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Protection diode across relay DS_1E

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Hello,

I am using the Miniature DS1E SPDT Relays in my circuit and want to protect the circuit with a protection diode across the relay. Each of the relays have different switching voltages namely 3.3V, 5V and 18V wrt GND.
Now what should be the diode voltage rating? Should it be like 3.4V, 5.1V and 18.1V across the respective relays? or could I use any common voltage diode (say 5V) across all these relays?
Should it be a zener or a regular diode is ok?

Thanks,
Kf
 

Assuming the diode is to suppress the back EMF, it normally isn't conducting so as long as it can withstand the relay operating voltage the PIV rating doesn't matter. The diode conducts briefly as the relay is de-energized and the collapse of magnetic field in the coil produces a reverse voltage across its connections. The diode will clamp the voltage to Vf which is normally about 0.6V for a silicon diode. In the case of such a small relay you can use almost any small signal or power diode such as a 1N4148.

There are special circumstances where a combination of Zener and signal diode are used across a relay coil but I don't think these apply in your situation.

Brian.
 

Most people use a simple free-wheeling diode. The voltage rating has to be higher than supply voltage, but isn't critical. You can use 1N4148 or similar SMD devices. If a very fast relay release is intended, you can connect all free wheeling diodes to the highest available supply voltage, if the respective relay drivers have a suffcient voltage rating.

Although relays are mostly driven by a transistor common emitter/source circuit, you can also use a common collector circuit - without a free-wheeling diode.
 
Thank you all.

Since the coil voltage would be 5V, I am planning to use a 1N400x diode across all these relays. Hope that's ok.

Thanks,
kf
 

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