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Re: Topband: Detuning Relay Flyback Diode

To: "Jim Garland" <>, <>
Subject: Re: Topband: Detuning Relay Flyback Diode
From: David Cutter via Topband <>
Reply-to: David Cutter <>
Date: Sun, 24 Dec 2017 16:00:07 -0000
List-post: <>

I agree with your idea about the zener and a diode but I disagree about using signal diodes on their own. In my early career I recall changing a few 1N4148/1N914s on small cradle relays. Changing to 1N4000 series stopped the failures. I also agree, that for small relays there is no need for 1N5400 size.


----- Original Message ----- From: "Jim Garland" <>
To: <>
Sent: Sunday, December 24, 2017 3:40 PM
Subject: Re: Topband: Detuning Relay Flyback Diode

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.


Jim W8ZR

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