One of the interesting challenges I am running into with building an irrigation controller is the EMI generated as a result of switching the solenoid valves.
There are really two sets of inductive loads in the project: the relays which switch the AC circuits (here I have flyback diodes integrated into the ULN2803a Darlington transistor arrays which switch the relays) and the solenoids themselves (which are running on 24VAC).
It is the EMI from the solenoids (mainly when they switch off) that I still need to deal with. The solution I have found, looking around online, is to put a MOV in parallel with the load, and possibly also a small resistor (47 Ohm in one example, 100 Ohm in another) in series with a small capacitor (100 nF to 220nF) in parallel with the load.
I was wondering if I am missing anything or if anyone has a better suggestion.