Dave Wilson writes:
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?
Hi Dave:
I'm a little late responding and I see that you have already received some
good advice.
The class of resistors that you need to focus on is typically called "pulse
withstanding resistors." Several of the major manufacturers of resistors,
such as IRC and RCD, have product lines of this type. It is very important to
carefully review the resistor specifications for a pulse-withstand
application. Any "real" pulse-withstand resistor will be well specified for a
variety of impulses, often including the specific waveform you mention.
There is a brief discussion about sizing resistors for pulse withstand in my
application note AN-6 that is posted on my web site, on the subject of
lightning protection.
You should be aware that there are several different technologies used to
implement pulse-withstand resistors, and their performance characteristics
vary widely. Generally, wire wound resistors provide good performance for the
cost and size, but they have an inductive characteristic that makes them
unsuitable for certain applications, such as the source impedance in a surge
generator. On the other hand, this inductance is usually not an issue when
used as part of a surge protection network.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com <http://www.randolph-telecom.com/>
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