what do I call this relay?

Question

I want to build a circuit to switch a 1 1/2hp pump ON and OFF. ON to fill a large tank. OFF when the tank is full. I want to use a mechanical start (push the button) with a switch to cut the power. I see this as a mechanical latching relay application. I prefer to have a 24vac coil to match other devices in the system.

I want latching so that the coil is not powered during the 3 hour fill time but only gets momentary power when the full float completes the coil circuit. After that, the coil and the pump lose power until a finger pushes button.

The catalogs I have read suggest the mechanical portion of the relay is an accessory. How do I spec this relay? I know I need to have contacts for two hot legs to the pump and contacts to control the power to the step down transformer for the coil. I am a hobbyist. Thank you for any clues.

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Control circuit.

Your question is a little confused but I think this meets your requirements.

• XFMR1 is the mains to 24 V AC transformer.
• RLY1 provides power through its NC (normally closed) contact to the rest of the circuit. When the float switch contact closes RLY1 is energised and power to RLY2 is cut.
• RLY2 runs the pump when it is energised.
• Pressing START will cause RLY2 to be energised and run the pump. A second NO (normally open) contact bridges out the start button and keeps RLY2 energised until STOP is pressed or the float switch contacts close or the power fails.

The relays are called 24 V AC control relay. Make sure the contacts are rated for voltage and current at least as great as your load.

If this isn't quite what your require then please edit your question to clarify and post a comment below to notify me.

Answer 2

Panasonic makes a line of latching relays with mechanical actuators (which can also be electrically controlled, of course).

http://www3.panasonic.biz/ac/e_download/control/relay/power/catalog/mech_eng_djh.pdf

Or, you could get a dual-coil latching relay and drive it with one-shots. A float switch on the tank triggers a one-shot which drives the reset coil; and your pushbutton triggers a one-shot which drives the set coil.

In either case, using latching relays ensures that the state survives power loss.

https://www.electronics-tutorials.ws/waveforms/monostable.html

Answer 3

Since you are switching mains power, Code requires that you must use commercial off the shelf mains control equipment, not random components from Mouser.

The obvious thing to me is to use exactly what you propose: a latching relay. The General Electric RR7 comes to mind. Anyplace you see light switches which are centered and you push them up or down to turn the lights on/off, that's an RR7 or competitor. It is made to control large banks of discharge lighting such as those found in the ceiling of any big-box warehouse store, typically HID lighting with transformer based ballasts with a wicked inductive kick. As such, the RR7 is well rated for inductive loads such as motor loads. It has two 24V coils, one for on, one for off. You smack one momentarily and it throws and stays.

RiB also makes some relays which mount in the knockout of a junction box.

You could also use a common off-the-shelf air conditioning relay, about $12 and fits handily in a 4-11/16 square deep box with extension. DPST, 30A throw and motor rated. This has a 24VAC coil, and you would need to hold that coil on at all times, it won't latch. You could do the latching in your control electronics, which can be low voltage, and therefore electronics-tier components from Mouser are allowed.

Answer 4

There is a possible easy solution to this: use an off-the-shelf Ground Fault Circuit Interrupter (RCD to our European friends).

Reset the GFCI to start the pump running. Have the upper float limit switch operate a relay that both trips the GFCI AND interrupts the power to the pump motor.

You can force a GFCI to trip by adding a load resistor from the (Hot) output terminal to the Earth Ground lead. This resistor needs to cause at least 25mA of imbalance current to flow.

The purpose of the upper limit relay removing power from the pump is in case the GFCI doesn't trip for some reason.