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Relay/Diode/Back EMF with a new twist

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Just remember that with a TVS you need to rate the switch (transistor) for the sum of the power supply voltage plus the TVS clamping voltage.
 
Here's a good example. A little 3W standard zener in a surface mount package:

https://www.electro-tech-online.com/custompdfs/2009/09/1SMB5913BT3-DPDF-1.pdf

Look at Page 4, Figure 2. It will handle 50W for a 10ms rectangular pulse if the starting temp was 25 degrees C. That's the sort of info you need to pick a good diode. You also need to have an idea of how long the pulse is going to last from the relay spike, kind of hard to know without a scope though. I would think 10ms would be a totally fine guess.

Also, I would personally use a TVS zener. Hardly any price difference when you're buying single unit quantities, and better at pulse handling (and they will ALWAYS have a chart for pulse handling in the data sheet).

thank you for your information
 
Mate, you need simply to understand that when power is removed from any coil the field breaks down and a voltage flows as it does so. The diode is to give that energy a one way path to ground. Why make so much of it?
 
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