Wiring car light relays with dimmable switches

Thread Starter

Elliott Balsley

Joined Aug 31, 2016
21
I'm adding some extra lights and accessories to my Subaru and I will install illuminated switches on the dashboard. I want these switches to dim with the factory dimmer circuit. On this vehicle, I believe the dimming uses PWM on the ground side. I don't know exactly how it's wired, but I found a place to tap into the dimmed ground and it does dim my switch correctly. So I drew up this diagram showing one circuit and I will build 5 of these. This is my first time drawing a circuit diagram. The only part I'm unsure about is will the relay work properly with a constant 12V coil voltage, considering this dimmer? (I drew the dimmer as a resistor because I'm not exactly sure what it is).Screen Shot 2019-02-26 at 01.00.24.png
 

mvas

Joined Jun 19, 2017
538
When your new "illuminated switch" is turned ON, it cannot be illuminated, as wired.

It looks like your Illuminated Switch was designed to be on the Hot side, not the Ground Side.
The right side wire, of the switch, should connect to fused + 12 Volts.
The middle wire, of the switch, goes to the relay coil, then the other side of relay coil connects to ground.
The left wire, of the switch, appears OK if your auto has "PWM Dimming to ground".

Typically, in an auto, the switch provides fused +12v power and the other side of the Load is grounded.
 
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Thread Starter

Elliott Balsley

Joined Aug 31, 2016
21
When your new "illuminated switch" is turned ON, it cannot be illuminated, as wired.
Is this because the power will take the path of least resistance and go straight to ground, not through the LED?
This switch is from Amazon and I can't find a data sheet. It has 5 pins labeled C, NO, NC + and -. So I think to make it function like the 3 pin switch you mention, I would connect + and NO and to the relay coil, connect C to ground, connect - to dimmed ground, and NC is not used. I should have clarified — I want the switch to be illuminated only when it's on.
 

mvas

Joined Jun 19, 2017
538
If you have a "grounded" dimmer circuit and
you want the Internal LED to be "Switched ON/OFF" then ...
.LED-BAR-small.jpg
 
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Thread Starter

Elliott Balsley

Joined Aug 31, 2016
21
Great, thanks for confirming! I do need a relay, since the LED bar pulls 15A.
These cheap switches said 12V so I assumed there was already a resistor on the LED, but nope, I fried it. Will have to be more careful next time and measure the LED.
 

mvas

Joined Jun 19, 2017
538
I did not know that some Illuminated Switches had built-in current limiting resistors, while others do not.
They should make that very clear on their packaging and in their example wiring diagram.
 

Thread Starter

Elliott Balsley

Joined Aug 31, 2016
21
I wired this up and found that it all works great when the dimmer is fully up. But with the switches turned off, they illuminate when I dim it down, and they get brighter the further I dim it.

I think I was wrong about the PWM. This car actually uses a "floating ground" for the dimmer, so at full brightness that wire carries 0V. And as you dim, it increases to a maximum of 9V which makes a 3V difference to the high side. So when my switch is off and the dimmer is all the way down, the LED gets +9V on the cathode while the anode is connected to ground through the relay coil. I'm not sure why this is happening, since the relay coil has a diode built in to prevent backwards current flow.
 

sghioto

Joined Dec 31, 2017
2,363
when my switch is off and the dimmer is all the way down, the LED gets +9V on the cathode while the anode is connected to ground through the relay coil.
Well somethings not wired correctly. There's no way a LED can illuminate when the cathode is at a higher voltage then the anode. Measure the voltage on both sides of the LED to confirm.
I wired this up and found that it all works great when the dimmer is fully up.
Wired what up exactly?
SG
 

Thread Starter

Elliott Balsley

Joined Aug 31, 2016
21
It's basically just like the diagram posted above by mvas, but I have three such switches, so apparently there is some unexpected interaction between them. The common pole of all three switches is fed by +12V from the IGN relay. All the cathodes are wired together to the dimmer (a.k.a. variable ground). On each switch, the LED anode and the NO pin are connected together and feed the coil of a relay, which then goes to ground.
 

sghioto

Joined Dec 31, 2017
2,363
But with the switches turned off, they illuminate when I dim it down, and they get brighter the further I dim it.
The only way I can see how this can happen is if the switch is illuminated by a lamp not a LED.
In the circuit below you can see the current flow when the switch is off by the red arrows. When the dimmer is at 0 volts or max brightness the lamp will be bypassed. At max dim or 9 volts the lamp will be on through the other dash lights.
SG
EEE subaru dimmer.PNG
 

Thread Starter

Elliott Balsley

Joined Aug 31, 2016
21
Hmm that makes sense, but if it's a lamp then I should be able to measure resistance across it. With nothing connected to the switch, resistance across the "LED" is infinite in both directions. If I use the diode check on my meter, it flashes 1.9v for a moment then shows infinite, same on both directions. I don't know what to make of that.
 

Thread Starter

Elliott Balsley

Joined Aug 31, 2016
21
Here is the full diagram; let me know if this makes sense. Factory car parts are shown in peach color. Thick lines represent bus bars. The relays and fuses are contained in a Bussmann RTMR box. The Subaru Crosstrek Hybrid has two 12v batteries — one to start the engine and the other for everything else. I've connected the winch solenoid to the the engine start battery, and everything else to the aux battery. Both batteries are grounded to the chassis.
The front LED is controlled by the factory fog light switch. The reverse LED will come on when the button is on and the transmission shifts to reverse. The winch can be controlled from either the cabin or a wired remote in the engine bay.

I didn't draw this because I can't see it, but I'm guessing there is a resistor inside the buttons because I measured the forward voltage at 12.1v with 14.9mA.

subaru2.png
 
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mvas

Joined Jun 19, 2017
538
Typically, LED's do NOT like to be reversed biased.
And they do NOT light up when reversed biased.

My understanding of the "problem", as wired ...
The new LED's Illuminate perfectly - Bright & Dim exactly like dash lights when the New Switch is ON.
But in all new Switches that are OFF, the the internal LED illuminates inversely / backwards per dimmer.
Ie "Back Fed" Power ?

So, is it possible that your new LED Illuminated Switches actually contain a Bi-Directional LED ( two LED's back-to-back )
so that one of the two internal LED's illuminates regardless of polarity?

If true, then add a 1N4001 ( or equiv ) in SERIES with your internal LED Lamp ( light-blue ) wire.
This will allow the Internal LED to illuminate only when the New Switch is ON but
prevent the Internal LED from illuminating when the New Switch is OFF - ie Stop backfeed from Instrument Lights.
 
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sghioto

Joined Dec 31, 2017
2,363
So, is it possible that your new LED Illuminated Switches actually contain a Bi-Directional LED ( two LED's back-to-back )
so that one of the two internal LED's illuminates regardless of polarity?
That is exactly what I was thinking.
Easy way to test is connect 12 volts to the LED section in both directions.
SG
 

Thread Starter

Elliott Balsley

Joined Aug 31, 2016
21
You're right! The two push buttons light up both ways. This makes me more angry at the seller for not including a diagram. I added a diode before the dimmer but that didn't help because they were still sharing current. So I added a diode on the cathode of each button (not the rocker switch because it doesn't light up backwards). Now it's mostly working, but I have one remaining problem. I've narrowed it down a little but can't figure out what's happening.

I disconnect all the output relays and the dimmer wire, so the only wires connected to my panel are the backup relay and the IGN relay. With the transmission in park, the backup relay is shorted to ground. So now I expect that nothing with light up, since the cathodes leading to the dimmer wire are open and they each have diodes. When I push the hazard button and the backup light button, they both light up. Do I need to add a diode on the wire leading to the backup relay? I don't understand how the current is flowing here.
 
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