tv power supply caught fire! why?

[philliesfan30m]

can someone tell me why this happened? i tried the hairdryer trick on the main cap it wasn't close to the board and it wasn't on the cap that long but 2 surface mount resistors .47ohms in parallel from the positive of the main cap to the drain of the transistor caught fire and burnt up also taking out the transistor next to the rectifier on the heat sink. anyone else ever have this happen? did it heat up the cap too much and raise the capacitance and blew the transistor? or am i just an idiot and did something wrong?

[budm]

Re: tv power supply caught fire! why?

Bad design, those should be Flame proof (MOX -Metal Oxide type), they are being used as fuse (Fusible resistor) in case the MOSFET
shorted out. May be the SMPS IC that drives the Gate has output stuck high to fully turn on the MOSFET in stead of switching ON and OFF.

[Kiriakos GR]

Re: tv power supply caught fire! why?

but 2 surface mount resistors .47ohms in parallel from the positive of the main cap to the drain of the transistor caught fire and burnt up

What happened is that by heating up the board the circuitry started needing higher current than usual.
And those resistors acted as fuses.

[philliesfan30m]

Re: tv power supply caught fire! why?

what could i replace those 2 resistors with? i don't have any .47ohm surface mount resistors can i swap them out with something else i have on another board? and if so how close do they have to be?

[Agent24]

Re: tv power supply caught fire! why?

Depends what they are for, but in general no, you will need to get new 0.47 ohm resistors. I'm sure you could buy them easy, eBay should have them if you can't find them anywhere else.

[senz_90]

Re: tv power supply caught fire! why?

What happened is that by heating up the board the circuitry started needing higher current than usual.
And those resistors acted as fuses.

would you mind to explained more specific why the current need higher when hot? thanks. i have never hear that and i want to learn. The ESR of caps could decreased to low as I know when i test it on failed caps with hairdryer.

[Agent24]

Re: tv power supply caught fire! why?

would you mind to explained more specific why the current need higher when hot? thanks. i have never hear that and i want to learn. The ESR of caps could decreased to low as I know when i test it on failed caps with hairdryer.

When components heat up, their parameters can change. An extreme example of this is an incandescent light bulb. When the filiament is cold, it has a very low resistance. As it heats up, the resistance increases to a higher value (For tungsten, about 15 times higher) and the current draw drops to a lower level. This is why lightbulbs often blow when you just switch them on, rather than during use. (high inrush current when cold)

Different components are affected in different ways. Some semiconductors have a lower resistance as they get hotter. If there is no feedback\compensation for this, they can go into a thermal runaway and blow up. I think Kiriakos GR is saying this may have been the case here.

[senz_90]

Re: tv power supply caught fire! why?

So it means the electronic circuit have a thermal dependent factor? that is always happened or just in case the components is faulty?

[Agent24]

Re: tv power supply caught fire! why?

Any circuit will change operation slightly with variances in temperature, but not usually to the point they would blow up.. that would either be bad design, faulty components, or some abnormally large temperature change.

That said, I am not an expert... Perhaps for philliesfan30m he did overheat and over-stressed an already faulty or marginal component. Or maybe it was just coincidence. I don't know.

[tom66]

Re: tv power supply caught fire! why?

Almost every component is thermally dependent. For example diodes typically change forward voltage by about -2.1mV per degree Celsius. That means a diode running 50°C hotter will have a forward voltage 0.105V lower than normal. (The normal measurement is made at 25°C, or room temperature, typically.)

Resistors have a temperature coefficient of around 1ppm/°C to 1,000ppm/°C. 1ppm is 0.0001%. A typical resistor might be 50ppm/°C. This means a resistor running 50°C hotter will measure ±0.25% from normal measurement. This may be negligible in most circuits but can cause issues in high precision stuff such as multimeters, voltage references, oscilloscopes, etc.

Heating components up can also lead to thermal runaway, causing failure.

[senz_90]

Re: tv power supply caught fire! why?

Thank you !! since Im still learn deeper about electronics this would make easier learn further. So sorry for other topic that I made, back to the thread !!