[AMRadio] Residual Magnatism

Donald Chester k4kyv at hotmail.com
Mon Sep 18 16:38:37 EDT 2006

I have had that problem with DC relays, but never with ac ones.  The 
magnetic field produced by the a.c. voltage should de-gauss the core, just 
like a tape de-gausser demagnetises recording tape.  A tape de-gausser is 
nothing but a large coil with 60~ a.c. applied.

The worst relays I have encountered with the sticking problem were dc ones, 
salvaged from ARC-5's and other command sets.  I have had limited success 
with a tape shim stuck across the pole piece.

I often operate 110 volt a.c. relays with about 32 volts DC.  I have heard 
of residual magnetism being a problem when running ac relays on DC, but I 
have never had any problem with mine.

If that ever does get to be a problem, perhaps an easy solution would be to 
periodically reverse the polarity of the dc voltage to the coil whenever the 
relay starts to stick.

I haven't used a Dow-key relay for antenna changeover since my Gates 
transmitter turned one into a solid block of charcoal.

I do use a bunch of smaller ones, built very similar to miniature Dow-keys 
but with BNC connectors and 28 vdc coils, for switching the VFO between 
transmitters, and for switching amongst receiving antennas.  They were made 
for military/industrial use and I suspect they originally cost a mint.  They 
have been extremely reliable for me ever since I picked them up in the mid 
70's; don't recall if they were in some of the stuff I once pulled out of a 
dumpster on the Harvard University campus, or if I found them in some odd 
junk I bought at a hamfest.

Don k4kyv

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