Topband: Detuning Relay Flyback Diode

Paul Christensen w9ac at
Sat Dec 23 11:15:45 EST 2017


If you use an LED with a dropping resistor across the coil to monitor relay
status, then you must use a fly-back diode.  The coil of the type you're
using will produce substantial EMF as power is removed and the coil field
rapidly collapses.  The collapsing field will produce E-I beyond the LED's
maximum ratings.  Failure occurs even when the LED is current-limited
through a series dropping resistor.   

On my remote-controlled homebrew balanced tuner, I was replacing LEDs until
I discovered that the collapsing vacuum relay field was immediately burning
out LEDs.  Never a problem after installing 1N400X fly-back diodes.

Paul, W9AC

-----Original Message-----
From: Topband [mailto:topband-bounces at] On Behalf Of Brad
Sent: Saturday, December 23, 2017 10:52 AM
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 <>
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