Components of LCD TV PSU

[wiru]

Hello,

I have an LCD TV power board IP-58155A.


I'd like help identifying a component on it. What may it be? It looks a bit like a germanium diode, it's marked SA101 on the board and has a symbol of a resistor on the underside of the board. It's connected in parallel with one coil of the power filter inductor.


And here's the underside:


The other component I'm interested in is a small transformer on the inverter side. What is the function of it? How does it work? I figure it's there for feedback purposes, but I don't get how it works. There are two of them on the opposite sides of the board. Marked T304 and T305, transformers are BL-25.

CCFL connections have two cables with two wires each. One wire goes straight to the big inverter transformer (which has failed in my case - resistance of two HV windings is different). Another wire goes to the small transformer. Why?

 

[davenn]

The SA is a spark gap device ( you wont fins any germanium components in that PSU )

why the interest in this component ?

The other component I'm interested in is a small transformer on the inverter side. What is the function of it? How does it work? I figure it's there for feedback purposes, but I don't get how it works. There are two of them on the opposite sides of the board. Marked T304 and T305, transformers are BL-25.

it steps up a small AC voltage to a much higher one approx. 1000V to feed the CCFL tube
note there are 2 of them .... so presumably 2 tubes one each across the top and bottom of the panel
( or maybe one each side of the panel)

CCFL connections have two cables with two wires each. One wire goes straight to the big inverter transformer (which has failed in my case - resistance of two HV windings is different). Another wire goes to the small transformer. Why?

your inverter transformer is mis-named ... its just a balance between the 2 tubes

power to the tubes is supplied via those 2 small step-up transformers

Dave

 

[wiru]

Thank you for identifying that component. I had just never seen it before and it had no obvious markings on it, and I was wondering what it was doing in parallel with the filter inductor.

So if I understand correctly, when there is sudden mains power loss while the circuit is under load, the input filter choke develops high voltage. In such an event the surge absorber SA101 conducts. This way the high voltage does not propagate further into the circuit. Is that correct?

Talking about the transformers, I can't agree that power is supplied by the two miniature ones. It doesn't make sense. The drain tab of the switching mosfet is connected to the bigger transformer. It has two secondary windings of many turns of thin wire which must develop the high voltage.


The resistances of the coils are:
Primary: ~0.1 ohm
First secondary: 1.629k ohm
Second secondary: 1.133k ohm

So it would make sense to me that this is the main inverter transformer and the smaller ones are used for some other purpose (feedback?).

 

[wiru]

I have found this schematic. It is not of the PSU that I have, but the CCFL connection seems to be very similar. As I can understand, these small transformers may be used to provide open lamp protection (? OLP1,2,3,4).

Here's an example:


I still do not understand how it is works though.