In article , Bill Turner
writes:
On 24 Nov 2003 22:50:40 -0800, (Jason Hsu) wrote:
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?
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You really need to ask the manufacturer of the resistor. They are well
aware of the problem - if you get to the right person.
Having said that, here is a generalization: If the resistor's element
is a solid block of material, such as in a carbon composition type, it
will have very good pulse power ratings. On the other hand, if the
element is a film, it may develop tiny hot spots during pulsing and
eventually fail.
Absolutely a good, pragmatic answer, Bill. MANY components can
survive a very short power overload. Carbon Comps do.
A "sacrifice" test can be done with a DC power supply, an electrolytic
capacitor, and a switch. Charge up the electrolytic, then dump the
charge through the resistor. It should be a trivial thing to calculate the
"pulse" of the initial charge-dump through the resistor for any under-
or post-grad engineering type...for a particular resistance, capacitor,
and voltage.
A rough time-constant of t = R x C yields the "pulse" time. Figure
the "pulse" power as half the capacitor charge voltage across
the resistor under test. Not precision, true, but pragmatic and
quick.
If it doesn't survive, well, too bad. [sacrifice]
If it survives, try it several more times, noting appearance, resistance
before and after each "pulse" test.
...or, everyone else can spend a lot of time at "intellectual arguments"
in here without ever trying it out on the bench... :-)
Len Anderson
retired (from regular hours) electronic engineer person