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Discussion Starter · #1 ·
I want to tap off my dome light, which opens anytime I open the door, in order to provide additional lighting throughout the car (trunk, feet etc).

I do not want to tap directly into the system, as the fuse won't be able to handle it, so I was wondering if I could essentially tap the relay circuit only into the dome light circuit and use the signal from the relay to power the lights, which will otherwise be connected to the battery with the appropriate fuse.

any input?

SCEMATIC.png
 

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Circuit looks good as long as the relay doesn't pull too much amperage.

But-
 
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You can certainly do this. Just add a resistor inline to the new relay. Use this;

Volts/Amps = Resistance
12 / amp rating of the relay coil = resistor value.

So, for a typical relay that requires 40 to 60 milliamps to actuate the coil;

12 / 0.05 = 240 ohms.

It is not necessary to be super exact. Just install an inline resistor from 230 - 250 ohms. Note; common resistor values in this range are: 240, 249, & 250. You are going to want a 1W rated resistor.
 

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It's hard to believe that you could pull so much current with a LED strip that you would need a relay at all. Most strip lights pull 14 watts per meter (1 amp) and you'll be using what? 4" to 6" pieces wired in parallel?

As for the relay, your wiring is correct. However the dome circuit on my '15 is on PWM circuit so your relay may not work as planned without a capacitor.
 

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i'm adding MORE lights. specifically, LED strips.



can anyone comment on using LED strips in series or in parallel?
Either way, you need to limit the current like I showed you. Your initial question was how to pull an input signal from an interior light.
 

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Yeah, there is PWM right before the TIPM switches off the light. I don't think it matters for LEDs - the LED will just flicker - but it may give the relay kittens depending on the frequency. The frequency is probably high enough that the relay would just shut off at a certain point.

There are 20W of incandescent bulbs on a 15A fuse, but the TIPM may have a current limiter. Like bobchase said, you'd be ok for around 2 meters of LEDs probably. What are doing? Running an indoor disco?
 

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Discussion Starter · #12 ·
Yeah, there is PWM right before the TIPM switches off the light. I don't think it matters for LEDs - the LED will just flicker - but it may give the relay kittens depending on the frequency. The frequency is probably high enough that the relay would just shut off at a certain point.

There are 20W of incandescent bulbs on a 15A fuse, but the TIPM may have a current limiter. Like bobchase said, you'd be ok for around 2 meters of LEDs probably. What are doing? Running an indoor disco?


It MAY be 2 meters total.

BTW I already switched out the incandescent bulb for LED so I'm probably saving even more wattage there.

If that's the case, maybe I just limit myself to 2 meters and call it a day. ¯\_(ツ)_/¯

Thoughts? Btw thanks for the feedback. I'm a mechanical engineer by profession but haven't had to deal with electrical work since college (somehow)


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