bulb voltage regulation

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rorschach

Member - Posts: 5

Joined: April 12, 2009

Location: Estonia

Posted: April 12, 2009 at 7:23 AM / IP Logged

Hello.

I've got a bulb in my taillights that functions as a parking light (always on) and a brakelight. Since I already have two other separate parking lights, I want to modify the third bulb to only work as a brakelight. Basically, the bulb is always on and it constantly gets ~3.5V and when I press the brake down, the voltage jumps to ~12V and the bulb gets brighter. I need some sort of a relay or switch that I can add to the circuit so that only when the voltage is over a certain value (since it gets ~3.5V as a parking light, I

d say something like over 4-5V would be good), the current gets through to the bulb.

i am an idiot

Platinum - Posts: 13,674

This member consistently provides reliable information

Joined: September 21, 2006

Location: Louisiana, United States

Posted: April 12, 2009 at 8:24 AM / IP Logged

I am pretty sure that a Bosch\Tyco relay will not energize at the low voltage state. It may not disengage when the brake is released. You will have to try one to see if it will work for your need. Simply connect the wire to terminal 86 of the relay. Then ground terminal 85. Have someone inside the vehicle and see if you can hear or feel the relay energize and de-energize as the brake is applied and released. If it does respond properly let me know. It will be a simple diagram.

I found this spec on one of their relays. The must Release voltage is 1.2V. I am pretty sure it will release at well above that. I am just not sure if it will release below 3.5

VF4-**F**-** 12 90 Must Enengize at 7.2V Must Release at 1.2V 22 17

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: April 12, 2009 at 9:34 AM / IP Logged

What kind of car is it?

3.5vdc parking light? Are the parking lights wired in series? If you remove one parking light it may cause more voltage to go to the other parking lights.

This would be a great application for an op amp comparator - I am an idiot posted a diagram for this a few weeks bach (it was for a low fuel light). Set the reference votlage to >6vdc and you'll be set.

Kevin Pierson

i am an idiot

Platinum - Posts: 13,674

Joined: September 21, 2006

Location: Louisiana, United States

Posted: April 12, 2009 at 10:23 AM / IP Logged

With the voltage he is getting on the bulb, they would have to have 4 lamps run in series. I find it hard to believe that they would have done that. If one goes out all quit working. Just to check, if you remove one of the lamps, does the other one continue to work?

rorschach

Member - Posts: 5

Joined: April 12, 2009

Location: Estonia

Posted: April 12, 2009 at 12:59 PM / IP Logged

The car is an E46 3-series BMW.

The tail light looks like this:

bulb voltage regulation -- posted image.

The connector from the car's wiring harness to the tail light looks like this.

Pin 1 - White with a brown stripe - Ground for the interior trunk light

Pin 2 - Blue with a green stripe - Positive for turn signal

Pin 3 - Brown - ground for parking and brake lights

Pin 4 - Red with a yellow stripe - Positive for the interior trunk light

Pin 5 - Gray with a purple stripe - Positive for parking light only

Pin 6 - Black with a green stripe - Positive for the combined parking and brake light

From what I can gather, it looks like the setup is something like this: from the Light Control Module, the wiring harness has a separate + for parking lights and brake lights. Somewhere in the body, the parking light + is split in two, one goes straight into Pin 5 (which powers the two bulbs in the lower red portion as *only* parking lights), the other one is *combined* with the brake light + and runs into Pin 6, which powers one bulb at the top red portion. That bulb, depending on whether I press the brake or not, has either ~3.5V or ~12V running into it.

I checked, if I take out the brake/parking light bulb (top red portion), the one on the other side keeps working like nothing has happened.

So I'm thinking if I put the relay right before the tail light connector between Pin 6 and the wire that goes into it, the relay should only affect the bulb on one side of the car (so I would need two relays, one for both sides).

Would one of the first two in this list work?

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: April 12, 2009 at 1:14 PM / IP Logged

So the bulbs aren't in series.

If the bulb is completely removed are there any "bulb out" indicators?

There isn't enough information given in the link you posted - you need to know the drop out voltage of the relay.

I wasn't able to find any relay that had a "drop out voltage' higher then 3.5vdc - it may be tricky to find one. The op amp circuit would eliminate that problem.

Kevin Pierson

rorschach

Member - Posts: 5

Joined: April 12, 2009

Location: Estonia

Posted: April 12, 2009 at 2:14 PM / IP Logged

Damn, I hadn't considered the indicator. Yes, when I take out the bulb and turn the car on, the LCM checks if the bulb is OK and then throws an error. Could the relay or an op-amp circuit give enough feedback to the LCM to trick it into thinking the bulb is OK?

rorschach

Member - Posts: 5

Joined: April 12, 2009

Location: Estonia

Posted: April 12, 2009 at 2:59 PM / IP Logged

Since I can't edit my posts..

I was thinking about this and came up with another idea. The car also has a separate brake light behind the rear window near the roof. Another possibility (if the indicator is a problem) is to just tap the brake light + from there (granted if I can get to it) and run it all the way to the tail lights. In that case, the only problem I can foresee is that the original parking/brake light + that goes into Pin 6 right now, would be left hanging and the LCM will still throw the bulb out error. But that should be easily fixed with a simple resistor to trick the car into thinking the bulb is there, like you you do with LEDs, no?

i am an idiot

Platinum - Posts: 13,674

Joined: September 21, 2006

Location: Louisiana, United States

Posted: April 12, 2009 at 3:55 PM / IP Logged

The 90 ohm coil of the relay may be enough of a load to trick the computer.

You can get a relay from any stereo shop for way less money than they want for those. 15 dollars in my opinion is outrageous.

rorschach

Member - Posts: 5

Joined: April 12, 2009

Location: Estonia

Posted: April 12, 2009 at 4:25 PM / IP Logged

The site is in English, but the prices are still in kroons, the currency here. Divide the price by 10 and you get the price in dollars. So the first 12V relay is actually about $4. I guess I'll just stop by the shop tomorrow, buy one and test the system.

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