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PTC thermistor

Discussion in 'Electronic Components' started by David, Aug 13, 2006.

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  1. David

    David Guest

    A friend gave me a TV that would not switch on. The standby light comes on and there is a
    kind of rattling noise coming from the circuit board near the power switch. I noticed a PTC
    thermistor was very hot to the touch and was showing signs of burning underneath on the
    circuit board. Is it normal for a failed thermistor to make a noise?

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  2. Eeyore

    Eeyore Guest

    I guess it might do that. Sounds like the power-on degaussing circuit.

  3. Mr. J D

    Mr. J D Guest

    It could make a fizzing noise from burning compounds in the component.
  4. Franc Zabkar

    Franc Zabkar Guest

    Yes. If it is a 3-pin PTC resistor, it will have two discs which can
    develop poor contacts, after which they will rattle. This kind of
    problem usually takes out the fuse, though.

    - Franc Zabkar
  5. ian field

    ian field Guest

    There are 2 basic types of 3-pin degauss posistor and some care is required
    to obtain the correct part! The less common type actually contains 1x PTC &
    1x NTC, the NTC element is wired in series with the mains in just as you
    would expect an inrush limiting NTC to be - the danger arises when this type
    is mistakenly used to replace the more common 2x PTC, as the NTC will then
    be directly across the mains in and will go off with a loud bang!!!

    In both cases the thermistor disc not directly associated with the degauss
    coils is one way or another fed directly with mains current to heat the
    degauss posistor above its knee curve so no residual current flows in the
    coil and causes picture wobble. The type with an NTC disc is heated by
    passing the current drawn by the set and the PTC only type one of the 2 PTC
    elements is directly across the mains so the power this consumes keeps the
    degauss PTC heated.

    The less common PTC & NTC combined type can go O/C on the NTC element and
    cut off the supply to the set - or if its fizzing it may be passing too
    little or irregular current for the set to start - this is the only cause I
    can think of for STDBY light on but not starting other than another fault
    elsewhere in the set.

    With either type its a simple matter to unsolder the degauss unit and shake
    it next to your ear - if it rattles fit a replacement.
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