[TowerTalk] Rotator Lightning Protection

[Paul Christensen]

Back in May, I was discussing the addition of lightning protection in a M2 
RC2800PX controller.  At the time, I had installed a vacuum relay at the 
controller output to isolate the switching MOSFET from lightning exposure. 
But, I later realized that a Polyphaser MOV short to ground on the 
controller's return lead could result in the rotator turning out of control. 
Recall that prop-pitch rotators don't normally use limit switches unless the 
user customizes his own protection system. This week, I added a second 
vacuum relay to isolate the +48V supply lead.  Now, two SPST vacuum relays 
form a DPST relay.  One pole isolates the supply, the other isolates the 
MOSFET drain lead.  You can see both relays in the photos below.   Due to 
cabinet constraints, the relays are not installed adjacent to each other.

http://tinyurl.com/po8u72y
http://tinyurl.com/oknt64n

In the first photo you can see the controlling MOSFET, the device known to 
ground-short and also cause the rotator to turn out of control.   I have 
seen many references to MOSFET failure in these controllers  Because the 
+48V supply side is always active to the prop-pitch coils, it's imperative 
to protect the rotator in the event of a source-to-drain short.  The MOSFET 
gate is normally pulsed with a PWM circuit.  The pulse duty-cycle is what 
determines rotator speed.  Now, even if the MOSFET shorts, the relays will 
activate/deactivate -- and certainly hot-switch, but the rotator will still 
stop at its target bearing.  The MOSFET is now mostly vulnerable only during 
a turn.  But even with the inclusion of the vacuum relays, "anything goes" 
with a direct strike.  For nearby induced surges, the relays should 
adequately protect the controller, at least better than the stock design 
that offers almost no protection at all.  With the remote site being 30 
miles to the NW of my control point, the lightningmaps.org website has been 
indispensible for letting me know when strikes are too close to turn the 
arrays.

In the controller's stock configuration, a thin Sil-Pad insulator is used 
between the MOSFET and chassis.  You can see the Sil-Pad is now replaced 
with a thick ceramic insulator that offers greater breakdown potential 
between the MOSFET and chassis.  In the first photo, a new terminal strip 
can be seen with a 2N7000 switching FET.  The small FET drives the relay 
coils and was only going to be used for brief testing.  However, according 
to the Fairchild data sheet, it will handle 4X more switching current than 
the present 100 mA draw.  So, it stays for now.

Paul, W9AC