Variac Transformers and switch mode power supplies.

[angelos]

Hello everyone,

I was wondering if anyone in the forum can tell me how they use the variac transformer to check a switch mode power supply that they had just repaired.
I wanted to know how they adjust the votlage, do they start from zero and turn it up slowly or rather switch on that about 60V ac ( My country we use 220V ac) and hope for the best.
Also what sounds the PSU should make, what sounds wrong etc. Also if i have a scope what can i check for.
The reason why i am asking is i have a Sony LED TV PSU whose PFC circuit shorted out and upon closer inspection a lot of SMD components had failed.
I have a donor board to cannibalise but i only have one shot at it.
Hope you understand my dilemma.

Regards
Angelos

[KYBOSH]

Angelos

I want to say you are doing this wrong...
I would suggest you not play around with something like this until you understand more fully what it is you are doing.
YouTube video, repair manuals and instruction manuals will be your best bet.

Off the top of my head, without seeing your setup, i think what you need is a Step Down Transformer (220v To 110v) in conjunction with a DIY Light bulb current limiter to test the SMPS.
AGAIN... please dont do anything unless you actually know what you are doing here as you can cause serious harm or injury to yourself and/or other people/property.

[angelos]

Hi KYBOSH,
How am i doing it wrong?
There are no schematics on this model. Sony made sure of it.
I have a 500VA isolation transformer for my protection and as for the power supply, it was blown up to begin with.
I simply need to understand the basics of how you ramp up the ac input on the switch mode power supply.
The PFC circuit was damaged. This included the mosfet, driver ic and a few smd components in between.
This power supply is avaialble on ebay or aliexpress as a second hand but they are mad with their price. I have all the time to fix it but i only have one try.
Can you explain to me? I am technical, i have been doing this job since 1996 but i came across this variac and want to use it.
I used the light bulb method in the past where i insert a light bulb on the ac input to prevent surges but that does not prevent the failure on the PSU.
Can you help, Do you know of a youtube video that talks about it?

Regards

[angelos]

I know that SMPS power supplies don't like a low AC input as they can draw a lot of current at startup. But what else can i do? Just give 230V AC and hope for the best?
🔥🔥🔥🔥🔥

[angelos]

Just an update, So I took your advice KYBOSH and I did not use the Variac. I instead opted to use a couple of incandescent light bulbs in series with the mains fuse to see what the current draw is initially and it seemed fine.
The tv works fine, thanks for your help.
Quite few parts replaced in this Sony KLV-32T400A power supply, Most damage was done to the PFC circuit.
I have added the pictures or your interest.

Regards

[KYBOSH]

Very happy ending!