Any electrical circuit gurus?

[CAMAN]

Long story short the bulb holder for the dome light in my truck melted, so instead of paying who knows how much to get a whole new dome light assembly I decided to use the existing frame and lense and make an LED array for it. It works great when on but whenever it should be off it stays on at about 10-20% brightness. Somewhere in the circuit there is a trace of power leakage, but instead of going through the entire circuit to find the leak and possibly screwing something else up I'd rather add something to the light.

My question is, is there something (small circuit, module, etc) I can install into the light that would completely cut the power to the light but turn on fully when powered on?

I know I can install a small relay, but I'd have to run a constant line to the fuse block and I'd like to prevent that if at all possible. I have an above average amount of knowledge of electrical circuits, just don't know of anything that would do this.

Thanks for any information you can give!

[Macgyver55]

Check with your local Radio Shack for a Zener diode. You'll have to figure out what size, possibly they can help you if you can tell them what the residual voltage amount is. It should stop them from glowing (which is actually hard on LED bulbs) Let us know what you find out, I've never used one myself but it should work in theory.

[delcecchi]

As an actual circuit guru, sort of, I would wonder first what the voltage and current looks like when in the "on" and "almost off" condition.

[low277]

As an actual circuit guru, sort of, I would wonder first what the voltage and current looks like when in the "on" and "almost off" condition.

I agree, the "almost off" or 10-20% glow may be what shorted/melted your original dome light.

[Macgyver55]

Without knowing the make/model and year, I guess I may have jumped ahead a bit too soon. I should have asked if it stayed glowing with the standard lightbulb? If not, it is my belief/assumption that the socket probably melted simply due to heat from the original bulb. (which is not unheard of) I also should have asked if the LED light stayed glowing indefinitely, or went off after a longer time.

It is my theory that the glowing is possibly due to the way that the body control module dims the lights. (if it indeed has that option)

Body control modules work in many different ways, (usually FM which stands for "frikin magic" to those of us in the trade) and it may not be sensing the lower amount of draw on the new light correctly, or that they are using a capacitor and the lights dim as they slowly bleed off the stored voltage and the LED takes much longer than a conventional bulb.

[CAMAN]

I will check the voltages today.

And no it probably wasn't what melted the original. The original had a halogen light (which gets hot) and a plastic frame which was "welded" to two pieces of steel which powered and held the bulb. In the picture you can see a few round gray dots on the steel bulb mounts, those are the welds that melted and released the bulb mounts, the back of the plastic frame also melted so fixing it with some rivets would have been difficult.

[CAMAN]

No it didn't glow with the original bulb. And from what I can tell (keep forgetting to check before hitting unlock while walking up to it at night after a long rest) the light remains on indefinitely. But will try and remember to check tonight.

And it's a 2005 Chevy Colorado

[CAMAN]

12.3v DC when on and 6.9v when off. That is considerably higher than I ever expected!

[CaptainMusky]

I like your idea of the LED bulb in there, but I wonder if there are variances in bulbs and what their "minimum" voltage is. Since you didnt seem to have this issue with the old bulb my thought is that it required the full voltage. IDK if that even makes sense, but an idea.

[delcecchi]

For the moment we are out of circuit land and into car land. Could it be that the car guys decided to run a small amount of voltage to keep the filament warm, but not glowing detectably in order to reduce the surge of current that would otherwise occur? (a really cold bulb will have a resistance much less than the same bulb after it has lit up)

If that is a design feature, it would be possible to design around it for the leds which don't have that characteristic.

[CaptainMusky]

Yeah Del, that is along the lines I was thinking, just probably didnt explain it well.

[delcecchi]

I posted this on the Automotive forum, where it probably belongs but what the hey, and got this response....From an actual technician..

Quote:

The BCM supplies power and ground to the dome lamp. There should be 12 volts on the orange wire (not sure if it is supplied all the time) and ground on the white wire (again not sure if it is grounded all the time). The BCM I'd either supplying voltage when the dome light is requested or supplying ground when the dome light is requested.

The BCM may be seeing something wrong with the circuit because of the low draw of the LED lamp. Have you tried a normal bulb to see if it works properly.

Or let us know what side still has power on it. Pull the bulb and see if there is 12 volts on the positive side and see if there is a ground.

The other thought is that this may be a dimming system where the dome light slowly dims out until it turns off completely. If this is the case than the LED may be interfering with the process.

Owner - Half Empty Graphic Design

ASE Certified Automotive Technician - Paramount Auto Service, Hastings Goodyear

13 year member of Hot Spot Outdoors, 1/17/01 - 1/17/[email protected]

[CAMAN]

Thanks Del, I just responded to Jeremys post in the automotive forum as well. I was debating placing this in the auto or equipment forum, but since I was just looking for something to control the light and wasn't necessarily an automotive question I placed it here, and now it has morphed into a vehicle issue.

Here is what I replied in the other thread. ..

Quote:

No it's not a dimming system, normal bulbs function properly. LEDs are just more sensitive and are able to stay lit at lower voltages.

Just checked with everything disconnected and light OFF; 2volts between the white and orange, 0volts between white and ground, 12 volts between orange and ground.

Light ON; 12 volts between white and orange, 0 between white and ground, 12 between orange and ground.

Now I'm he11a confused.....

[delcecchi]

Back to circuit land. Try putting a resistor, maybe a couple hundred ohms in parallel with the LED. That way if there is a little leakage it will go through the resistor and not the LED.

What kind of LED did you use?

[CAMAN]

I just had some strip LEDs leftover from a project at work. Ran 4 strips parallel to each other on a piece of acrylic and soldered them together in series.

May not be the most professional looking if you take the lens off but for zero cost and an hour of my time they are about 10 times brighter than the old light.

I checked out the Zener diode and a few diagrams and can't really understand how or if they would work.

Would placing a standard diode on the neutral wire prevent the back flow that is now taking place? As my understanding of them is they only allow electricity to flow one way.

[delcecchi]

Strip LEDs? Like are used for decorative type lighting and in fish houses and stuff?

An LED is a diode so backwards polarity shouldn't make light unless they put two in, back to back.

[CAMAN]

Strip LEDs like the flexible ones with adhesive on the back.

And yeah I understand an LED is a diode, I was just thinking maybe these strips are designed without a polarity and allowing them to light up from the neutral being powered.

I have a bunch of single round LEDs sitting around here, I may just experiment with one to see if it fixes the issue.