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Thread: using a relay for a latching circuit

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    using a relay for a latching circuit

    I've reupholstered the Defender seats and included seat heaters as my wife raves about them in my other car. The standard wiring that comes with the heater kit includes a fairly normal rocker switch albeit with a nice diagram of a seat heater. Problem I have with this is that if the heater is on when the ignition is turned off, it will come back on next time the ignition is turned on and will remain on until I remember to switch it off. I don't like this particularly for the passenger seat which could be inadvertently left on when unoccupied.
    I want to set it up so that the heaters will not come back on after the ignition is switched off, unless they are again switched on. I believe I can achieve this with a simple 4-pin relay wired as shown in the attached diagram, with a 'start' (N.O.) and 'stop' (N.C.) switch.
    I'd appreciate if the brains trust could check my diagram and confirm I have this correct (been a while)
    I'm thinking also that the start and stop can probably be combined in a single 2-way momentary rocker switch with a N.O. and a N.C. function combined. If anyone can point me in the direction of such a switch that would also be great.
    seat heater wiring.jpg

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    Can you post an image that’s not squashed.
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    What you've drawn up will work however the switchs will briefly be required to carry the full load

    I'd add a resistor between the stop switch and the relay if the seat heaters pull full.cirrent on startup.
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    Quote Originally Posted by p38arover View Post
    Can you post an image that’s not squashed.
    Yeah dunno what happened there. Trying again.
    seat heater wiring.jpg

    Regarding carrying the full load, wouldn't that be for a poofteenth of a second before the relay latches? do you reckon that would be a problem? I can use this circuit to switch a second relay.

    I was looking at Carling switches with an on-off-on momentary function, there are some that have eight terminals- would I be right in thinking that this makes one of the positions selectable between N.O. or N.C.?

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    Quote Originally Posted by POD View Post
    Yeah dunno what happened there. Trying again.
    seat heater wiring.jpg

    Regarding carrying the full load, wouldn't that be for a poofteenth of a second before the relay latches? do you reckon that would be a problem? I can use this circuit to switch a second relay.

    I was looking at Carling switches with an on-off-on momentary function, there are some that have eight terminals- would I be right in thinking that this makes one of the positions selectable between N.O. or N.C.?
    Yes it's only a fraction of a second but that fraction of a second is all it takes to arc the contacts and fuse them infused often enough.
    Dave

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    Yes good call. I'll run the heat power through a second relay.

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    Or use a latching relay with two sets of contacts,one for the load,using a hard 12V.
    Just make sure the relay can carry the current needed.

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    Quote Originally Posted by scarry View Post
    Or use a latching relay with two sets of contacts,one for the load,using a hard 12V.
    Just make sure the relay can carry the current needed.
    I guess that would be a DPDT relay- I'm going to start to get confused sorting out the separation of the circuits! I might be better off using this latching circuit to feed switching power to a second relay.
    The challenge seems to be finding a NO / NC rocker switch. I'd far prefer a rocker switch for each seat to a pair of push switches.

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    I answered before but had a brain fart, this is try 2.

    You need a dual pole relay (or 2 relays, just parallel the coils) . The seat load need to come from battery not Ignition, dont forget to fuse it.

    You need a push button switch for seat control , Not a toggle, Carling have push buttons.

    IMG20240106105617.jpg

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    Quote Originally Posted by goingbush View Post
    I answered before but had a brain fart, this is try 2.

    You need a dual pole relay (or 2 relays, just parallel the coils) . The seat load need to come from battery not Ignition, dont forget to fuse it.

    You need a push button switch for seat control , Not a toggle, Carling have push buttons.

    IMG20240106105617.jpg
    Thanks for that. A problem with that schematic though, unless I'm missing something, is that the only way to turn the heater off is to turn the ignition off. Still leaves me needing a 'normally closed' switch to break the coil circuit.

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