Avalonnw:
Hi,
I'm trying to switch 4 relays simultaneously with arduino. Essentially, I have these relays' coils connected in parallel. I tried connecting them with every relay having its own rectifying diode and it works just fine. The question is, is it necessary to have a diode on each of the relays, or can I use one (maybe larger) diode for the whole lot?
Yes you can.
How do I select it (assuming it is possible)?
Depends on the relays. What diodes are you using now?