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November 25th 03, 05:04 PM

(Jason Hsu) wrote in message om...
It's part of a design for a T/R sense circuit for a noise cancellation
device.

I won't bore you with too many details. One thing I noticed in a
design I'm looking at is that 1W resistors could be subject to as much
as 50W of power apiece during the time it takes for a relay to
respond. This response time is 7msec. The overall duty cycle will be
low (well under 1%).

Can the 1W-51 ohm resistors handle this 50 RF volts 0-peak (about 50W
PEP) for .007 sec? 50W over .007 seconds is .35 Joules. .35W for 1
second is also .35 Joules, which a 1W resistor should have no trouble
handling. Can the resistors be damaged during that .007 seconds?

Also, how much time does it take to damage a toroid? If it can handle
X units of flux density continuously, how much flux density can it
handle for .007 seconds with a low overall duty cycle (like well under
1%)?

Jason Hsu, AG4DG
usenet AAAAATTTTT jasonhsu.com

Resistor pulse ratings can be difficult to obtain from manufacturers.
As stated by another poster, peak power ratings on datasheets are
often about 10x continuous, sometimes even lower. For high value
resistors, the voltage across the resistor may be the limiting factor.
Anyhow, I've destructively tested lots of low value resistors. For
some intermediate range of pulse durations, their failure point tends
to follow the famous 'I-squared-t equals K' curve, which describes the
ideal tradeoff between time and power level. At some short pulse
duration, the failure threshold becomes roughly constant. The
resistor reaches a power density that causes rapid destruction of the
conductor. For wirewound and metal oxide resistors, this power level
can be 100s of times the average power rating. For carbon composition
resistors, 50x may or may not be destructive. Carbon film and thick
film resistors have failure points that vary all over the map. This
is partly due to the way that they are trimmed: scribing or laser
cutting produces a narrow area in the resistive element, which
concentrates power in a small zone. Failure is usually due to
cracking of the element as it expands more rapidly than the substrate.
This is why pulse ratings for these type resistors are usually very
low multiples of their continuous rating. So, it depends.
Paul Mathews