Topband: Detuning Relay Flyback Diode

Brian Pease bpease2 at
Sat Dec 23 11:30:31 EST 2017

You can put the diode directly across just the LED if the relay speed is 
important.  The LED series resistor won't slow it much.

On 12/23/2017 11:15 AM, Paul Christensen wrote:
> Brad,
> 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
> Denison
> 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.
> Thanks,
> Brad W1NT <>
> _________________
> Topband Reflector Archives -
> _________________
> Topband Reflector Archives -

More information about the Topband mailing list