Topband: Detuning Relay Flyback Diode
jim.thom at telus.net
Sat Dec 23 12:26:02 EST 2017
Date: Sat, 23 Dec 2017 10:51:57 -0500
From: Brad Denison <brad.denison at gmail.com>
To: topband at 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.
<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.
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