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'[EE] Resistor power averaging'
2011\03\21@010049 by

I have an application which will be dissipating a significant amount of power through a resistor - for a very short period.  Say a dozen or so microseconds.

Assuming I don't exceed the current specification, how long of a period can I average the power dissipation over?  Is there a way to calculate this?  I realize that it will probably depend on the amount of 'overpowering' I'm doing...

Mainly I want to be sure I'm not going to cook a resistor by dissipating quite a bit of power through it for 20-30us - and have no idea where even to start to validate this from a mathematical/engineering standpoint.

-forrest
On Mon, March 21, 2011 12:02 am, Forrest W Christian wrote:
> I have an application which will be dissipating a significant amount of
> power through a resistor - for a very short period.  Say a dozen or so
> microseconds.
>
> Assuming I don't exceed the current specification, how long of a period
> can I average the power dissipation over?  Is there a way to calculate
> this?  I realize that it will probably depend on the amount of
> 'overpowering' I'm doing...
>
> Mainly I want to be sure I'm not going to cook a resistor by dissipating
> quite a bit of power through it for 20-30us - and have no idea where
> even to start to validate this from a mathematical/engineering standpoint..

This varies a lot depending on the type of resistor you are using.  Most
through hole and SMT resistors you find will be thin film or metal film
resistors- where there is a layer of resistive material that is laser
trimmed to the proper value.  I've had the best luck with carbon
composition resistors in a pulsed power application, but they are a lot
more expensive. The trimmed resistors usually include a very small gap
that, in high voltage pulse, has a high potential difference and makes the
resistor prone to arcing. This is one of the places where you will need to
carefully read the datasheets and look for peak current ratings. Most
resistors are designed/tested for just plain dissipation of power over
time with a DC-ish load without really pushing the voltage.

Regards,

Matt

Matt Bennett
Just outside of Austin, TX
30.5,-97.9

The views I express are my own, not that of my employer, a large
multinational corporation that you are familiar with
Ken: Very short large current pulse handling in resistors.

> This is one of the places where you will need to
> carefully read the datasheets and look for peak current ratings.

Philps of yore used to produce reports and even data sheets on such things.
Much such may be on web.

For newer stuff a web search should help

Riedon want to be your friend (but want you to use wire wounds)
The paper is worth reading as they discuss why the various ratings
exist and provide graphs.
Common sense MAY almost allow extension to other technologies with due care..
(Caveat Incinerator)

http://www.riedon.com/us/technical-information/application-notes/pulse-rating.html

As do TT

Including

The current limit is defined by the current density,
and 100 A per mm2
are considered absolute
maximum in power electronics. With a 0,8 mm
diameter copper wire, the current limit is 50 A.
With respect to reliability, the welding junction
between copper and resistive alloy should not carry
more than 20 A continuously.
The 50 A limit may not be exceeded even under
pulse condition

And

This limits also the pulse handling capacity.
50A can be carried by the internal contacts, but energy
must not exceed 625 mWs

http://www.vitrohm.com/files_stat/design_notes.pd

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