Relay triggering 120v Pump

[Prince]

This is for my pond pump.

I have a 250 watt solar panel with a 20amp controller powering a 24v 100w pond pump.

I also have my 120v pond pump.

What I would like to buy is a relay that I can use to turn on my 120v pond pump when my solar panel does not provide any power.

I think my solar controller output can provide the lack of power to a relay.

Where can I get a relay (and what do I call it) that will trigger from lack of voltage to open a circuit to turn on my 120v pump?

[jflorey2]

Where can I get a relay (and what do I call it) that will trigger from lack of voltage to open a circuit to turn on my 120v pump?

Almost anywhere. Radio Shack, Digikey, All Electronics, Jameco etc. Size the relay contacts for the worst case load by the motor (usually on the spec sheet.)

If the controller has a "lighting" output it can run the relay directly. If not you'll have to connect it to the solar panel output. You may need an additional diode between PV panel and controller to prevent the battery from backfeeding the relay coil; depends on the controller.

You can use 12 volt relays if you want; just wire 2 in series then use the contacts on just one of them. (Alternatively just use a resistor equal in value to the coil, and equal or greater in power.) It's good practice to put a snubber diode across each coil but not critical in this application.

In general you will need an SPDT or DPDT relay (DPDT is probably safer.) You will be connecting to the "normally closed" contacts.

[Prince]

Thank you. I just ordered a 24v dptd 10a relay.

Can you direct me to a schematic I can understand in order to wire it up? All the ones I see through a Google search are very cryptic.

[FNG AZ]

Wire the 24vdc relay coil in parallel to the pond pump output. (I would throw a 1 amp fuse inline to the coil.) Then on a N/C set of contacts break the hot wire to your 120v pump. do not break the neutral or ground wires, just the hot.

This will turn on the 120 volt pump when the 24 vdc relay drops out.

[Prince]

Thank you again.

I appreciate your help.