Minor Issue With Back EMF? Ripple current?

Today I fried an Arduino and I am not sure why. I suspect it has someting to do with my big forklift relay.

I have an SBEC (UBEC?) which powers my Arduino and the big forklift relay with 9V and usually 0.6A. The Arduino and the Relay share the same power supply. Could it be that the relay produces some kind of current in the Arduino when it turns on and off? It worked about 10 times in a row and then the Arduino was toast.

I never got this whole back EMF in school, and I don´t get it now. Did this kill the Arduino?

The relay coil is essentially an inductor. An inductor (the relay coil) cannot change it’s current instantly, so when you switch the relay off you get a voltage spike. Most likely this voltage spike killed the arduino. You should add a ‘flyback diode’ parallel to the coil, this provides a path for the current when the coil is switched off.

Thanks. I have a range of diode sizes. Is smallish enough?

It should be able to conduct the full relay current(on the input side), thats about it. I looked at your schematic of when you first wired your electronics at How shall I power the safety cutoff relay? - #15 by MaxMaker - Electronics (ESC, remote, batteries) - FOIL.zone. It was not quite clear to me how you separated the 12v for the big relay or derived from it the 5V for your Arduino. It appears to me that only your activation relay could really damage it. You are sure that the relay current is low enough for your arduino?

My UBEC/SBEC only goes up to 9V, so it is powering both, the Arduino and the 12V forklift relay. I tested it, and the 9V are enough to close it.

The Forklift relay gets 9V from the UBEC, the same 9V also goes to the Vin of the Arduino.
The 9V circuit of the Forklift Relay is normally open and gets closed with an Arduino relay module. I have two of those blue Arduion relay modules and they switch fine, even if they just have USB power.

They are connected like here, just that the blue lampshade would be the forklift relay and that the power supply at the bottom right also powers the Arduino with 9V.

There are many solenoids. I use Gigavac solenoid. Seem to be waterproof compared to Albiright . Also it does not create EMF , some versions anyways don’t require diodes

This setup seems correct to me
your small relay board already includes a flyback diode to protect the arduino switch. The output of the small relay is potential free, the contact of this small relay may suffer from the switch off voltage coming from the large relay, but should not affect the arduino directly. Better put some flyback diode for the large relay to protect the small relay output. A medium sized diode should work. Place it directly on the contacts of your large relay inputs.
Is the arduino sensitive to power supply fluctuations?

Thats what I did tonight and it worked fine. So far at least. Thanks.