Go Down

Topic: LED glows when it should be off (Read 773 times) previous topic - next topic

Helicopter12

May 24, 2019, 04:36 am Last Edit: May 24, 2019, 05:41 am by Helicopter12
Hello,

I am having an issue in my car where the LED light I put in will glow very dim even when the car is off and there should be no power to it. The same thing is happening to the third brake light on the back of the car, it will glow very faintly when it's supposed to be off but work normal under full power.

I assume there is some sort of voltage leakage that is causing this to happen. I know diodes require just about 0.7 volts to conduct so I assume LEDs are the same way and there is at least 0.7v being supplied to them when they are "off". I also know LEDs change their brightness with the change if current and so I was thinking a diode in series would drop the voltage enough to have the LED turn completely off. Does anyone have any advice or ideas? This is a 12VDC system with an alternator that can add transients and boost the voltage to over 14V, but even with the car not running this happens.

Thanks

Coding Badly

Does anyone have any advice or ideas?
Yes.  Determine the source of the problem so you don't end up with a damaged or dead battery.

That, and post a schematic.


Koepel

#2
May 24, 2019, 08:15 am Last Edit: May 24, 2019, 08:16 am by Koepel
If you have 25 minutes, perhaps this video might be helpful: https://youtu.be/2yFh7Vv0Paw

wvmarle

I suspect there's an error in either your circuit or your code.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

ReverseEMF

#4
May 24, 2019, 05:29 pm Last Edit: May 24, 2019, 05:35 pm by ReverseEMF
I am having an issue in my car where the LED light I put in will glow very dim even when the car is off and there should be no power to it. The same thing is happening to the third brake light on the back of the car, it will glow very faintly when it's supposed to be off but work normal under full power.
Was that happening to the "third brake light" before you added your "LED"?  Or is that something that showed up after?

I assume there is some sort of voltage leakage that is causing this to happen. I know diodes require just about 0.7 volts to conduct so I assume LEDs are the same way and there is at least 0.7v being supplied to them when they are "off".
You have the right idea, about how LEDs behave like diodes, but the forward voltage on an LED is a bit higher.  For example here is a short list of Forward Voltage values, typical for different kinds of LED:
- IR: 1.1V
- Red: 1.7V
- Super Bright Red: 2.1V
- Green: 2.3V
- Super Bright Green: 3.3V
- Blue LED: 3.0V to 3.4V

Yes, they are a type of diode [though rarely, if ever, used in that way], but no, their Forward Voltage is nothing like "0.7V".

I also know LEDs change their brightness with the change if current and so I was thinking a diode in series would drop the voltage enough to have the LED turn completely off.
Even if that works, is it going to fix your brake light, too?  I would be more interested [even worried], in what's going on with that brake light -- doesn't sound normal.  But, I /we need an answer to my first question [above], to even begin to ponder those particulars.  In fact, we need a LOT more information, as other's have already indicated. 

To learn more about how you can help us, to help you: Have a look at: How To Use This Forum
"It's a big galaxy, Mr. Scott"

Please DON'T Private Message to me, what should be part of the Public Conversation -- especially if it's to correct a mistake, or contradict a statement!  Let it ALL hang out!!

Helicopter12

The interior lamp worked with the incandescence bulb from the factory, but switching to the LED it did not. Clearly this is because there is a 7V across it even when the doors are both closed (seem image below). The tail light remains untouched and I never noticed it in the past so I can't say for certain if it is a new issue or an old issue. From the factor I believe the third brake light is a LED. The schematics below seem super straight forward and so what I believe might be happening is one of the door switches is bad. The light should be floating with both of the doors closed, and then connected to ground when one of the doors open, thus completing the circuit. I am going to take a few more measurements later today and double check there is a solid ground and 12V on the other pin.


Interror Light glowing slightly showing 7v with both doors closed:


Brake Light Schematic:


Interior Lamp Schematic:

Paul__B

Water (and grease) in the door switch.

MrMark

There might be a parasitic and moderately high resistance path to ground through the A11 pin on the "Alarm Unit (M99T2)"  One could test this by removing or disconnecting that unit.

Helicopter12

Hey guys,

I checked the door switch connections and they were fine, but just to be sure I hit them with some electrical cleaner. I measured the voltage across the leads of the door switch and got 4.7V, when it should be 0V, see image below;

Door Switch Reading:


Light Reading with Door Switch Unplugged:


Light Resistance to Ground:



I found this video which is the exact issue I am having with my tail light and also the interior light. This seems to be a common problem and this guy is fixing it by putting a resistor is parallel with the light.

https://www.youtube.com/watch?v=OR04EWcibYA

My original though to fix the interior light was also a parallel resistor of 1k - 10k ohm, or two series diodes. Which do you guys see fit and why?

JohnRob

I did not study the schematic but your vehicle might have a monitoring circuit to sense if the bulbs functional.

A quick test would be to leave the LED and Bulb out for some time an see if the vehicle complains.

Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

Helicopter12

I do not see any monitoring circuit in place so it seems like there is a voltage leak somewhere. I am still between using diodes or a parallel resistor.

TomGeorge

#11
May 30, 2019, 12:44 pm Last Edit: May 30, 2019, 12:44 pm by TomGeorge
Hi,
Can you show on the diagrams you posted, EXACTLY where you connected your extra LEDs including the polarity of the LED?

Thanks.. Tom.. :)
Everything runs on smoke, let the smoke out, it stops running....

Go Up