I need to determine what power rating a resistor has to be to adequately
dissipate 0.5J (a 1.2/50us surge of 500V from a 500 ohm source). Anyone have a
conversion formula?
Like Mike I'm puzzled by 0.5 J number as well.
The Exponential Impulse waveforms Annex of the ATIS 0600338 Electrical
Coordination of Primary and Secondary Surge Protection for Use in
Telecommunications Circuits states the i^2*t of an exponentially decaying
current impulse of amplitude I and 50% decay time Td is 0.72*I^2*Td. The
energy in a resistance R will be R*0.72*I^2*Td.
I assume you are trying to dimension the 500 ohm source resistor. A 500 ohm
resistor with a 1 A peak impulse current decaying in 50 µs will have an
energy dump of 500*1^2*50E-6 or 25 mJ. Although the peak VI power is 500 W the
average power at one impulse per minute is only 0.4 mW (25 mJ/ 60s) - so John
is right continuous power rating isn't important
If you are using a 1.2/50-8/20 combination wave generator then the impulse
waveshape with a 500 ohms load will be 1.2/50 (least with the generators Mike
was associated with)
I would look for some high voltage pulse resistors. For these sort of energy
and voltage levels I would start with thick film versions. Composite carbon
resistors do have good HV withstand (carbon film variants don't have good HV
withstand), but for the long term other technologies are preferable.
Mick Maytum
UK
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