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I have a friend who has brought me a circuit board from her flat screen TV. She assumed it was bad because the TV no longer turns on and there was off-white stuff leaking out of some of the components. When I got the board, the off-white stuff turned out to be that adhesive they use in asia to secure components, so I explained this to her.

But I did notice something strange about the board. On one of the components, the off-white glue was blackened as if it were burnt. The component in question was marked DSC 5D-15... which turns out the be an Inrush Current Limiting Thermistor.

Now, I would expect this thing to get kind of hot, (being a thermistor) and I understand that it provides a different amount of resistance depending on what temperature it is (being a thermistor). But should it really get hot enough to burn the glue?

I tried some tests on some glue I peeled off from another area of the board, and it took much more than the mean max operating temperature (280C) to make the glue burn or turn black.

She also reported that the TV wouldn't turn on until you repeatedly hit the power button... but after a few months of operation like this, it wouldn't turn on at all. I would assume that the action of repeatedly hitting the power button probably caused this inrush thermistor to stay hot and therefore, not protect anything any more.

I was hoping I could just replace the inrush thermistor (if it is damaged). But I suspect there is more damage than this. Currently, I only have the Power supply board.

So, my question is, should the thermistor get hot enough to burn asian component glue? And also, any suggestions on how I can test the thermistor to verify if it is working? And any other suggestions would be appreciated.

clabacchio
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Spencer Renosis
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  • Can you post a photo? – posipiet Jan 30 '12 at 01:54
  • I could, but not sure what you want to see. If you want to see the burnt glue, I had to scrape it off the component because it was partially covering the markings. The component itself looks pretty ordinary... (no deformation or anything). I have also removed it from the board, so I could verify the markings. – Spencer Renosis Jan 30 '12 at 02:24
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    This seems off-topic to me as it is debugging consumer electronics, not designing them, just trying to fix a TV. Many times before the point has been brought up that this site probably has some very qualified members for such a task but this does not change the goal of the site. – Kortuk Jan 30 '12 at 04:53
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    @Kortuk it's just an opinion, but these tasks could also help understanding how consumer electronics are made, and I think that also opening stuff and repairing can be "Engineering" activity...at least hacking – clabacchio Jan 30 '12 at 10:53
  • @clabacchio, this goes to other questions we have closed before also. A lot could be be learned by many in troubleshooting a software on the site. Think how much you could learn that would be applicable to writing firmware, it does not make it on topic. – Kortuk Jan 30 '12 at 11:34

1 Answers1

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The failure fairly likely is not related to the charred part.

Inrush thermistors could get very hot and having one charg glue woul not be a surprise BUT would probably be poor design of care-less manufacturing.

Having glue over a thermistor MAY reduce its heat dissipation and lead it to get even hotter than it was designed to - but having one exceed its max rated temperature would be no great surprise.

If you know what the thermistor is you may have a data sheet. But even without one -

  • Measure thermistor resistance in still air.

  • Place thermistor in hot air flow from a fan heater.
    Resistance should change markedly.

  • Remove from hot air flow - resistance should change back exponentially towards original value

  • When the thermistor is at room temperature, clenching it in your hand while measuring its resistance will probably produce a noticeable change in resistance.

I had a TV that behaved much as described. This was unrelated to an inrush thermistor. Some power supply capacitors went low capacitance with age. Regardless of why, replacing them fixed the problem. There is no good reason to think this is/was the problem with the set concerned - just a parallel example.

Russell McMahon
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