I am attenuating an audio frequency (50 KHz) signal that is pulsed at 666 W for 300us with a period of 300ms. I estimated the power dissipated averaged over a period to be approximately:
\$\frac{300 \times 10^{-6}}{350 \times 10^{-3}}\times 666 = 0.5709 \approx 600\text{mW}\$
In the past, I have had no issue with going smaller than the peak power I expect to see on the power rating for the resistor as long as the average over the (reasonably small) period was less than power rating for the resistor. The resistors warms up, but they are ok long term.
Now I am working with some mini circuits attenuators (VAT-XX) that are rated for 1W with various attenuations (1dB – 20dB). Running this signal through these attenuators results in "POP POP POP" and some dead attenuators. Before the failure, measuring the DC resistance gave some reasonable value around \$300\Omega\$ for the chain of attenuators. The DC resistance is now in the 10s of mega-ohms. The failure happened about 5 or 6 periods after I turned on the source and the attenuators did not heat up. The popping continued after the first pop and has a period that audibly matches the signal period.
I am skeptical that the failure is due to exceeding the power rating and I think that it is possible that the attenuators were damaged by the voltage during the pulses, which is ~850V peak-to-peak (perhaps the coax dialectric was broken down).
I would appreciate any helpful suggestions on why the attenuators failed.
Best Answer
You have exceeded the pulse power handling of the attenuators.
Some resistors, like bulk water, or wirewound, dissipate the heat in a very large volume of material, so can have very high pulse power ratings.
Some, like thin film, have a very small volume of resistive material deposited on a more-or-less thermal conductor. Unfortunately, the thin film requires heat to be conducted away fast enough to keep its temperature reasonable.
The difference between 666W and 600mW may be key to your problem. I often exceed average power, but by a factor of a few, or maybe 10. More than a ratio of 10, you really need to look into the pulse energy rating of your resistors. You are trying for a factor of 1000!
Now some attenuators, and I know this from when I blew them up as a junior engineer, have a profile that concentrates the heat in just one tiny part of the film, and they drop like flies with the merest pulse overload.
You need to multiply 666W with 300uS, and find an attenuator rated for that number of Joules, as well as the average power.