# How long does a resistor last?

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When run at its rated power?

I'm referring to 1/4 - 10 watt resistors in TVs and PC power supplies,
probably in 30-50 Celcius surrounding air.

I thought that resistors were supposed to be chosen for twice the
actual power, but I've seen many 0.5W to 1.0W resistors run at almost
exactly their rated power continuously.

Re: How long does a resistor last?

How long is a piece of string? I think you need to rethink what you're
asking. More to endurance than duration.

Re: How long does a resistor last?

Lostgallifreyan wrote:

What additional information should I specify, other than rated power,
actual power, and ambient temperature?  I'm not dropping high
voltages.

Re: How long does a resistor last?

Materials. What I was getting at is that resistors aren't always nicely
behaved metal/ceramic thingers, and carbonisation of an epoxy or urethane or
other coating might affect a high-value resistance enough to consider it
failed even though the resistive element is fine. Basically it can get
complicated, so you need to think of the specific conditions to eliminate
guesswork.

Another way of looking at it is, if it's a ceramic/metal type and isn't
glowing with heat, or melting solder if mounted that way, then it's likely to
outlast everything else in the circuit and is therefore the least of
concerns...

Re: How long does a resistor last?

On Fri, 9 Oct 2009 02:15:50 -0700 (PDT), "larry moe 'n curly"

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That's because it costs more to use a larger resistor than is necessary.

Let's say that you can get 1/2 watt resistors for a penny in quantities
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Re: How long does a resistor last?

Designing for longevity is a thing of the past. Making a buck is. Most
problems in electronics is caused by under rated components (not enough
power or voltage headroom). Caps are a big culprit.

Re: How long does a resistor last?

..only if you are stupid enough to not increase the sale price of the
widget by 50 cents per one watt resistor used.

Re: How long does a resistor last?

On Fri, 09 Oct 2009 21:40:42 -0700, Robert Baer

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Then your widget will cost more than an identical widget designed to be

Re: How long does a resistor last?

John Fields wrote:

--
The movie 'Deliverance' isn't a documentary!

Re: How long does a resistor last?

The life at rated wattage will depend on many factors.  The legs of
the resistor will conduct the heat away.  How much copper they hook to
will control how well this works.  The brand of the resistor also
matters.  One from Uncle Chin's Resistor Company will last much longer
than those from the Shaw Dee Resistor Factory because Uncle Chin uses
better ceramic.

If you use that urethane conformal coating that the US navy seems to
like so much, it will turn dark and look very ugly long before the
resistor fails.  The ceramic materials will do their best when clean.

Re: How long does a resistor last?

On Fri, 9 Oct 2009 02:15:50 -0700 (PDT), "larry moe 'n curly"

Depends on what it's made of and how well it's cooled. We use 0603
surface-mount resistors at half a watt, because we heat sink them
well. Half-watt carbon resistors will die at half a watt in a confined
space.

Enameled wirewounds are very tough, up until the enamel melts.

Some resistors will die from temperture cycling stress.

But in general it's good to derate 0.5 maybe.

John

Re: How long does a resistor last?

There is another dimension to consider: duty cycle.  I've been told
(but don't have first hand experience) that old fashioned carbon
carbon film resistors.

--
These are my opinions, not necessarily my employer's.  I hate spam.

Re: How long does a resistor last?

hal-usenet@ip-64-139-1-69.sjc.megapath.net (Hal Murray) wrote in

They're also a lot better at high voltage. I'm just mentioning this because
when I used some for a HeNe supply current limit resistor, they really
impressed me. Most other things I tried eventually burned. Basically, a solid
mass is better than a film for HV, it seems.

Re: How long does a resistor last?

Except for the horrible voltage coefficient.  Carbon comps' resistances
decrease by about a quarter at their rated voltage.  That's okay for a
HeNe, since the tube's load line is pretty steep.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
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Re: How long does a resistor last?

Well that's cool cos that's about all I ever use them for. :) But they were
the perfect answer in this singular case.

Re: How long does a resistor last?

Veering swiftly offtopic... this interests me because people often rave about
the sound of valve amps. I don't mean audiophool ravings, I mean those who
like the effect especially in overdriven amps for bass guitar and other
instruments where the sound is appropriate, and very good. It's normally
attributed to the harmonic distortion and saturation effects in the valves,
but now I'm wondering how much of it might be due to that voltage coefficient
in carbon resistors. Maybe designers deliberately tried to reduce the effect
by careful biasing, or maybe there is a strong effect that is overlooked, I
really have no idea...

Comments welcome, so long as they come from either experience or interest in
this subject.

Re: How long does a resistor last?

Noise?

It's well known that adding rumble and pop noise to a recording makes
it sound more appealing to the phonograph crowd.  And apparently makes
them angry when you tell them that's what you've done.

You aren't going to see much nonlinearity, and resistors certainly
weren't well picked -- 470k is close enough, and with 20% resistors,
who cares anyway.  What scares me more is, for instance, how a common
Magnavox 6V6 PP amp uses 0.047uF for the coupling cap to one 6V6, and
0.0047 for the other.  And the phase splitter doesn't split anything,
it's just a cascaded stage.  The only balancing is due to a voltage
divider.  Ewwww.  I'd be willing to bet they did that intentionally,
since LF distortion makes phantom bass.  They used teensy output
transformers, too.

Tim

Re: How long does a resistor last?

Proven!  I was using some 100 Ohm resistors to limit current in a
spark-gap ignitor for mercury short-arc lamps.  The film resistor just
BLEW off its resistance film on the first shot.  I mumbled "oh yeah, I
KNEW it was going to do that" and replaced with a bulk carbon resistor,
and that held up for the life of the unit.  You can guess, the pulse
current was 20,000/100 or about 100 A.  200 A ^ 2 * 100 = 4 mega-watts
for a couple microseconds.

Jon

Re: How long does a resistor last?

Resistors can last for well over 100 years...

Re: How long does a resistor last?