How do I limit voltage from two power sources?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. AC source switching.

How it works:


If AC1 (supply 1) is off the relay is de-energised. If AC2 (supply 2) turns on it is passed through the normally closed contacts of the relay to the irrigation valve.
AC AC1 turns on the relay energises and AC1 supplies the irrigation valve.


You need a relay with 24 V AC coil and contacts rated at 24 V AC/DC minimum and rated at the switching current or greater.