ok to use mini relay for extra zone?

[greglarious]

i have an 8 zone system with a master valve and dont want to purchase an expansion board just for one.
it seems like 24v ac would be within the limits of the relay but i have not seen any discussion of this. the main concern i have is about the unique issues of controlling a solenoid and the possibility of back EMF causing damage or interference.

also, i’ve hunted the forum for posts on this topic and dont see an example of code to use the relay. any hints?

[rederikus]

The relay used on the board is a very small device. It would be better to make this little relay drive a bigger relay (yes I know it is cumbersome) and have that drive the solenoid.

There are two reasons for saying this.
1). the little on-board relay might do the job for a longer or shorter while, maybe forever. However if it does fail then you just lost your master valve so the entire irrigation system would stop.
2). It is easy to replace components that are not on the board. If the on-board relay fails then you will have to either replace the entire board or do some soldering iron surgery.

[Ray]

There are two types of the mini relays: the earlier version used a black 110V AC / 2A relay, and the current version uses a yellow 250V AC / 3A relay. Both would work perfectly fine for driving a sprinkler solenoid, which only needs 24V AC / a few hundred milli-amps. I don’t see any big issue as the voltage / current both fall well below the relay ratings. The back EMF could be an issue, but the relay pins are relatively far away from the rest of the circuit, so it’s probably not a big concern in practice.

Dan’s interval program now provides a plug-in that shows how to control the relay. The OSPi user manual now also includes information about the pin that the relay uses:
http://rayshobby.net/docs/ospi14_manual.pdf
Basically outputting a high on that pin activates the relay, and low deactivates the relay.

[Author]

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