!

Thanks for your help

 

Good.
I still don't understand why you couldn't get -5V, especially on two supplies. Just one more mystery I guess.