> I'm a little confused about lightning suppressor theory.
>
>
>
> Many of us have installed lightning suppressors from Polyphaser, ICE,
> K5FD,
> etc. to protect control lines running from towers to the shack. The
> Polyphaser units contain MOVs that turn on at 82V. ICE offers two MOV
I don't have my GE manual at hand but IIRC:
Something to remember about Metal Oxide Varistors. They are a whole
conglomeration of diodes mechanically mixed and packed together. Unlike
Zener diodes that have a more or less set voltage, they do as you say, start
to conduct. Normally when an MOV actually does something they receive
*some* damage so they tend to age over time, depending on how hard and often
they are used. If they take a healthy whack from a nearby strike (or closer)
they may end up a one-time-use. Polyphasers and the GE MOVs are like this.
I'm not familiar with the other brands.
> voltage levels, 50V and 20V. K5FD offers three voltage levels: 65V
> (standard), 35V and 17V. I have the Polyphaser and K5FD standard units in
> my
> suppressor network.
>
>
>
> So, if components in a control box are connected to these suppressors, the
> voltage on the pins can rise as high as 65V-82V with respect to ground
It's the job of the suppressors in this case to bypass most of the voltage
spike while absorbing the energy within it, before it gets to the unit
connected to it. It may or may not do anything for other things in the
shack. If you are using DC the MOV can be as little as 20% above the
regularly applied voltage. With AC you need at least 1.414 times the RMS
value plus about 20%. Each conduction will tend to lower the starting
voltage a little (or a lot) depending on how hard it has to work. So you end
up having to decide just how much slack (the 20% in this case), you are
willing to give between your operating voltage and what will do damage. As
far as a direct hit, the MOV is usually replaced by a scorch mark and a pair
of bare wire leads. I've been within 6 feet of one rated at 50,000 jouls
when it left leaving a pair of #8 wires sticking straight out. It's quite
spectacular, but a bit hard on the ears...and nerves... and attached SCRs.
> before the suppressor starts to do its job. Some bipolar transistors can
> take that kind of voltage but many FETs and ICs can't. While the
> suppressors
Also that rated voltage is the knee of the curve. It isn't a cut off as with
a Zener diode. The MOV is rated in volts and jouls or the amount of energy
it can absorb. As the voltage continues to rise the resistance basically
goes down until the MOV is destroyed. Most, but not all I have seen fail did
so shorted. OTOH a few did fail open (not counting the ones where only the
leads were left)
If the strike is close enough (not necessiarily a direct hit) to impart
enough energy the MOVs will probably do little to save the equipment, but
they do increase your odds from lesser strikes.
> probably keep things from melting and vaporizing, isn't it probable that
> semiconductors attached to the line will be damaged?
>
That's one of those, "It all depends" but in general the answer is probably
...Yup!
However, I do not use them in the lines to the rotator and with all the
verified hits since it's been up, there has been no damage to any rotator or
control box due to lightning. I should note that more than likely that tower
has taken more direct hits since it's been up than most ham's towers do
druing the hams entire life. I did have one Polyphaser short out (in the
coax from the 440 array) and I had one F to F N-connector fail at the 75
meter half wave sloper.
>
>
> I ask because we took a massive lightning hit here about three weeks ago
> that damaged at least a dozen boxes in my shack. I've got an extensive
> ground system and suppressor network. The only way I can explain some of
> the
> damage is that the suppressors allowed the voltage to rise too high for
> the
> semiconductors connected directly to the control lines. Does this make
> sense?
MOVs are not infallable.
>
>
>
> I'll post a complete report on the damage, which was extensive, my theory
> on
> the path taken by the surge(s) and the recovery when I get a chance. Too
> busy fixing things right now.
>
>
>
> 73, Dick WC1M
73 and good luck on the restoration.
Roger (K8RI)
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