Topband: Detuning Relay Flyback Diode

David Cutter d.cutter at
Sat Dec 23 13:36:40 EST 2017

The back emf diode slows the relay a lot and you can improve that with a 
resistor in series, fudge as required.
Here's another method:
in the drive transistor circuit, fit a small capacitor between collector and 
base.  As the back emf builds up, a current will be fed back to the base 
which will tend to keep the transistor on.  This results in a slower rise 
time and this can be controlled with the value of capacitor used.  A 
resistor in series with the capacitor provides even more control.  Fudge as 


----- Original Message ----- 
From: "Brad Denison" <brad.denison at>
To: <topband at>
Sent: Saturday, December 23, 2017 3:51 PM
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 <>
> _________________
> Topband Reflector Archives - 

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