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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.

[JohnO]

This has evolved over several years. Raw, wet, dirty oil starts in the big blue container. It gets pumped into the highly modified Bird Waterer dryer, which dribbles the dried oil into the Sump. The Sump is a can with two float switches - one starts a pump when the sump gets "full", the other stops the pump when the level reaches "empty". The pumped oil goes to a Luber-Finer filter, then into a tote for storage, ready to use as SVO fuel.

The sump pump runs as long as it takes to push about 1.5 gallons of freshly dried oil through the Luber-Finer filter. A fresh filter offers hardly any resistance, so the pump only runs for about 30 second. If the filter is nearing change-time, the pump may run for several minutes.

A paper prefilter sits on top of the sump. They cost about $0.05 each, extending the life of the $16 Luber-Finer filters. I change Luber Finer filters about every 600 gallons. The paper filters get changed every few gallons of dirty oil, or several tens of gallons of pretty clean oil.

The paper filter is also equipped with a float switch, on a movable frame that also holds the filter open. When the paper filter gets clogged and fills up to the float switch, the system shuts off until I notice and change the clogged filter. The float switch is positioned so that it will also shut off the system if the Sump fills all the way up, such as might happen if the Luber-Finer filter becomes clogged, or the pump stops working, or something else malfunctions.

The paper filter float switch relay has been connected either to a latching relay, so it permanently shuts down the system until attention is paid, or connected to a "normal" relay, so it restarts if/when the oil level drops back down. The later approach saves process time, but means the system will start up again only to shut down as soon as a little more oil is dribbled into the clogged paper filter, wasting energy. I prefer the latching relay system.