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[JohnO]

I've mentioned "Latching relays" for controls. Here's how to make the circuit. I use this to heat oil for processing.

What it does: The heater circuit is normally off, until a switch is momentarily turned ON. The heater then stays ON until the thermostat clicks OFF. The heater stays off, permanently.

I use this to control a propane solenoid valve, but it can also control an electric heating element, or a pump, or any other electrical device.

How it works: The momentary switch closes the relay, which sends power to itself, so it stays ON (Latched) until the thermostat opens and breaks the circuit. The relay then turns OFF, which kills power to itself and the heater. The relay stays off, so the heater stays off, even when the thermostat contacts close again.

If you leave the switch ON, the heater will start up again.

I use variants of this circuit to control pumps. One pump feeds an automated oil dryer from a barrel of raw WVO. The barrel has a float switch that shuts off the pump and dryer when the level runs low. I don't have to watch it. Another pump pushes the dried oil through a filter, sucking the dry oil from a small (2 gallon) can with 2 float switches - one to start the pump, one to stop it.

I've also used float switches in other steps of the process. Most float switches and inexspensive temperature switches don't handle much current, so a relay is necessary. This is also true of most PLC's - their outputs can only handle 1 amp - plenty of power for a relay, but not enough to directly control a pump. Although I design PLC controls for a living, I don't use one for my processor because the logic necessary can be done just with relays, so the PLC in unnecessary, at least the way I process. Still, they're fun to play with.