Al wrote:

In article ,
Bill Turner wrote:


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?

________________________________________________ _________

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.



Many years ago I analyzed a problem with resistors which were
discoloring in the field. This involved two identical circuits with
identical singal inputs. In one circuit, a resistor kept turning brown
over time, in the other it did not. We were getting field returns as a
result of troubleshooting that focused on the brown resistors.

What was the difference? The resistor which did not discolor was resting
on two metal traces which ran under it, the one which discolored, was
resting on the PCB. That tiny amount of heatsinking provided by the
traces was the difference. Obviously, the circuit needed to be
redesigned, but it worked great on the breadboard when it went into
production.

The resistor should never have been run at that close to its maximum
dissipation to begin with. That's a lack of what they call a
conservative design.

Al



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