I have a standard irrigation system with 24VAC solenoids. I have built a simple Arduino + ethernet shield connected to a 4xopto-isolated-relay board for controlling each of my 4 sprinklers. I have the 24VAC from the irrigation wall-wart feeding into a rectifier and then voltage regulator to give me 5VDC which I am using to power the Arduino and drive the input circuit on the relay board. The Arduino digital output pins are driving the relays directly. The relays are switching the 24VAC to drive the solenoids.
It seems to work fine most of the time but every now and again when a solenoid shuts off, the Arduino restarts. After some reading I think this is due to EMF spikes caused by the solenoids which results in arcing across the relay contacts and all this *noise* is enough to cause the Arduino to restart. Further reading indicates I need a *snubber* circuit across the relay contacts (capacitor + resistor in series) to dampen this EMF.
However I am unsure of what size C + R to use and if I can buy a pre-packaged device? Any tips on what I might need and where I could get it from?
I found these on the Element14 site - http://nz.element14.com/roxburgh/re1202/rc-network-250v-0-2uf-120r-pcb/dp/2336111 - but I am not sure if they are big enough or even suitable? Also, shipping costs about 5 times the actual device...