On Fri, 4 Sep 1998, w8ji.tom wrote:
(among other things)
> I found that carbon film and metal film resistors, no matter what the size,
> failed almost every time a storm occurred. They fail even when the
> lightning is cloud to cloud, or cloud to ground miles away.
>
> What I did after changing resistors a few times was go to carbon
> compositions. They are made almost exclusively by Allen Bradly, and are
> special order parts. They are generally used in parasitic suppressors and
> surge suppression applications. These resistors, when cut open, have a
> solid carbon core instead of the thin carbon or metal spiral all other
> resistors have. I have yet to lose a carbon comp, even after direct strikes
> on my towers. On the other hand 5 watt metal films and regular two watt
> carbons opened up like clockwork without nearby strikes.
Carbon composition resistors are well-known for their ability to handle
pulse overloads. The "bulk" core that forms the resistance has enough
thermal mass to avoid destructive hot-spotting typical of the film
resistors. I specify them in ECG protection circuits because of their
ability to absorb and dissipate the intense pulse overloads associated
with cardiac defibrillation into sensitive front ends. They can do this
over and over without deteriorating (within limits, of course) as the MOVs
do over time. For very short pulses, the power dissipation can be as high
as 10-times the steady-state rating without damage.
These resistors, once the mainstay of the industry, are becoming
increasingly difficult to obtain as the film and SMT devices become the
standard. Drawbacks to carbon comps include sensitivity to moisture and
higher intrinsic noise (not noticeable at HF). Carbon comps are also
non-inductive (lead inductance not included) to very high frequencies.
Anywhere you expect to get short periods of overload, such as protective
circuits, static dissipators, and so on, the resistor of choice should be
a carbon composition.
73, Ward N0AX
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