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Forum ยป Pls advice about volts, resistors n shiet (reps)
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Thread: Pls advice about volts, resistors n shiet (reps)

post 1687459403 07-30-2023, 10:36 AM
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Pls advice about volts, resistors n shiet (reps)

I fitted some LED rear lights to a trailer. A couple of the indicator LEDs are faintly lit constantly (even when vehicle lights are off), although I can't find any power to them with the multimetre. I have an idea to add some resistors to the indicator wire to draw that power down and keep the LEDs off. However I have no idea how many ohms n shiet I need.
Power supply is 24V.

I thought it might be the hazard warning light switch not working properly, but I unplugged that and they're still lit.

Will rep for knowledge
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post 1687459733 07-30-2023, 10:44 AM
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post 1687459963 07-30-2023, 10:50 AM
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Check your ground connection. Is it grounding to the trailer frame. If so clean the chit out of those connections.
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post 1687460293 07-30-2023, 10:57 AM
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LEDs already have resistors to limit current, usually the LED will have a voltage drop around 1V. You would want to figure out how much current you want the LED to get, and divide 23V by that. That will the resistance of the resistor since it will have the same current as the LED it's in series with. Although the resistance already there would complicate that a bit of course, you'd need to factor that in.

If you don't want the lights on though, why even leave them connected in the first place?
post 1687460713 07-30-2023, 11:09 AM
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In just to see what the solution is in case I ever run into something similar
post 1687460723 07-30-2023, 11:09 AM
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Originally Posted By yewotm8⏩
LEDs already have resistors to limit current, usually the LED will have a voltage drop around 1V. You would want to figure out how much current you want the LED to get, and divide 23V by that. That will the resistance of the resistor since it will have the same current as the LED it's in series with. Although the resistance already there would complicate that a bit of course, you'd need to factor that in.

If you don't want the lights on though, why even leave them connected in the first place?
Some of the indicator (blinker) LEDs are faintly lit all the time, even when vehicle and lights are turned off.

No idea how much current the LEDs are drawing tbh. I'll take the multimetre and get some more info.
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post 1687460803 07-30-2023, 11:10 AM
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Originally Posted By tvr123⏩
I fitted some LED rear lights to a trailer. A couple of the indicator LEDs are faintly lit constantly (even when vehicle lights are off), although I can't find any power to them with the multimetre. I have an idea to add some resistors to the indicator wire to draw that power down and keep the LEDs off. However I have no idea how many ohms n shiet I need.
Power supply is 24V.

I thought it might be the hazard warning light switch not working properly, but I unplugged that and they're still lit.

Will rep for knowledge
Just buy whatever is smallest.
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post 1687461093 07-30-2023, 11:17 AM
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Most likely a short.


Are you wired directly to battery? Does trailer have its own. In circuit you can see bleed
Over from truck battery.
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post 1687461163 07-30-2023, 11:18 AM
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Ok so there is a 0.01A amp@3.5v current draw on the indicator (blinker) wire. This increases to around 2.3 amps @24v when you actually turn the indicators on.

There is no draw on any of the other wires (side lights, brake lights etc) when the lights are off.

The fact that there is actually a supply of 3.5v when there should be nothing seems to be the problem. But what is causing it? is confusion
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post 1687461263 07-30-2023, 11:21 AM
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Originally Posted By multiplemiggs⏩
Check your ground connection. Is it grounding to the trailer frame. If so clean the chit out of those connections.
No it grounds through the plug on the vehicle it's hooked up to. I cleaned out the connectors on the plugs and no change. They weren't dirty.

Trailer has no battery.

The wires run right from the plug at the drawbar all the way to the lights on the rear with no interruptions
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post 1687461333 07-30-2023, 11:22 AM
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Originally Posted By tvr123⏩
Ok so there is a 0.01A amp@3.5v current draw on the indicator (blinker) wire. This increases to around 2.3 amps when you actually turn the indicators on.

There is no draw on any of the other wires (side lights, brake lights etc) when the lights are off.

The fact that there is actually a supply of 3.5v when there should be nothing seems to be the problem. But what is causing it? is confusion
It's not a problem, it's made that way for one or two reasons, either it's just cheaper that way, or it's for the incandescent lights to always consume a bit of power as a way for the system to check if the bulb is burned out or not.
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post 1687461403 07-30-2023, 11:25 AM
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Originally Posted By Godfrd824⏩
It's not a problem, it's made that way for one or two reasons, either it's just cheaper that way, or it's for the incandescent lights to always consume a bit of power as a way for the system to check if the bulb is burned out or not.
Ah yes, truck has a message on the dash that shows up when a bulb is gone. So that's what's causing the issue then ? How to fix?

Yes the rear lights on the trailer were previously incandescent. Since I've changed them to LEDs I've had this problem.
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post 1687461643 07-30-2023, 11:30 AM
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Not quite sure what you need and why you need resistors. Dim lights, usually a bad ground but could be power too. Voltage drop testing is the nominal way. While it is under load, measure the voltage from the source to where the load starts. You should read under a volt of drop, honestly less than that, maybe half a volt. Then measure from the ground side of the LED's to the battery ground, voltage drop should be 0.2 or so, preferably less.

Here is a good resouce:

https://www.techshopmag.com/electric...kes,12%2F2%3D6).
post 1687461723 07-30-2023, 11:32 AM
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Originally Posted By Godfrd824⏩
It's not a problem, it's made that way for one or two reasons, either it's just cheaper that way, or it's for the incandescent lights to always consume a bit of power as a way for the system to check if the bulb is burned out or not.
no.
there should be no power going to the lights when your car is off.

opie, are you using actual trailer lights or did you just buy some leds off amazon?
why are you self powering the leds?
why wouldn't you get 12v from the trailer harness coming off the vehicle?
actual led trailer lights hook up just like incandescent bulbs.
you don't have to futz with external resistors.
post 1687461813 07-30-2023, 11:34 AM
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Originally Posted By tvr123⏩
Ah yes, truck has a message on the dash that shows up when a bulb is gone. So that's what's causing the issue then ? How to fix?

Yes the rear lights on the trailer were previously incandescent. Since I've changed them to LEDs I've had this problem.
Oh I see, you switched to LED's which is causing issues. Look up "bulb resistors for switching to LED" and amazon sells them. Either that or get the factory bulb and measure resistance and then the LED resistance, subtract those and then install that resistance level
post 1687463073 07-30-2023, 11:56 AM
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Originally Posted By NitrogenWidget⏩
no.
there should be no power going to the lights when your car is off.

opie, are you using actual trailer lights or did you just buy some leds off amazon?
why are you self powering the leds?
why wouldn't you get 12v from the trailer harness coming off the vehicle?
actual led trailer lights hook up just like incandescent bulbs.
you don't have to futz with external resistors.
Conflicting opinions...

Actual led trailer lights
What do you mean "self powering". The trailer is just plugged into the lighting socket at the back of the truck.
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