Topband: Detuning Relay Flyback Diode

[Jim Garland]

I'm always hesitant to challenge anything Jim VE7RF says about amps, 
because he's almost always right, but here I have to respectfully 
disagree with a couple (not all!) of his points.  I, too, call 
protective diodes "back EMF" diodes, because their purpose is to clamp 
the inductive voltage spike that occurs when a relay coil is interrupted 
too quickly. As Jim notes, that voltage spike can be much larger than 
the relay operating voltage and can quickly fry, e.g., a switching 
transistor that operates the relay.

In the olden days, builders just put a diode directly across the relay 
coil (cathode connected to the positive coil terminal), and this clamped 
the inductive spike to about 1V, which is the forward, turn-voltage of 
the diode. The breakdown voltage rating of the diode only needs to be 
greater than the coil voltage rating, so fast signals diodes like the 
1N914, or 1N4148 diodes work fine.

The problem with this simple approach is, as Jim noted, the diode alone 
slows the relay's release time. The forward-connected diode acts like a 
low value resistor R when the back EMF drives it into conduction. The 
current through the coil decays with a time constant of L/R, where L is 
the coil inductance. Depending on the diode and coil inductance, this 
time constant, which determines how long it takes for the relay contacts 
to open, can be many msec.

Jim's fix is to put a resistor in series with the diode. This raises the 
R in L/R and shortens the release time somewhat. Unfortunately, it only 
partially clamps the reverse voltage spike, so breakdown of the 
switching transistor can still happen unless R is chosen carefully. The 
trick to pick a series resistor that is as large as possible without 
exceeding the breakdown voltage of the transistor. I've tried this, but 
under best of circumstances, I still end up with an undesirably long 
release time. It's a tradeoff between zapping the transistor or 
excessively delaying the release time.

My solution is to put a small 24V zener diode in series with the 
clamping diode. This automatically limits the back EMF to 25V (24V from 
the zener, 1V from the forward-biased diode). I've done numerous 
workbench tests and find this is the best compromise solution I could 
come up with. It still delays the release time slightly, but much less 
than a resistor-diode combo.

Two final points: First, none of this discussion pertains to the closing 
time of the relay contacts. When the relay coil is keyed up, an 
inductive EMF fights the buildup of current through the coil, but 
doesn't generate destructive voltages.(The exception is if you try to 
switch the relay on with a high impedance current source, but nobody 
would do that.)

And lastly, there's no need to use a big rectifier diode (1N5408, etc.) 
to clamp a relay coil, and especially no need to series several diodes 
to increase their breakdown voltage. The diode is always forward-biased 
by the inductive spike, so its breakdown voltage is immaterial, so long 
as it is at least 100V or so. The momentary peak current through the 
diode will never exceed the steady-state current of the relay, which is 
100 mA or so. Thus a single small 1/2W zener (1N5252) and a small signal 
diode (1n4148) will work just fine.

73,

Jim W8ZR



On 12/23/2017 10:26 AM, Jim Thomson wrote:
> Date: Sat, 23 Dec 2017 10:51:57 -0500
> From: Brad Denison<brad.denison at gmail.com>
> To:topband at contesting.com
> Subject: Topband: Detuning Relay Flyback Diode
>
> ##  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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