Topband: Detuning Relay Flyback Diode

[Paul Christensen]

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 contesting.com] On Behalf Of Brad
Denison
Sent: Saturday, December 23, 2017 10:52 AM
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.

Thanks,

Brad W1NT <http://w1nt.com>
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