Wiring 12v automotive relays to need two hots to trigger - would this work?

In My Humble Opinion
1

I’ve been puzzling this in my head, and I think I have an idea, but know there are Dopers with much more comprehensive auto and electrical knowledge than me, so I thought I’d ask for advice.

The context is wiring a 12v device in my car that needs a beefy power supply. Typically, I’d just throw a Bosch SPST on there with the right trigger wire, and call it a day. However, I’d like this relay to need two distinct hot triggers to trigger the main power circuit.

After thinking of it, my thought was get two SPST relays - wire the first one up so that when triggered by the first hot (at post 85), it shows ground (at post 87, via 30 being grounded), then connect the other end of that ground to the ground side of the other SPST trigger circuit (post 86, relay 2). Then wire my second hot to the hot side of that SPST-grounded trigger circuit (post 85, relay 2), and my actual hot out (87 relay 2) to the device on the triggered end.

Would this work? Is it a bad idea for any reason?

Are there simpler or more elegant solutions (maybe using a single SPDT relay?) Thanks to any who post!

2

Why not just put two SPST switches in series with the coil of relay?

3

I think putting the two SPST in series is basically what I’ve proposed, but I’m no electricity-talkin’ guy.

And I am curious if there’s any hidden detriments to doing it, if anyone else has experience - like interfering with the existing circuit I trigger the hots off of, or anything like that.

4

I wouldn’t think so, but you might want to draw up what you have in mind and post it.

5

I would use diodes as discussed here but two relays work too.

6

I feel like I am missing something here but if your main goal is to just have two switches to operate one device why not just use a switch controlled ground.

7

You’re trying to make a mechanical AND gate. Easy-peasy, even if I don’t know the pinouts of Bosch relays. Trigger A goes to the coil of Relay A, the other end of the coil is grounded. Trigger B goes to the coil of Relay B, and the other end of that coil is grounded. Wire the SPST contacts of the two relays in series, and you’re done.

Only detriment I can think of is that the triggers have to stay hot for as long as you want the whatever it is to be on.

8

Thanks everyone!

And beowulff is right, I should probably be drawing pictures.

So the idea I had was this: http://i.imgur.com/cqpAoil.png

But it sounds like what everyone was suggesting is this: http://i.imgur.com/zLcHAsz.png

I’m totally happy to go with that one instead given wiser heads are recommending it, but is there any particular benefits of that versus the first one?

I thought the first one because it minimizes heavy gauge hot connections to one relay only, but I really don’t know much about this stuff.

9

Thanks for the diagrams.
The first one is correct. The second one would also work, but would increase your contact resistance by 2x, and require heavier wires connecting the two relays.

10

Ah good, I was just misunderstanding what folk were saying then. Thanks for the confirmation and advice, the first method makes more sense to me, too.