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I am using a Zener diode along with a flyback diode with a relay in order to speed it up a bit. This works very well but I am currently using a rather over-sized zener from the inventory.

For proper design, I'm trying to determine what the power (or current rating) of the zener diode in the above schematic should be. The coil current of the relay is 80mA. Let's assume the zener has a breakdown voltage of 24V.

If the coil current is 80mA and zener voltage 24V, then power dissipated is approx. 2W - but this is a continuous rating, isn't it? How do I choose a zener with enough current and power rating that it will withstand the kickback effect of the relay?

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How to choose

The first part is consideration for the instantaneous power dissipation: \$24 * 80mA\$ As long as your Zener can handle that it will not die immediatly.

The next is the total energy.

The Inductance is key here & to a lesser degree its resistance.

The use of a zener ensure a constant voltage during decay as oppose to an exponential if it was to naturally decay via a freewheel diode & the coils own resistance.

\$V = L\frac{\Delta i}{\Delta t}\$ V = Zener Voltage L = coil inductance di = 80mA

Leaving dt

Now you know the energy stored in the coil: \$E = \frac{1}{2}Li^2\$. This energy needs to be transferred from the coil to the Zener.

With the time required from the previous calculations you now know the total power that needs to be dissipated \$P = \frac{E}{\Delta t}\$

With the power for one dissipation known you can then start short listing Zeners

Finally ... duty. Do you want the zener to be chosen such it can just handle one discharge event and then takes time to cool or do you need to tolerate a minimum enable-disable duty. Equally how many short succession enables-disables.

This increases the rms power that the zener needs to tolerate

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  • \$\begingroup\$ Yes, but L is not given in most relay specs. I could measure it, I suppose but is valid to assume it will not change much between relays? Also, you say that if the Zener can handle 24*80mA, it will not die immediately. Will it die eventually if I pick it based on that rating alone? \$\endgroup\$ – Saad Jan 25 '15 at 10:57
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    \$\begingroup\$ The conservative route would be to size it based upon VzII but this would produce a larger zener. You must have consideration for the power and its temp rise \$\endgroup\$ – JonRB Jan 25 '15 at 11:03
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    \$\begingroup\$ yup sorry, double-tap there. P = V*I \$\endgroup\$ – JonRB Jan 25 '15 at 11:26
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    \$\begingroup\$ what cancels out? At the end of the day you are trying to remove the stored energy in the inductance of the Relay. This is either passively done via the resistance of the winding or with assistance of a Zener (if you require a faster discharge). This is all about its inductance \$\endgroup\$ – JonRB Jan 25 '15 at 15:33
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    \$\begingroup\$ Ahh I see where you are coming from. Yes for a complete reduction you are correct. However, the L is required to know how long it takes & how long it takes is a driver into duty & other power dissipation concerns. \$\endgroup\$ – JonRB Jan 25 '15 at 15:44
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If you're able to access the relay armature you could eventually lock it in place and measure τ with a known series resistor, square wave generator and oscilloscope. Or eventually directly with an LCR bridge.

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