[TowerTalk] Relay lightening protection

[donovanf at starpower.net]

This discussion of relay reliability and switching speed is irrelevant for a relays in a phased array that should never be hot switched.

The spike suppression components don't need to be outdoors, they can be in the control box near the solid state devices being protected.

73
Frank
W3LPL

---- Original message ----
>Date: Thu, 29 Jan 2009 06:51:18 -0800
>From: jimlux <jimlux at earthlink.net>  
>Subject: Re: [TowerTalk] Relay lightening protection  
>To: "Wolfert, William R." <WWolfert at columbuspolice.org>
>Cc: towertalk at contesting.com
>
>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.
>
>
>First question: Why do you want to put a diode across the relay coils? 
>Are you driving the relays with something that is transient sensitive? 
>If you're just driving them with a mechanical switch, etc., then I 
>wouldn't worry. Who cares if there's a 50 or 100V transient when you 
>open the switch. It's not like you're running an auto ignition at 
>hundreds of breaks a second.
>
>Second question: Diodes could fail either short or open. I assume your 
>DC power supply for the relay coils has some sort of overcurrent 
>protection, so a shorted diode just draws more current and triggers the 
>OCP, just like any other fault that occurs. If you have a fault, you'll 
>have to hike out to the box anyway, so it's not clear that knowing in 
>advance that it's a blown diode saves you much. The fuse or CB pops, or 
>the power supply current limits, and you know something's up.
>
>If you're worried about lightning damage to the coils, that's a separate 
>issue.
>> 
>> 
>Jim
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