Forcing on Power Supply with PS_ON

[budm]

Re: Forcing on Power Supply with PS_ON

The voltage is 4.5V on pin 3 of the PFC controller IC. Here are some pics of the PFC circuit.

That is OK, the trip point is if it is below 1.40V, so next you need the scope to look at the Drive Output pins GD-M 19, GD-S 17.

[tech2014]

Re: Forcing on Power Supply with PS_ON

ok I will check those. I don't have an isolation transformer, will it be safe to use my scope as long as I have it is grounded on primary ground.

[budm]

Re: Forcing on Power Supply with PS_ON

ok I will check those. I don't have an isolation transformer, will it be safe to use my scope as long as I have it is grounded on primary ground.

NOPE! you need to plug what you are working on into the completely isolate AC outlet of the isolation transformer.
If you put you scope GND on the primary circuit ground (negative leg of the main cap) you will see nice sparks and smoke.

[tech2014]

Re: Forcing on Power Supply with PS_ON

HAHA glad I asked lol. Any idea where I can find one on eBay?

[Andyy]

Re: Forcing on Power Supply with PS_ON

If your scope is battery powered, you won't need an isolation transformer (effectively the scope is a fancy glorified multimeter, whose ground reference is floating).

But an isolation transformer is good anyway for safety's sake.

[tech2014]

Re: Forcing on Power Supply with PS_ON

My scope is not battery powered so I am SOL. I measured the 3 parts on the heatsink in the PFC circuit, 1 diode and 2 FETs. They all measure ok in circuit. I also measured all the surface mount low ohm resistors and diodes and they all measured good. So only other thing is the PFC IC, unless I missed something?? I watched a video on youtube regarding this p/s. The tech used a 220 ohm resistor from the PS_ON pin to ground. I just used an alligator clip connected to ps_on pin and other end connected to secondary ground. It shouldn't make a difference since the voltage is low enough shorted directly to ground.

[budm]

Re: Forcing on Power Supply with PS_ON

You already know that the PS-ON signal (ACTIVE LOW) is fine otherwise you will not have VCC for the PFC IC, that VCC for PFC is fed through a switched Transitor which is turn on when PS-ON signal is present.

[Andyy]

Re: Forcing on Power Supply with PS_ON

My scope is not battery powered so I am SOL.

Borrow a car battery and inverter

[budm]

Re: Forcing on Power Supply with PS_ON

Plug the TV into a UPS with TRUE SINE wave that is not connected to the outlet and safety GND.
Running Scope with Battery will make your scope GND at line potential, still not safe if you touch scope GND/chassis and Earth, it is the same as cutting off the Safety GND pin of the power plug of the scope.
You want to float the load, not the instrument, your scope has exposed metal, you meter is all plastic.
You must remember this, the primary side circuit GND ref. (Negative leg of the main filter cap) is not at GND potential with ref to safety GND!
Differential probes come in handy when work on the HOT side of the circuit, that is what I use in the lab at work.
Safety first!

[tech2014]

Re: Forcing on Power Supply with PS_ON

I am going back to school in September, so I cannot afford to drop hundreds of dollars on a differential probe, but I can not afford not to have an isolation transformer. I did find one on eBay. Made an offer, waiting for response.

I agree budm, safety first.

[tech2014]

Re: Forcing on Power Supply with PS_ON

So I removed the 2 mosfets in the PFC circuit and tested them. I put my (-) on the source pin and the (+) on the drain pin. Reads OL, which is normal. I then touched the (+) to the gate for a second, then back to the gate. Should I not get a reading close to 0? I get 0.4V. When I put my finger between the gate and source, the voltage goes back to OL.
Is a voltage drop of 0.4V ok, or is it too high?

[budm]

Re: Forcing on Power Supply with PS_ON

MOSFET should usually shorted out when they fail, and as I indicate, I doubt that both of them failed at the same time.