[AMRadio] Reduce relay contact arc

[Rob Atkinson]

Jim,

Interestingly, my relay experience involved my HT-20, which I think
you wrote about in ER 68?   The 3PDT toggle tx switch on the front
panel had long ago given out, the previous (and first) owner had put
in a relay in the 1950s but it was no longer usable so I tried a
replacement, and the new open frame relay arced on either the high v.
primary or the low v. dc, can't remember which.  The front panel
toggle switch switched the ~340 v. low B+ to the driver stages and 807
screens.   Hallicrafters used one transformer for the filament, low B+
and bias supplies.  They couldn't switch the whole transformer off
between rx and tx so they had to run the low B+ through that toggle
switch.  There were a few other odd things in my opinion like lack of
bleeder resistors on the low B+ power supply filter caps, and the
thing with the high v. power supply filter choke on the CT of the high
B+ secondary.  Well, I'm risking hijacking the thread so I'll stop
now.

73

Rob
K5UJ

>
> As I recall, there are relay contact materials that are better at switching AC arcing loads.  Silver cadmium oxide comes to mind.  Mouser stocks a bunch of relays with AgCdO contacts that are rated to switch many amperes at up to 240 volts AC or more.  Do a search on "Silver Cadmium Oxide (AgCdO) General Purpose Relays | Mouser" and you should find a whole bunch of them.
>
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> Once upon a time (1970 to 1989), I used to work in a group at Bell Labs that designed relays built by Western Electric and identified good commercial relays for purchase and use in Bell System equipment.
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> 73,
>
>
> Jim Hanlon, W8KGI

>