Normally Closed Latch Relay

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

• Does a latching relay even have a "normal" position? Commented Feb 25, 2021 at 10:37

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

Invert your logic and it's very simple using a standard relay latch circuit.

simulate this circuit – Schematic created using CircuitLab

Figure 1. A standard relay latch circuit.

• When power is lost the relay "latches" off.
• When reset is pressed the relay latches on until power is lost.

Use the NC and NO contacts as required.

• FWIW: North American practice will often call this arrangement a stick circuit, based on the name it acquired from its frequent use in relay-logic based railway signaling work Commented Feb 26, 2021 at 4:44

Have you had a look at the Double coil latch type / 2-coil latching type ? Have a look at mouser or digikey. You will have to control two different coils though.

• How would you make one of those change state on loss of power? There's a schematic button on the editor toolbar. Commented Feb 24, 2021 at 23:45
• Surely this is the only type of relay that behaves like an SR latch? One set coil, one reset coil. OP does say "separate signal to reset or unlatch". Commented Feb 24, 2021 at 23:53

They make bistable relays that do exactly what you need. One coil closes the contact and then it will stay that way forever until you trip the other coil.

There's also a variation where pulsing the coil in one polarity sets the contact while pulsing in the other way resets it.

There are a couple advantage with these: you don't need to be continuosly powered so

• The coil doesn't heat up the relay (precision signal relays can alter the signal when heated up)
• You can matrix wire them (like keypads, only using relays)

for this reason you find lots of them (like, 96 in a board) in signal acquisition switches.

There is however an intrinsic problem (other than they cost somewhat more): most safety standards forbid them for machinery or actuator control since you can't E-stop them (that's also one reason for not finding them with many Ampere contacts)

• "Specifically I need one that will latch when power is lost to the coil ..." How would you implement that feature with an SR relay? Commented Feb 25, 2021 at 9:08
• If a relay is used to select which function a machine is performing, the fact that it latches shouldn't preclude e-stopping if there's an upstream relay that cuts power that would be fed through the latching relay. Commented Feb 25, 2021 at 14:20
• Clarification: they are not completely forbidden, but in must cases you can't use them. And mode selection at least for the EU machinery directive has quite complex requirements, by the way Commented Feb 25, 2021 at 15:33