Re: [SI-LIST] : Heating resistor: RMS or average current (was Re: Effect of low Zo for unterminated lines)

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From: Larry Miller (ldmiller@nortelnetworks.com)
Date: Tue Oct 26 1999 - 06:17:26 PDT


RMS IS the heating effect in terms of watts; good RMS meters use bolometers
to measure it with. "Average" is the wrong answer.

If you want to know how hot a resistor will get you need a model of its
heat flow, conduction, and radiation. You have to come up with an
equivalent Theta(case-to-ambient). I think this kind of info is fairly
readily available for common resistor types and is easily measured with DC
in any case. Then the temperature rise is simply P(rms) *
Theta(case-to-ambient). Dimensional check: Watts * Deg/Watt = Deg.

Larry Miller

At 05:27 PM 10/25/99 -0400, you wrote:
>Raymond.Leung@qsa.idt.com wrote:
>>
>> D.C.,
>>
>> Your discussion remind me a while ago there was a discussion
>> whether the temperature effect of a heating resistor is proportional
>> to average current or rms one. Yet I can't remember if here was the
>> forum or not, please forgive my faint memory in this issue. However,
>> I still remember the answer was remained non-conclusive.
>>
>> As for me, I would stick to average current side :-).
>>
>> Raymond
>
>Well, from my (rusty) memory of a college course, one of the ways of
>measuring RMS current involved a resistor heating a thermosensitive
>coil connected to the meter needle. Whether it's the combination of
>the coil and the heating resistor that measures RMS, or the heating
>resistor alone, or just the relatively slow response of the meter to
>changes in current, I can't recall.
>
>I get the feeling I'm about to find out one of the many differences
>between college and reality. :-) The more, the merrier!
>
>BTW, thanks to all the chip designers and other employees of IC (etc.)
>vendors that contribute to the discussions here. Oh, and everyone
>else, too.
>
>Silly idea for a way of getting people to at least think
>about SI: Hamster Habitrails (long clear tubes for hamsters)
>as Transmission lines. Only problem, is that hamsters only
>reflect off the ends of closed transmission lines. And
>the rise times are kind of fuzzy.
>
>Later,
>--
>Laurence Michaels
>Simulation Support, General DataComm
>
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