I'm looking for an off the shelf, self-contained component/device that I guess is similar to a latching relay, except that under normal conditions, it will have low voltage DC at the coil all the time and the contacts will be closed. When the low voltage DC at the coil stops, I want the relay contacts to open and stay open even though the low voltage DC may be restored in a second or two. Once losing the coil voltage, the only way the contacts can close again is for the coil voltage to be restored AND some type of manual action has occured. If necessary, the "normal" mode could be reversed to be no voltage present and the "shutoff" mode changed to having voltage present, i.e., NO instead of NC.

I want to use this as part of a method to cut power to a water well pump if a leak is detected. I can use a leak sensor like this one...

http://www.grisk.com/specialty/2600water_sensor.htm

...to detect the leak and provide low voltage to the component I'm looking for either when there is no leak (NC sensor) or when there is a leak (NO sensor). The relay contact side of the component will use low voltage DC to control this "relay in a box"...

http://www.functionaldevices.com/pdf/RIB24S2.pdf

...which will control the water well pump and provide an override switch that will bypass the leak sensor stuff.

Hope that makes sense.

Thanks,
Ira

 

Look up a SCR, it latches as long as DC is applied, then resets when power is cut, waiting for a signal to a gate.

You can wire a simpler version of the same thing from transistors, since it is a PNPN device.

Commercial models can be quite heavy duty, large voltages, large amps.

 

Bill,

Thanks for the info. After much reading on this site and lots of Google'ing, I think I understand the basic concepts, but I'm going to try to put it in my terms.

Low voltage DC goes thru the NC leak sensor (while no leak is detected) then thru the SCR to the relay coil to keep the NO relay contacts closed, thus allowing power to the pump. To start things off, a trigger voltage must be sent to the SCR gate via the momentary NO switch. As long as the leak sensor does not detect a leak, current will continue to flow thru the sensor and SCR to the relay coil to keep the pump powered.

When the sensor detects a leak, it's contacts will open stopping the low voltage there, so no power will get to the relay coil and the relay contacts will open which stops power to the pump.

When the leak is fixed (or if it dries up without being fixed), the sensor will once again close on its own (since it is NC and doesn't detect a leak anymore), sending power to the SCR. However, the SCR being what it is, the power will not go thru it to the relay coils (to close the relay contacts and provide power to the pump again) until the SCR gate has received a trigger signal from the momentary NO switch.

Did I get it right?

Now my question is how do I find the appropriate SCR? All of the low voltage stuff will be done by DC wall warts, so I've got a lot of flexibility with voltage and amps. I've been looking at various web sites, but the selection is very confusing for a novice. I've also yet to find anything that's more or less "self contained" (like the "relay in a box") so that all I have to do is hook up a few wires. Any suggestions?

Thanks,
Ira

 

I've also yet to find anything that's more or less "self contained" (like the "relay in a box") so that all I have to do is hook up a few wires. Any suggestions?

You may use any DPST (NO contacts) or DPDT type relay to do the job. If the current flows through the motor of the pump can be handled by one pair of contacts only, you won't need to get another latch device such as DPST relay or SCR. The attached wiring diagram using the mentioned water sensor and the relay is for your reference. The only additional part is a momentary push button switch. Please note the contacts of the water sensor can just handle 500mA 30V, the relay used should employ a coil within that range of rating.

 

OK, if there is a leak you want to open the circuit, but if there isn't you want it to stay working. I can't argue with the previous post, but you will get much more energy efficient results with something like a SSR. This may be more expensive that you want to go though.

What is the power requirements of the motor you're turning on and off?