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November 25th 03, 02:56 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

Depends on the resistor's construction. It is a rating that will be
published in the mfr's data sheets. Cement-coated resistors have a low
peak stress rating. Enamelled and welded wirewound, composition and
ceramic resistors can have high pulse power handling capabilities in a
single surge event that is characterised as being less than a half
cycle of the AC line frequency ( a common surge stress rating).

Toroids are basically thermally limited to their material construction
class. If wire doesn't fuse, the em force applied doesn't cause
mechanical damage and you don't care if volt-seconds are exceeded
(saturation), a 7mSec overwattage isn't going to hurt the part
permanently.

RL

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