[TowerTalk] Relay lightening protection

[Kipton Moravec]

On Wed, 2009-01-28 at 09:20 -0500, Wolfert, William R. wrote:
> Gentlemen
> 
>  
> 
> I'm putting together a switchbox for a phased array. I had many spdt relays on hand so I'm using them. 2 relays per antenna will work fine. The array is an 8 circle 
> 
> (for 80m, hung off the top of the 160 Vertical) and I've used ON4UN's switching scheme (on page 11-65 of latest edition of LBDX). I'm located in central Ohio, with moderate thunderstorm activity in the summer. Since unused antennas must float, the relay contacts are spoken for. The 160 Vertical has a spark gap and I intend to put a spark gap at the base of each wire. The 160 antenna has 120 ¼wl radials and each 80 m element will have 64. Control wires are underground from the shack to the towers. I have bypassed the control wires at the phase box. The feed coax is grounded when not in use. Adding a third relay per element would allow me to ground the system when not in use, but I'd rather not have 3 relays per element. Are diodes across the relay coils necessary? And if so, how can I detect a failed diode (a short, correct?) before turning on the power? The entire system (house, towers, shack, etc) is properly grounded by SPG. Your input is appreciated. 
> 
>  
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> 73, Bill WR8K 
> 

I can answer the diode question.

It is generally good practice to put a diode across the relay because of
the inductive spike when turning it off. I have seen the spike go 3-4X
the input voltage. If you may have a very long line to the relay the
spike may be more. 

There are a couple of of ways to mitigate it. The easiest is a diode.
When the relay is turned off the diode sends the excess energy back into
the coil to dissipate. 

The DC coil does have some resistance so looking for a short can find a
shorted diode. But if the diode dies open you will not know it until the
spike comes back to you. I have never lost a diode in this application.
Size the diode to be more than the voltage you are powering, and more
than the current through the coil, and you should be alright. You can
use pretty much any diode. Like a 1N4002 for example. If I am worried
about a large spike and my switch is near the voltage limit, I will use
a fast switching diode.
 
Kip

-- 
Kipton Moravec AE5IB
"Always do right; this will gratify some people and astonish the rest."
--Mark Twain