Topband: Detuning Relay Flyback Diode

Jim Thomson jim.thom at
Sat Dec 23 12:26:02 EST 2017

Date: Sat, 23 Dec 2017 10:51:57 -0500
From: Brad Denison <brad.denison at>
To: topband at
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 <>

##  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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