|[AMRadio] Residual Magnatism |
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.
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