jimlux wrote:
> john@kk9a.com wrote:
>
>> I too have replaced many many potentiometers, hash filters and a few motors
>> on TIC ring rotators after lightning strikes. Is there a way to protect
>> them? Do MOVs protect the motors?
>>
>>
Yes and no. The selection of the proper value taking into account the
spikes created by a motor may make the clamping voltage too high to be
of much help.
Metal Oxide Varistors (MOVs) have a number of drawbacks. Where lightning
is concerned they are generally one shot devices. They also age. IOW
with minor conduction (voltage spikes that contain little power and are
only slightly overvoltage) the break down voltage will slowly get
lower. MOVs are a mechanical mix of tiny metal oxide diodes and some of
those diodes fail with each conduction. Another problem is motors
create spikes from the large inductance within, so it'd take a healthy
MOV to swallow those spikes without a premature failure. Also MOVs may
fail either open or shorted. I've seen both. GE used to have a very
comprehensive manual pertaining to the theory and use of MOVs. I've not
looked to see if it's on-line, but if any one finds it, the thing is
well worth reading. I have/had one of the original manuals around here,
but I'm not sure if I can find it.
73
Roger (K8RI)
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