This (below) agrees with what I have learned from talking to Bird
Electronics and Altronics Company. Thin Film resistors on ceramic tubes,
like used in some of their loads, are not tolerant of overvoltage, and
peak voltages much beyond their CW rated voltage can be destructive. So
its not just an issue of better cooling.
However, they must derate their loads somewhat to be able to quote things
like CW and AM power ratings that are similar, as AM would have a much
higher peak voltage during 100% modulation (or higher).
73
John
K5PRO
> Message: 3
> Date: Tue, 4 Apr 2006 18:56:29 -0700
> From: "Pfizenmayer" <pfizenmayer@worldnet.att.net>
> Subject: Re: [Amps] sealed air cooled dummy load in oil??
> You want to be careful of the maximum current and voltage - heat is not
> the
> only issue - I remember some "50KW" Bird water cooled loads - and 50 KW
> was
> it - If you ran 100 percent amplitude modulation at 50 KW the load would
> pop - Bird said it was a voltage issue.
>
> I once did some fusing current measurements on nichrome thin film
> resistors - various sheet resistivities - and you could see little
> "streamers" in the element before the element would fail .
>
> Anyhow - just getting the heat out may or may not allow the load to
> survive.
>
> FWIW -- Hank K7HP
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