This question is similar to Zener Diode Selection for Relay but the focus is in how to select the voltage for optimum or near optimum release time.

With the circuit below, how does one select the zener voltage ?


simulate this circuit – Schematic created using CircuitLab

I've removed the values from the circuit because I'd like to know how to select a zener for any relay with any coil voltage.

Added After thinking about it for a bit, is the follow reasoning sound ?

$$ P_{zener} \geq V_z * I_{coil} $$ $$ V_z \leq \frac {P_{zener} } {I_{coil}} $$'


1 Answer 1


Please define "optimum release time". Does that mean that you want the fastest possible release time?

If so, simply allow the flyback voltage to go as high as possible. In other words, pick a zener value that is somewhat less than the maximum voltage that your transistor can handle.

And remove catch diode D2 - you don't want it if you need fast relay release.

  • \$\begingroup\$ Yes, that's what I was looking for. It's what I read from some online source, but I didn't know how trust worthy it was. That is what I'm looking for. \$\endgroup\$
    – efox29
    Commented Apr 5, 2015 at 1:34

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