In-rush resistor failed

Sat Sep 24, 2011 3:16 am   

Hi Trevor,

I've been busy elsewhere, so away from the computer.

Thanks for the heads up re the fusible nature of any replacement
resistor for the in-rush current resistor.

The original resistor was 2W wirewound, and the replacement is 5W
wirewound. WES doesn't actually have a branch in Adelaide, but the WES
closest replacement in wirewound is 3W.

My thoughts are that if I put a 2.2 ohm 5W wirewound resistor across
the mains (as in assuming all other series components in the power
supply failed short circuited) then it would be momentarily
dissipating 24KW and it wouldn't last long before it vapourised. I
concede it might last a tiny bit longer than a 2W wirewound resistor,
but my thoughts are the difference would not be significant
practically speaking.

The remote control for the LG DVD/VCR combo was literally removed from
the council wheelie bin, and the combo unit itself was already on the
council hard waste pile, so just to have it working again for however
long it might work is a big plus for the owner in any case.

If (say) the electrolytic capacitor fails catastrphically short
circuit and the 5w wirewound resistor fails too slowly then I guess
the unit will just go back on the hard waste pile, so no great loss
there.

Those are my thoughts anyway. I don't see it as being a fire hazard in
catastrophic failure mode with the 5W wirewound resistor, since the
case is metal anyway.

As a more general comment I last worked as an "on the ground"
electronic design engineer back in the middle 70's. PC boards back
then had pretty wide track widths and multilayer cards were the big
thing of the time. An 8080 processor was the latest LSI of the time.

Looking at the build quality and line quality of the PC cards etc
inside this LG unit I have to say I think it is just a miracle of cost
engineering to sell something of this quality so cheaply.

If by some miracle of time travel we could have seen this unit 30
years ago we would have been in awe at both the achievement and the
reliability of current electronics.

We've come a very long way in a such short time.

Ross

Sat Sep 24, 2011 3:54 am   

On Sat, 24 Sep 2011 03:16:15 GMT, rmd_at_invalid.invalid (RMD) wrote:
Oh, and just by the way, I remember when metal film resistors first
appeared on the market. They replaced carbon resistors which weren't
very reliable at all and quite given to going higher in value.

Anyway, some co-workers of the time were quite interested in seeing
how good these new metal film resistors were.

So they overloaded these resistors so much they were eventually just a
smoking black ruin. However these darn resistors still measured
exactly the same as when they were undamaged.

Metal film resistors are pretty tough guys!

Ross

Sat Sep 24, 2011 3:55 am   

"RMD"


** Resistor wattage ratings and their ability to withstand large but brief
overloads are NOT related. Even WW resistors vary enormously in design and
things like the gauge of wire used for an given value and wattage rating.

Reason being, there are a large number of different resistance metals that
could be employed inside.



** It will blow the AC fuse in the item - or trip the AC supply breaker.



** Far more common is failure of the switching device or diodes in the
bridge.



** It will be quick.



** WW resistors are the most common used for inrush surge limiting - they
usually fail instantly and with no visible sign.

So called " fusible " resistors are really just flame proof types, when
overloaded they glow red and emit some smoke but do not catch fire
THEMSELVES.

See pic here ( taken by me).

http://sound.westhost.com/articles/incandescent.htm#norm


.... Phil

Sat Sep 24, 2011 4:42 am   

"Phil Allison" <phil_a_at_tpg.com.au> wrote in message
news:9e52ksFq8qU1_at_mid.individual.net...


There's a wealth of what look like good quality articles & info tucked away
there as well. cheers.

http://www.sound.westhost.com/articles.htm

Sat Sep 24, 2011 5:14 am   

Hi Phil,

<<<<** WW resistors are the most common used for inrush surge limiting
- they usually fail instantly and with no visible sign.>>>>


I'd have to say that I've had various electrical things failing on me
for nearly 50 odd years.

Usually the worst I've seen is a dark brown smell, or maybe a burn
mark, but mostly things just fail silently and with no obvious damage.

While catastrophe can happen, it is rare. Luckily for all of us! :)

Ross

Sat Sep 24, 2011 5:31 am   

"RMD"


** Wish I could say the same.

Burning resistors are a real hazard, they can easily set PCB material alight
and that means black soot all over the inside of things. Carbon soot is
highly corrosive and takes the plating of the chassis and any metal parts it
touches.

Another real nasty is when a big electro decides to bleed its juice all over
the PCB. That stuff is corrosive and conductive enough to start a fire if
enough energy is available.

Then there are items that have had drinks ( beer, OJ or Coke etc) spilt over
them - I even saw one case where someone had puked on the front of a Disco
mixer. It stank.


.... Phil