How do latching relays work?

**Mik** · 03-01-2004, 02:55 AM

I think I want to use a mechanical latching relay, but not sure have done a bit of

and this is where I herd about latching relays, but seem to have come to a dead end.

I have a single push switch on the dash of my car which blends in with everything and I want to use it as a SPDT toggle switch.

From what I can gather a mechanical latching relay has the same inputs as a normal relay, +12v -12v Common and NO/NC etc.

If I wire a mechanical latching relay up to the push switch tom make the break in the +12v, so the relay energises when I push the button and the relay wont draw any power what so ever when not energised. Will every time I push the button will it toggle the output of the relay, or do I have to apply a reverse current like you do on some door lock motors?

Hope this makes sense; don’t hesitate to ask for any clarity!!!

Thanks for any help

Mik

**deadweasel** · 03-01-2004, 04:55 AM

I think what you're looking for is what's called a Dual-Position Double Throw (DPDT) relay. It provides two positions (completes two circuits, one or the other) and has a double throw action. What this means is that when you apply power once, the relay flips to one side, which in this case, could be left open, thereby cutting the circuit. When you apply power again, it throws to the other side, completing the circuit you want to apply power to.

Placing a on/off pushbutton on a single throw relay means that in order to keep power going through the circuit, you have to keep power in the relay, and that's not a good thing for the relay. The coil will burn out after a short while that way.

Best place ever for answers to questions like these:
HowStuffWorks : Relays

**djg** · 03-01-2004, 10:46 PM

Mik,

Here's a relay that works as you describe you want: http://www.oldvolkshome.com/headrelay.pdf
It is used on VW Beetles to toggle between low and high beams so it will handle a fair amount of current. It mechanically latches in alternating positions each time you push the switch connected to the terminal labeled "brown wire" in the drawing, with the other side of the switch connected to ground. The wire labeled white with black stripe would be connected to the +12V source thru an appropriate fuse. The coil is only energized when you push your switch.

Hope that helps.

Dave

**Mik** · 03-02-2004, 02:06 AM

Originally Posted by djg

Mik,

Here's a relay that works as you describe you want: http://www.oldvolkshome.com/headrelay.pdf
It is used on VW Beetles to toggle between low and high beams so it will handle a fair amount of current. It mechanically latches in alternating positions each time you push the switch connected to the terminal labeled "brown wire" in the drawing, with the other side of the switch connected to ground. The wire labeled white with black stripe would be connected to the +12V source thru an appropriate fuse. The coil is only energized when you push your switch.

Hope that helps.

Dave

sounds exactly what is am looking for, trying to google for other makes, hope fully going to find one in the uk