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Need some help, car LED dome light related

raymcshake

Jun 17, 2016
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I'm a trained electrician, but over here (Norway) electricians rarely work on specific electronics components, mostly complete pre assembled products, so I've sadly forgotten most of the small electronics knowhow I got from school. Hope some of you can help me with this.

I've swapped out the incandescent bulbs in the dome light in my car for LED plates from eBay with 48pcs 5050 SMDs per plate. Being eBay stuff, there's not very much information or specs, least of all watts, amps and the likes.

The car is from 2007 and every module in the car is on a CANBus network, meaning when the car is running and the network is active, the LED plates glow dimly. This irritates me something fierce. I've been led to believe I can solder in a resistor either in series or parrallel with the LED plates to solve this, but my electronics skills are like I said, next to irrelevant. Any help here would be greatly appreciated. What ohm value would the resistors need to be, and in series or parrallel? What specs would you need on the car or LED plates to calculate this?

Voltage drop when LED plates are lit? Supply voltage? Amp pull?

Appreciate any help.
 

shrtrnd

Jan 15, 2010
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There's not much information here, and apparently you don't have much information there, on what you bought.
It SOUNDS like you've got a voltage regulation problem (current is also involved). What people are suggesting to
you is a also an uniformed suggestion (with no good input information), to try to stabilize the current though your LEDs, by
using a resistor to provide a stable load to the complete circuit.
The problem I see here, is that we don't know what-all is in your 'LED plate'. It may just be made-up of LEDs, it may be
composed of LED's with their own dropping resistors, or there may be some other circuitry in there that we don't know anything about. (As in: maybe the plates were designed for some specialized system, and not for your application)
The LED's themselves, because there are so many of them, might be drawing excessive current in your application, causing the dimming effect.
You asked about calculations, and maybe someone here will look for information on the specific SMD LED's you listed.
What I would do first. is (you're an electrician) put an ammeter in-line with the LED module, and find-out what the
current through the LED plate is.
What my mind keeps going back to, is that because we don't know if there's any circuitry in-with the LED plate, I
wonder if any circuitry in there might be interfered with by other electronic systems in your car.
In this case, and maybe somebody else on this site has a better idea, I would find out the current draw of the
LED plate, and especially if you are using multiple LED plates in this circuit.
Somebody here will hopefully have additional helpful input, but for starters, that's mine.
 

raymcshake

Jun 17, 2016
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Thank you, your reply is much more specific in terms of procedure than replies I've gotten elsewhere. Gonna do the ampmeter today, as normal ohms law / watts law don't apply here (5050SMDs are by Wikipedia 0.24W each - which multiplied by 48 (SMDs on the plate) rolls out at over 11A), so I'll do the measuring.

The plates are intended for the purpose, as they came with a Festoon-style connector to plug right into the dome light. However, ebay, cheap manufacturing and so on are unknown variables, almost always.

I'll get back to you, thank you very much.
 

shrtrnd

Jan 15, 2010
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You sound like you're well aware of the variables here.
I don't know your car. Is there any type of electrical system, electronically/computer auto-protect circuitry involved.
(As in: is there some type of crowbar circuit that would kick-in to limit current flow in an overload condition?)
It's an oddball thought, but I try to do the simple elimination guess-work first, then narrow the troubleshooting search.
 

raymcshake

Jun 17, 2016
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Well, this light is run off a CANbus module called UEM (Upper Electronics Module), the car is a '07 Volvo. OEM bulbs are 12V/15W. I would assume there are overload protection in the various modules, but I can't say for sure - there are four or five fuse boxes around the car, but I'm pretty sure they're there to cut the power in a short circuit situation, not a small overload

On my way home now, hopefully I'll get you some values soon.
 

shrtrnd

Jan 15, 2010
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So now we've got the UEM circuitry involved in this circuit.
Are the battery terminals to the car clean and corrosion free? The connecting power cables clean?
Right about now, I'd seriously consider taking one of your LED plates and connecting it (you're an electrician, so
I will highly recommend a fuse too) directly across the battery. Just to see if you get full brightness, or if there
just might be a problem with the LED plate itself.
Large trucks over here run on 24V, Cars at mostly 12VDC. Maybe your LED Plates were designed for truck lighting
at 24VDC?
 

raymcshake

Jun 17, 2016
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Ahh, I have screwed up in the first post.

The plates light up just fine when they are supposed to, when opening the doors or pressing the dome light switch. It's when they're not supposed to light up, that they glow dimly, when driving for instance. I've been told this is because the CAN network always have a small current/voltage flowing to monitor circuits - and that the incandescent bulbs just won't light up on this voltage. Sorry about that.

Home in a few (long drive today), I'll get right to it.
 

raymcshake

Jun 17, 2016
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Ok, this monitoring voltage is ca 7volts. The plates light up nicely under 12volts, when I hit the switch. My amp meter doesn't do inline measuring, but rather field measuring (it's a Fluke TS 1000), and it registers 0.2A @ 12V.

Funny thing, when I hit the sunroof switch, the LED plates go completely dark for a split second, when not set to light up the cabin. The sunroof motor pulls a high current, so I'm thinking the plates go dark because of the voltage drop when engaging the sunroof motor. Meaning, the 7V monitoring voltage is seemingly enough to light up the plates. Assuming my thinking is correct, should I solder in a resistor in series, to reduce the voltage across the LEDs, or in parallell? And what value resistor? Don't want to create a heat source either.
 

shrtrnd

Jan 15, 2010
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OK, whole 'nother problem.
In that case I'd side with some of the advice you got from those other websites.
Put a 1K ohm 1/2 W resistor across the LED plate.
If what I think is happening, is happening, that should do it without causing any heat problems.
If you do that at you still have LED dimly-lit-when-off problems, we'll need to revisit this problem and
consider other options.
 

raymcshake

Jun 17, 2016
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Thanks, I still feel your answers have been better and more thoroughly explained than other places.

So, 1k Ohm 1/2 resistor in parallel with the supply leads ( 12V and ground) to the LED plate?
 

shrtrnd

Jan 15, 2010
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Yes. I would try that and see if it works (it will probably have to be at night to be sure the LEDs are dark)
I don't know the proper terminology for the perfectionists out there, but I think you're seeing current leakage simply
due to the characteristics of LEDs (Which are Light Emiitting DIODES).
If this doesn't work, this will take more digging for the problem.
 

raymcshake

Jun 17, 2016
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Success! Got the 1kOhm resistors in the mail a few days ago. LED plates without resistor:



Resistor tested on one of the plates:



Completely dark. There has to be one output from the Upper Electronics Module to both these two and one dome light in the back seats, because adding this one resistor made all three go completely dark when switched off. Soldered in and shrink wrapped:





Completely dark when off, lights up like hell when on.






Thank you very much for your help. I drive alot at night, and this has been a real nuiscance.
 

shrtrnd

Jan 15, 2010
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Glad this was an easy fix, and that as electrician you knew how to modify the circuit with neat physical modification work. Sometimes it's difficult to troubleshoot problems remotely because people will tell you a specific fault they see,
and neglect to mention something important that is a contributing factor. It was nice talking to someone with an electrical background, that can take measurements and understands electrical circuits.
All your photos of your work are greatly appreciated. I'm sure others who encounter this problem will be glad
to read your post and how you solved the problem.
 

raymcshake

Jun 17, 2016
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Thanks, and again - I reallu appreciate your help. I built a stereo amplifier back in school and did at one point consider majoring in electronics, but as it happens I didn't. Went on to building installations, and one thing you learn quickly there is that if you find yourself with a problem you don't know how to solve, ask someone who does. Most of the electronics know-how beyond the basics I forgot a long time ago.

Gotta say, it seems like a grest forum, this. Most other forum regardless of topic is cluttered with stuck up knowitalls who don't know how to talk politely to people. Thanks again;)
 

shrtrnd

Jan 15, 2010
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I agree. I came here many years ago looking for information after checking several other forums.
The moderators here don't put up with insulting individuals, and we have a lot of knowledgeable people here.
I stayed on this forum because people want to help. So many others just want to make themselves feel important
by telling others that they're brilliant and the person asking the question is stupid. They can have it.
Showing us exactly how you built your modification will be very helpful to anyone else who comes here asking about
the problem you encountered, and that's a big plus for this forum. Thanks.
 
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