Date: Sat, 23 Dec 2017 10:51:57 -0500
From: Brad Denison <brad.denison@gmail.com>
To: topband@contesting.com
Subject: Topband: Detuning Relay Flyback Diode
<I have a couple of 6ms 12V Kilovac relays wired up as a detuning circuit
<during RX. One question comes up before implementation regarding flyback
<diodes - would they be needed? Seeking opinions on this question. The
<relays are powered during TX and not powered for RX direct keyed by the
<radio with no sequencer.
<Thanks,
<Brad W1NT <http://w1nt.com>
## I call em back emf diodes. Use a single 1N5408 or 6A10 diode, reverse
connected across the 12 vdc winding. Heres the simple fix to SPEED UP up the
RLS. Wire a resistor in series with the back emf diode, EITHER LEG, the
value of this resistor will be equal to 2 to 3 times the DC resistance of the
12 vdc coil.
## IE: resistor in series with a rvs connected diode..and the entire mess is
in parallel
with the 12 vdc coil. Use a peak reading dvm, like a fluke 87-A to measure
the
actual back emf, without the diode + resistor installed. If the back emf is in
excess
of 1kv, then use 2 x diodes in series + the resistor..across the winding.
## What ever you are using to key the relay coil, will be fried by the back
emf of the
vac relay, if no suppression device used.
## I have tested the above technique,and it works good. Simple and effective.
## On a side note, I use the above diode + resistor combo on the INPUT vac
relay
on my hb amps. On the OUTPUT vac relay, I use just the diode. I want the
input +
output relay to operate fast, but want the output relay to have a longer rls.
You can also
fine tune the rls time by juggling the value of the resistor in series with the
diode. The
higher the value, the faster the rls.
Jim VE7RF
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