Resistor Power Rating Spec

[Jim]

I need to spec a 1 Ohm resistor to handle 4.5 Amps.
Ordinarily,
(4.5^2)*1 = 20.25, and I'd spec a resistor with the appropriate power rating
depending on the environmental conditions.
In this case, the current will only last a fraction of a second (0.1 s, max)
and will not be repeated for a while (probably 5-10 minutes at the
absolutely fastest). All of this will be occurring in an air conditioned
environment.
Can I get away with specifying a lower power rating? I'd be concerned
going with 1/8W, but I'd think that 2-3W should be able to handle it (but
they may get a bit warm).

TIA,
Jim

 

[John Popelish]

Jim wrote:

I need to spec a 1 Ohm resistor to handle 4.5 Amps.
Ordinarily,
(4.5^2)*1 = 20.25, and I'd spec a resistor with the appropriate power rating
depending on the environmental conditions.
In this case, the current will only last a fraction of a second (0.1 s, max)
and will not be repeated for a while (probably 5-10 minutes at the
absolutely fastest). All of this will be occurring in an air conditioned
environment.
Can I get away with specifying a lower power rating? I'd be concerned
going with 1/8W, but I'd think that 2-3W should be able to handle it (but
they may get a bit warm).

TIA,
Jim

A 2 watt should do fine, but get a wire wound, not a metal film.
These have more resistor metal mass to soak up the heat pulse.
--
John Popelish

 

[Joe McElvenney]

Hi,

And mount it with plenty of air all around so that if the
system latches-up it won't set fire to anything. I used to employ
a similar trick (high peak power but a low wattage resistor) to
delay the closing of some relay contacts on a largish power
supply and found that they made wonderful fuses.


Cheers - Joe

 

[DarkMatter]

On Fri, 18 Jul 2003 15:58:39 -0400, "Jim" <gt0975b@prism.gatech.edu>
Gave us:

I need to spec a 1 Ohm resistor to handle 4.5 Amps.
Ordinarily,
(4.5^2)*1 = 20.25, and I'd spec a resistor with the appropriate power rating
depending on the environmental conditions.
In this case, the current will only last a fraction of a second (0.1 s, max)
and will not be repeated for a while (probably 5-10 minutes at the
absolutely fastest). All of this will be occurring in an air conditioned
environment.
Can I get away with specifying a lower power rating? I'd be concerned
going with 1/8W, but I'd think that 2-3W should be able to handle it (but
they may get a bit warm).

TIA,
Jim

At that duty cycling of 0.1 sec with a 5 minute rest in open air,

even a 2 watt job would probably work.The work period and rest period
are the key. That is a 0.33 percent duty cycle! :-] The very short
duration on cycle would almost certainly work into a 2 watt Metal
Oxide or a wirewound version resistor. The lead on those devices can
handle it, and I'd be willing to bet the device can without excessive
or any abnormal heating to speak of. Still, if there is any chance of
the on period getting stuck on, or such, I'd go at least 5 watts, if
not the full monty... err watty. :-]

 

[meirman]

In sci.electronics.basics on Fri, 18 Jul 2003 15:58:39 -0400 "Jim"
<gt0975b@prism.gatech.edu> posted:

I need to spec a 1 Ohm resistor to handle 4.5 Amps.
Ordinarily,
(4.5^2)*1 = 20.25, and I'd spec a resistor with the appropriate power rating
depending on the environmental conditions.
In this case, the current will only last a fraction of a second (0.1 s, max)
and will not be repeated for a while (probably 5-10 minutes at the
absolutely fastest). All of this will be occurring in an air conditioned
environment.

I can't help you with the rest, but in the scheme of things, I don't
think it matters much that it is AC. The difference between 90 and
70F or even 110 and 70 seems like a lot to us because it's the
boundary between comfortable and not. But I think 20 or 40 degrees is
not so great when compared with the melting or burning temperature of
most electrical components. Well, maybe AC would matter here???, I
don't know, but I just wanted to raise the point in general.

Can I get away with specifying a lower power rating? I'd be concerned
going with 1/8W, but I'd think that 2-3W should be able to handle it (but
they may get a bit warm).

Try it a few dozen times and then see how hot it gets. First hold
your fingers near it. If it's not radiating too much heat, then lick
your fingers and be quick, like people do with a clothes iron, so you
won't burn yourself.

TIA,
Jim

Meirman

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