Re: Who Has Used Resistors as Fuses
- From: bill.sloman@xxxxxxxx
- Date: Mon, 25 Jun 2007 06:50:40 -0700
On Jun 25, 1:00 pm, Jan Panteltje <pNaonStpealm...@xxxxxxxxx> wrote:
On a sunny day (Sun, 24 Jun 2007 17:12:29 -0700) it happened
bill.slo...@xxxxxxxx wrote in
<1182730349.111120.175...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>:
Just do not use metal film, I did, and it burned a hole in the peeseebee.
Carbon is OK.
But the energy dissipated in it when a sort circuit or overload
happens must be high enough to evaporate the carbon.
Carbon is not okay. It has a negative temperature coefficient of
resistance and if things go wrong, you can find your 10k carbon film
resistor carrying about an ampere at a voltage drop of a few volts or
less - all the current is flowing down a narrow, red-hot channel
through the carbon film
Not my experience,
I watched while my boss set up the situation and measured currents and
voltage myself once he'd got the resistor into the stae he wanted.
This was back in 1975, and the guy was an active member of the group
controlling the "intrinsic safety" rules for electronic equipment to
be used in areas wherre the was a risk of igniting inflammable gases
or liquids. I've forgotten exactly how he got the current to
concentrate itself into a red-hot filament in the carbon film - he had
a procedure that worked reliably, probably picked up at an instrinsic
safety committee meeting.
and anyone can get a couple of 10 Ohm 1/4 watt carbon
resistors and verify that on a LAB supply.
They can probably replicate your experience more easily than mine, but
the two situations aren't mutually exclusive.
Actually you can also test you strands of wire from a flatcable that way,
to see how many you need for say 3A (burn out).
Most metals have a postive temperature coefficient of resistance, so
this is irrelevant.
You are probably referring to composite carbon resistors, like 'Vitrohm'
used to make, they would open split apart, and perhaps occasionaly short.
No, it was a good quality Philips carbon fim resistor.
Normal carbon resistors these days have ceramic body with a spiral
carbon track around it.
The track width and number of turns sets resistance, and thickness.
In case of overload the track just opens.
Mostly. Raise the power dissipation at the right rate and you can
create and sustain a low reistiance hot channel through the carbon
film.
Metal fim and metal oxide resistors do at least have a positive
temperature coefficient of resistance,
Irrelevant, even if TC was 10% per degree C, it is going to blow up so who cares.
An incorrect and potentially dangerous misconception.
They are designed to run hot
when dissipated their rated load - somewhere around 250C - so if you do
want to use them as fuses, bend the leads so the resistor body sits a
couple of millimetres above the printed circuit board.
Sure, that is normal practice, and a metal film mounted like that burned
a hole though my peeseebee.
It wasn't normal practice anywhere I've worked - axial lead resistors
were supposed to contact the printed circuit board at two points. If
they were mounted out of contact with the board, they became
susceptible to vibration - with body of the resistor acting as a mass
on the end of the notionally springy metal leads - and people fussed
about eventual fatigue fractures in the leads.
--
Bill Sloman, Nijmegen
.
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