As Ken points out, MOVs degrade a bit every time they absorb a surge.
This can lead to either no protection or a dead short. A nearby ham had
a major fire created by a shorted MOV inside a plastic outlet strip.
Fortunately the energy in rotor or antenna control lines is less likely
to cause a fire.
Note that the better whole house surge protector units have monitoring
of the MOV and a fuse in series should any one short.
As a result I use the TVS (Transient Voltage Suppressor) diodes for
control/low voltage circuits. The RT-21 has TVS diodes on each line,
inside right at the terminal strips.
As with MOV's the power absorption capacity comes in many sizes. I'm
pretty sure that TVS are able to repetitively absorb surges at a
specified power level without degradation. TVS are available
unidirectional or bidirectional. Be aware as they are essentially high
power zener diodes there is significant junction capacitance, so that
needs to be considered.
The Littlefuse/Vishay 1.5KE series are available at Digikey, Mouser,
etc. Specs there also. These are fairly high capacity devices for
control circuits. Check the specs. About $0.75 each qty 1.
Grant KZ1W
On 8/25/2021 15:17, Kenneth R Goodwin wrote:
You have two options in rotator control line solutions. My source on this
topic is Jeff at Green Heron Engineering. MOV's protection is normally at 60
- 80V and a spike of that magnitude will undoubtedly nail any electronics.
You will protect your house but the rotator controller is not going to
survive. MOVs also degrade over time with no means of evaluating them other
than what is likely to be destructive testing. I suspect the good
engineering procedure is to replace them at some point in time but knowing
amateur radio operators they will probably have degraded to the point where
they will provide no protection at all . Green Heron uses TVS diodes which
will short circuit at their rated voltage which means they will blow and
need to be replaced after doing their job. They are not cheap. I have had
the TVS diodes in my Green Heron RT-21 controller blow even though I have a
PolyPhasor IS-RCT for the rotator control lines mounted on a copper plate
which is part of a single point ground system located at the base of my
tower. The RT-21 version that I have uses 82V TVS diodes in the motor
control lines (CW and CCW). I am not sure where the 20V TVS diodes are
connected without opening the RT-21 controller box but I think the lines are
marked POS and REF on the back of the box.
I have never had a direct hit but have experienced four or five indirect
lightning hits (nearby trees, TV/Internet cable) and the lesson I've taken
from this is that you can not fool mother nature. I've thrown a pile of
money into protection devices and such things as single point ground systems
(as close to single point as possible), copper ground straps, piles of
ground rods, etc. and yet when lightning is nearby, I disconnect everything
at the single point ground. I also isolate my computer from the radio
system by disconnecting the USB 3.0 industrial hub from the computer that
connects to the numerous radio USB ports My last two failures have been the
radio equipment USB connections. The most likely energy path into the house
has been thru the TV/Internet cable (where I have a high frequency rated gas
tube protection) and the telephone lines where I also have the telephone
lines on a surge protection device. The house has an MOV protection system
on the power lines coming into the house but again the only successful
solution I've had is to disconnect. I have great respect for the commercial
systems on the top of a mountain that easily survive but I bet they design
their protection system into the building from day 1. Ken K5RG
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|