# Question about pulse transformer test(computer psu)...

#### UnnamedUser159

Joined May 3, 2016
501
Hi i am rereading psu repair guide for 100th time

Its written "check if the primary coil is healthy"

I am watching now one old PSU - Delux 400W and see 3 coils which enter the board and can see them solder on the back side of the board.

Why there are 3 not 1 and which and how to check?

thanks!

#### LesJones

Joined Jan 8, 2017
3,259
Transformers always have more than one winding or at least a tap on a single winding. (An auto transformer) You need to read up on the theory of transformers as the fact that you think it should have only one winding shows you know nothing about transformers. It is not practical to teach you transformer theory in a reply. As you show your occupation as an electrical engineer I would have thought you would have been tought transformer theory.

Les.

#### UnnamedUser159

Joined May 3, 2016
501
i didnt learn transformer theory and dont want to/right now/.

i talk about primary and secondary windings.In guide is written like "there is only one primary winding. check its healthy"

#### LesJones

Joined Jan 8, 2017
3,259
There will be only one primary but is is just possible it could be made up ot three windings connected in series or parallel. Think of the way the primary is normally wound on a normal mains transformer that can be configured for 110 or 220 volts input. For 110 volts the windings are connected in parallel. For 220 volts they are connected in series. I would say that checking just the primary winding is pointless. I would check all of them. It is very unlikely the HF transformer in a switch mode power supply is faulty. I have never found a faulty one.

Les.

#### UnnamedUser159

Joined May 3, 2016
501
In computer PSU its used to have the output voltages like +5V +12 and 3.3V.

Let say is meaningless. just tell me how to make basic check.

#### AlbertHall

Joined Jun 4, 2014
11,393
In computer PSU it`s used to have the output voltages like +5V +12 and 3.3V.

Let say is meaningless. just tell me how to make basic check.
Transformer (or choke) windings should read very low resistance. Reading high resistance or open circuit indicates a fault. You do need to be able to identify the winding connections to do that.

I'm with Les above. The transformer(s) are a very very low probability of causing a fault.

#### ColinPatra

Joined Jan 27, 2018
28
You can check for continuity with a multimeter, but that will not show shorted turns where insulation has broken down. You do not say if this is a straight mains PSU or switch-mode.

#### UnnamedUser159

Joined May 3, 2016
501
continuity between which and which
You do not say if this is astraight mains PSU or switch-mode.
What is the different ?

This is atx-400w p4

#### AlbertHall

Joined Jun 4, 2014
11,393
continuity between which and which
Continuity between the ends of each winding. Without a schematic and a layout diagram you have to trace the circuit and work out where the winding connections are.

I say again, don't worry about the transformer(s). Any faults are much more likely to be caused by capacitors or semiconductors but also check fuses and varistors which may be damaged by the effects of the fault.