Make Latching/Toggle SPDT relay from single DPDT

I’ve sometimes had an application that needs a latching relay that can be toggled by a remote momentary button switch. If you search for this kind of thing, on many DC DIY electrical websites you’ll find a description of a logic array that requires 4 single-pole dual-throw (SPDT) relays to accomplish this one simple function. It’s a horribly ungainly way to get there, requiring a lot of space and many interconnections. I knew there had to be a more compact and elegant way to do it, but I could never find anything on the web or in any of my books.
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So as a challenge I set myself the goal to use a single dual-pole dual-throw (DPDT) relay with a remote momentary pushbutton switch to create a latching / toggling / self-canceling circuit, employing my rudimentary understanding of electronics (I’m great with electrics, as in power circuits, which isn’t electronics despite the careless use of the word to apply to anything electric; I’ll be muddling thru the latter for a long long time, I suppose, but having a good time learning more and more).

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The relay I used is a very common small 10A DPDT you used to find at Radio Crack stores and is sold by McMasterCarr, etc.


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Here’s a sketch of the solution, it achieved my original goal very nicely. The sketch uses common symbols for components, lines can cross but small circles indicate connections. See if you can figure out how it works! (or cheat and read my explanation at bottom).

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There are base sockets available for this relay with screw lugs like below, or smaller ones with male quick-disconnect tabs. With a base the extra circuits can easily be wired between the base lugs, or soldered between tabs, so the relay itself is easy to change out.

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This setup uses one of the poles as a control to switch and hold the other. The second, working pole can be used for anything as a simple switch or a selector, like any dual-throw relay. The working pole is electrically isolated from the control side.

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When the power source is hot, there is a steady ~22mA current thru the control side, limited by the 390ohm resistor plus the coil impedance (12/(150+390)). The circuit barely gets warm so it can remain live indefinitely. The current the momentary switch handles is at most the coil current, about 80mA, so a tiny remote switch and wire can be used even at long distance. When the 12V supply is removed the relay defaults to its open position, so if you put it on an ignition switched circuit or the key-in Su circuit it will self-cancel with ignition off or key removed, respectively. It requires 11V minimum to work.
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I haven’t experimented with scaling it up to a larger relay, but by knowing the relay’s coil resistance and with a little experimenting to find it’s minimum pull-in voltage and minimum hold-in voltage, and applying some simple math you could find the right combo. If you need to control more than 10A you could take that path, or the second pole of a small relay like this one can simply control a larger conventional relay, or even several (a common DIN SPST or SPDT automotive “cube” relay coil only uses about 130mA).

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When it’s activated, the coil current is only about 1/4 what it would normally be, applying a lighter pull on the moving contact, so I would derate the second pole contact. You could easily measure the resistance or voltage drop across it to come to your own conclusions as to how much you want to let it handle.
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Anyway, this provides a commonly needed function for control circuits and takes up less than a third of the space the 4-relay array does, and is actually much simpler to wire up. If you’re budget conscious it’s also probably cheaper if you needed to buy the components retail.

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How it works:

  1. When the 12V supply goes hot, the 390-ohm resistor holds the coil current below the pull-in threshhold, meanwhile the side circuit thru the normally-closed throw on the control pole and then the 1kohm resistor juices the 25V 100microfarad capacitor.
  2. Pressing the momentary switch dumps the cap into the coil after the 390ohm, so the coil gets full voltage and pulls in. The constant current thru the 390ohm, which wasn’t enough to pull the coil in, is enough to hold it in. The control pole goes to the normally-open throw so the cap circuit is open.
  3. Pressing the switch again grounds the up-circuit side of the coil feed. The 1kohm in that path limits that current to a safe level for the switch. With the feed voltage dumped, the coil field collapses, and everything returns to the first condition.
  4. If 12V feed goes low while the coil is pulled in, it opens, so the circuit self-cancels with power off on the feed.

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