latching-relayrelay
Are there any relays that essentially have the same logic as an SR Latch? Specifically I need one that will latch when power is lost to the coil and then require a separate signal to reset or "unlatch" the signal. I have found many normally closed latching relays but those would not work for this situation because when power is lost to the entire circuit the relay unlatches.
Thanks
All this hanging contacts around the relay coil can and does work .The red and green start and stop buttons seen in machine shops are older than me .There is another way that uses the fact that the pull in voltage is always much greater than the dropout voltage .Get your relay and a DC power source thats the same as the coil volts .The relay coil will off course pull in reliably.Now place a series resister thats the same as the coil resistance in your relay coil power supply circuit.The coil now only gets 6V and wont pull in .If you momentarily short the resister the coil will pull in .If you momentarily remove the power the relay will drop out .So you can make a normal relay have a bistable latch function .The time to do this rather than the orthodox approach is when you need the contacts for other things and you are not worried about power.I have tried this with small DC relays but have not done this on AC relays.
A very simple way to do this involves two diodes and a regular DPDT relay. The relay could also be a DPST relay with normally open contacts.
The idea is that the input 12V pulse comes and powers the relay coil for the duration of the pulse. During that time the relay closes and supplies power to your load. At the same time the other pole closes and gates power through the second diode to maintain the relay coil in the energized state as long as the NC unlatch switch is closed. A benefit of the circuit is that if the 12V supply goes off the circuit automatically resets itself.
This topology can be made somewhat simpler if the whole thing is powered from the same voltages and suitable relay coil voltage is selected.
Best Answer
If power consumption is not an issue, you can use the "relaxed" contact of a normal relay with changeover contacts.
Here an example of latching a relay once you hit the button (instead of a button you could use another relay if you wish) and with a dual changeover contacts to connect/disconnect what you need on the second free one.
simulate this circuit – Schematic created using CircuitLab