Zener across source resistor

Discussion in 'General Electronics Chat' started by inwo, Feb 23, 2014.

  1. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    2,433
    315
    Tv had a shorted MOSFET. 4.5amp/650 volt.

    Drain connected to B+ thru transformer primary.
    Source to 0V thru .33 ohm 1/2 watt.

    What purpose could a zener across this resistor (.33ohm) serve?

    It's broken beyond reading.
    I'm trying to find zener value for a replacement.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,507
    2,367
    A clamp for some reason, I guess you don't know the value at this point?
    I would think something very low.
    Where did you get the info it was a zener? Schematic?
    Max.
     
  3. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    2,433
    315
    I wish! No schematic.

    Marked ZD100 on board.

    I don't know if I broke it. Noticed it was broke when probing with "diode check" meter.

    Ohms law comes up with less than a volt using static values.:confused:

    I have more serious problems with it anyway.

    Making my 3rd parts order today.
    Hope I'm getting close.

    If anyone has info on LG 47LB5D or PS board info specifically it would help me out..........
     
  4. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    2,433
    315
    This should be close.........................................
     
  5. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    2,433
    315
    This is the tv from hell.

    Every time I replace the shorted parts, I find more parts shorted in another part of the circuit.

    I've had past experience of causing my own problems by damaging during testing under power. This seems not to be the case.:cool:

    If it wasn't my own I'd accuse cx of not telling me about a lightning hit.

    It's working now, but I don't trust it. Haven't found a root cause.

    Unless a bad PS diode led to the bad caps. Which led to more shorted diodes downstream along with there driver fets.

    Still haven't got an answer about a zener across a .33R resistor.

    The fet with the .33/zener combo on the source seems to run warm.:(

    Don't have a clue on finding correct zener value!:confused:
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Only a guess, but wouldn't the resistor/zener combination provide a RL time constant until the current through the resistor produced a voltage equal to the zener value, then the zener would carry the current.
     
  7. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    2,433
    315
    The way I see it, is that the voltage across a .33ohm would never reach even 1 volt.

    It's only a 4.5amp rated device.

    I'm hoping someone with inside knowledge of this tv notices my plight.

    I emailed LG service. Not holding my breath!

    Maybe something for the event of resistor failure?
     
  8. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    2,433
    315
    Tv from h*** is running.:D

    Ordered parts 3 times and got stuck when I could not find a H.F. power supply transformer.

    Ordered new board from three places online. All orders were canceled in a couple days. Not in stock!:mad:

    Sears had one. Order went fine until UPS handed it off to USPS in the wrong city.:mad:

    Call to Sears parts got it reshipped.:)

    It did eventually come thru.:)

    Sears parts= A+++ Contrary to online reviews.

    Price was more than China but much less than other USA mainline suppliers.
     
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