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?
Answer
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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