Any electronics boffins?

[Andy Clap]

I have a flat screen monitor with a burned out component on the PCB at the power input.
It's so badly charred I can't identify what it is yet alone rating.  I'm guessing a resistor.
A long shot I know, but can anybody confirm from the pic if it would be a resistor on the power input, and guess at a typical value and wattage?  The input is 12V dc, the component appears to bo on the input to a rectifier which I guess is dropping to 5V.



Cheers
Andy

[Andy Clap]

Just noticed the annotation F1.  I'd have expected this to be a fuse, but then why would it burn up instaed of blow?

[Andy Clap]

And I meant regulator, not rectifier. Doh!!  (where has the Modify post button gone?)

[bigralphie]

It looks like a vitreous enamel resistor as to value sorry no idea

[goldfazer]

Cut it out - there may be some colour underneath. If not, see if you can remove the burnt coating (may just be on top!) with some Jif/Cif/TCut/fine sandpaper to see the colours, or any other identifying marks.

[Andy Clap]

Cut it out - there may be some colour underneath.

Good idea mate, there was just enough left to identify it as a diode (though sadly not enough to rate it).
Guess it's to protect against using a PSU with the wrong polarity, in which case if I make sure I use the correct PSU I should be able to stick a bit of wire in place of the diode?!  I'll let you know if I get electrocuted or the house burns down!

[AdieR]

Diode may be reverse-polarity protection - but it may also be simply acting as a fuse. Its not something I'd replace with wire however.

It may be burned, but it might still be semi-functional - it can happen.

If you have a magnifying glass, you may be able to make out any identifying characters (eg 1N XXXX).

Final point: although you've identified the burned out diode, it'd be a good idea to make sure you've identified any underlying faults to prevent the new one going the same way.

[Andy Clap]

Diode may be reverse-polarity protection - but it may also be simply acting as a fuse. Its not something I'd replace with wire however.

It may be burned, but it might still be semi-functional - it can happen.

If you have a magnifying glass, you may be able to make out any identifying characters (eg 1N XXXX).

Final point: although you've identified the burned out diode, it'd be a good idea to make sure you've identified any underlying faults to prevent the new one going the same way.

Fuse - well that would explain the F1 annotation.
It was smoking with power applied and burned to a crisp, not funtioning at all I wouldn't think.  Fell apart as soon as I clipped the legs, only readable bit was "6KE".

[Major Rant]

Looks like a fuse to me ... Problem is that a fuse is either "there" or "not there" and in order to blow like that (obviously hot!) it must have passed a LOT of current in the few milliseconds before it blew. Which means that its a symptom of something else.

Expect something horrid downstream of that fuse.

Have a close look at all other components on the board with a magnifying glass. You might find something simple to replace and THEN replace the fuse.

OR ... if someone had reveresed the polarity of the power supply, or plugged in the wrong one by mistake - that might happen with no other defects.

[Captain Haddock]

If it's drawn lots of current replace the regulator as well as that should be the limiting factor, a diode makes sense for polarity protection, check the datasheet for the regulator and you'll know the max current that the diode should have to deal with.
Maybe the mains adaptor has gone wrong and put unregulated voltage to it.

[clayt74]

The 6KE series is a type of diode know as TVS (transient Voltage Surpressor) it protects the regulator and other sensitive circuits from high input voltage spikes. (the type of spikes normally of very short duration caused by high current switching or lightning strikes)
these diodes are designed to dissipate these spikes as heat, as yours is so badly burnt i would have to assume that what ever power supply you connected to  is wrong or faulty. it looks like a much higher supply than 12V has been applied to it.

RS sell the diodes here http://uk.rs-online.com/web/c/?searchTerm=6KE&sra=oss

good luck

[Andy Clap]

Thanks for all of the help guys.  I think there is discolouration at the regulator, but to be honest it's a multi-layer PCB and I don't have the equipment/skills/inclination to fix it, so I think I'll just bin it.


Cheers,
Andy

[Major Rant]

TVS (transient Voltage Surpressor)

He's probably right! ... I had one go short circuit in my central heating timer. Simply cutting the "dead thing" out of the circuit solved the problem.
(and of course replacing the fuse upstream that it had blown).

Timer still works 5 years later.

Worth a try I think.

[pointer2null]

Looked like the (remains) of a fuse to me - some of the on board ones look like a diode (and in some cheap kit they use a forward biased diode)

Bet you anything the regulator (3 legged thing next to it) is short.

[clayt74]

Looked like the (remains) of a fuse to me - some of the on board ones look like a diode (and in some cheap kit they use a forward biased diode)

Bet you anything the regulator (3 legged thing next to it) is short.

its definitely a TVS diode! and it is there to protect the internal circuits from external voltage spikes.
the TVS diode is rated at 600W approx 50A at 12V.
if the regulator had gone short the fuse in the EXTERNAL 12V supply would have blown (and poss the |Reg burnt out in the process)
the TVS has burnt due to too much EXTERNAL voltage applied.


check the external 12v supply is indeed still 12v.
then solder in a new 23p TVS diode and all is good!!!!
good luck