With a DC relay coil, the diode suppresses a voltage spike that happens when you turn off the relay. That spike can damage other electronics in the area, especially the transistor driving the relay coil (if there is one). For an AC relay coil, it might be a part of some cutesy trick to make a normal relay a latching relay, but I'm just guessing there.
Can't comment on whether the circuit will do what you want, because you haven't told us what you want the circuit to do.
ak
Don't think so. Every half-cycle of the 24 Vac it presents a dead short to the driving circuit or power source. With a current-limited source it could be used to allow a DC relay coil to function in an AC environment, but I'd think the contacts would chatter.the diode is a noise suppressor to protect the float switches and reduce noise in the ckt.
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