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
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