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Resistor with very short, high power pulse will overheat?

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treez

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The datasheet for the SG73P2E 47R, 1210 resistor says that over a 1us interval, it should dissipate no more than 900W…

SG73P2E pulse rated resistor datasheet:
https://www.koaspeer.com/products/resistors/surface-mount-resistors/sg73p/

…That is, an energy of 900 uJ.

Now suppose that it dissipates 900W for 1us, then dissipates nothing for the next 99999us
…that means the resistor dissipated an effective power of just 9mW over that 0.1second.

…that’s a ridiculously low power level. –So how can they say that the power must be limited to just 900W over a 1us time interval?
I cannot believe that the resistor is going to heat up to a damaging temperature in 1us? Would it? How hot would it get?
 

In "short time" you can consider that the energy left
in the resistor film (which is not the whole body) is all
of the pulse energy and none escapes by conduction
(the adiabatic approximation). In this case you have a
power-to-blow which is greater than steady state, but
does not come close to duty cycle averaged (this
assumes conduction balances input power, which at
short pulses is not true).

You can do the calculations if you can get manufacturers'
information on the volume of the resistive material and
the temperature at which Bad Things Might Happen.
It could be that a thin (or thick) film resistor simply
evaporates and entrains to the the encapsulating layers
(as integrated thin film NiCr resistors do), it might be
reaching the material liquidus such that the "fuse"
just melts and pulls open, it could be some metallurgy
thing that simply results in an unacceptable (to some
criterion) drift in value or stability.

Joule energy of the pulse, resistor active volume,
material density and specific heat are all you need
to figure the temperature rise. The "fail" temperature,
the criteria and so on reside (if anywhere) in the bowels
of the manufacturer's reliability organization. Probably
all sandbagged to hell, in the spec you got.

But it's a real thing and needs to be respected /
understood / quantified.
 
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I agree with dick_freebird that there's no alternative than follow the specification in the first place. If you don't like the specification, use a different resistor with dedicated pulse rating.

There are different physical laws determining the pulse rating of a resistor. You can guess that the constant power part of the pulse rating curve below 10 µs is due to incomplete characterization rather than being validated. But that's only one of several possible interpretations.
 

A little bit of test-to-fail would tell you a lot.

It might be that the mfr's applications folks have
data in pocket if you acted like a customer and
asked them all friendly-like.
 

Thanks yes i see your points.
Its just that when you explain to your people that you cannot use this resistor becuase it dissipated 1200W for 1us, and then virtually zero watts thereafter, they look at you as if you are mad.
I presumed that it is the heating due to the pulse that was the sole problem.....but as dick_freebird says, there are metallurgical aspects. I suppose the dreaded "electron migration" also comes into play?

- - - Updated - - -

This pulse rated family of resistors is "thick film"...i presume "thick film", as opposed to "thin film" means it can better withstand high power pulses? The resistor contains no bond wires so i presume the thickness of the film gives it the higher pulse rating?

SG73 series pulse rated resistors
https://www.farnell.com/datasheets/66099.pdf
 

Film thickness goes to film volume and mass, so a
higher pulse energy for same temp rise "in the
moment".

Of course there may also be differences to film
composition (max tolerable temp) and these may
be deliberately optimized if the product's claim
to fame is pulse power handling.
 

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