Controlling a dual relay with one switch

Thread Starter

Hardbody

Joined Dec 27, 2022
8
Hello, Have a question about using a diode with a double relay.

What I am thinking of doing is I want to control 2 pairs of lights using a double relay and one on/off/on switch.

switch left "on" would turn on 1 pair of lights
switch right "on" would turn on both pair of lights
middle would be all off

I would like to flip the switch to the left "on" to turn on 1 pair of the lights, than if I flip the switch to the right "on" it would turn on both pairs of lights.

Problem I see is since the right side of the switch would need to be connected to both sides of the relay, when I did turn on the first pair of lights (left "on" on the switch), being the 2 sides of the relay are connected, turning on the left side would feed the right side.

Was thinking if I used a diode on the wire going to the second relay trigger terminal, it would block the power from going to the second relay terminal, but when I turn the switch to the right side "on" position, power would flow through the diode to trigger 1 causing all 4 lights to come on.

Never used a diode so wanted to ask about using one in this configuration.

Does this sound correct?
 
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Thread Starter

Hardbody

Joined Dec 27, 2022
8
I realize this would be very easy to do with using 2 separate switches, just was thinking of using 1 on/off/on switch.
 

Tonyr1084

Joined Sep 24, 2015
7,965
As I said, it's easy to do as long as you get the right value components. And you don't need a double relay.

Here I've drawn a SPDT Center Off switch with a diode bridging the left and right pins. When you throw the toggle to the right only the two left lights will be on. When you throw the toggle to the left then all four lights will be on. The diode needs to be able to support the amount of current the lights will draw. Also, the switch needs to be robust enough to handle the current.

1672155818472.png
 

Tonyr1084

Joined Sep 24, 2015
7,965
You posted while I was composing my response. I don't think that's a "Double" relay. I think - and don't know this for a fact - the S terminals are Common, Normal Open and Normal Closed. The two L terminals are likely the coil connections that activate that switch. It probably does not have the ability to be off at any time. Either S1 & S2 are connected or S2 & S3 are. I don't know the internal connections of the relay so I can't state with any kind of authority. There are others here who have vastly more experience than I have. Someone will chime in and possibly add clarity. For now the diagram I drew should do what you ask. Since they are LED cubes their amperage is likely very low. I suspect the 1n4000 series diodes will probably be robust enough to handle the current.
 

Thread Starter

Hardbody

Joined Dec 27, 2022
8
As far as I know it is a double relay, it has 2 separate coils inside. It takes, as I said a common power in, and has trigger 1 trigger 2, Lights 1, Lights 2, with a common ground.

S terminals activate each side of the separate coils, L terminals are the outputs, so S1 sends power to L1, S2 to L2

I've used these relays before to power multiple pairs of lights.

I'm used too using relays since the lights I've used in the past are much higher amps (being they were not LED's), I would use a small switch since the trigger current required is very low to activate the relay.

Being these new lights are LED and low amps, a setup like your diagram would probably work fine with a normal sized switch.

I also know back in the day before relays were a common item to use in vehicles, aftermarket lights were wired, battery to heavy duty switch (20 or 30 amp) than out to the lights. Problem with that was the power had to go from the battery under the hood, into the cab to the switch and back out to the lights (where ever they were mounted), I guess doing it with out a relay there is some power loss.
 

Tonyr1084

Joined Sep 24, 2015
7,965
We need to know how much current the two lights that are on when the switch is in either ON position. Any regular diode that can handle approximately twice that current should be fine. If - for example - your LED cube lights draw 2 amps then you'd typically want a diode that can handle 4 amps. Keep in mind that is an example, not a recommendation. You want more than enough current carrying capability in the diode, otherwise the diode will burn and fail.
 

Tonyr1084

Joined Sep 24, 2015
7,965
The two lights would draw 5.4 amps. Here's one you might consider. Cost is $1.24 (US). Given that much amperage (wattage is amps times volts so - 13.8V x 5.4A = 74.5W) I'd recommend a heat sink as well. But with that much current there is probably a far better option. Give me a bit of time and I'll bang out another option. Just remember the switch needs to be capable of handling ALL the amperage for all the lights you're switching.

[edit] forgot to add the link to the diode.
 
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Tonyr1084

Joined Sep 24, 2015
7,965
OK, here's using a SPDT relay. When the toggle is left the right lights are on, so is the relay. When the relay is on it conducts power from C (Common) to NO (Normally Open) and all four lights will be on. When the toggle is thrown to the right only the two left lights will be on and the relay and the two right lights will be off. No need for a heavy diode and heatsink.
1672162581881.png
 

Tonyr1084

Joined Sep 24, 2015
7,965
Here I'm using a DPDT switch with Center Off. The upper portion of the switch has power going to both legs while the lower portion goes only to one of the legs. The common legs are each wired directly to the lights. In the up position only the right lights are on. In the center position (square) all lights are off. In the down position all the lights are on. No relays, no diodes, nothing special other than the DPDT Center Off switch rated for 12V @ 10A or more.
1672163427814.png
 

Tonyr1084

Joined Sep 24, 2015
7,965
And yes, my diagram does not show LED lights. Just lamps I grabbed from my stock of lights. They only represent the load, not the type of lighting involved.

Sometimes it takes me a few minutes to consider better designs. I think this would be the route I'd go if I were doing what you're doing.
 

MisterBill2

Joined Jan 23, 2018
19,065
Certainly the DPDT center-off switch, as shown in post #13, is the simple way to go, if the switch is rated to handle the current. Been there and done that and it works very well. But some styles of switches are not available in a DPDT center-off arrangement.
 

Tonyr1084

Joined Sep 24, 2015
7,965
The switch listed in #17 doesn't give a DC rating but I suspect that if you're switching (per circuit) less than 3 amps to light LED's at 12V you should be good to go.
 

Thread Starter

Hardbody

Joined Dec 27, 2022
8
Thanks, can you post a better diagram showing what you posted in post 13, as far as which wire would go to each terminal?

I found a diagram online but it shows running 2 pairs of lights individually and not as I was looking to do.

Thanks
 
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