VW Rear Bulbs

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VW Rear Bulbs

Postby Peter704070 » Fri Sep 09, 2011 8:43 pm

Hello

This is not exactly to do with a Picoscope but I hope to use the scope once I understand how the system works. Many new cars are using a single filament bulb on the rear, it does the rear light and brake light. I remember being told that these bulbs are controlled by Duty Cycle (or OCR) I understand the basic principle of duty cycle but would like to know how this makes the bulbs dimmer (when the lights are on, but no brake) and brighter when the brake lights are on.

Does the voltage remain the same (12v) or does that change. Any help would be great.
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Re: VW Rear Bulbs

Postby Robski » Fri Sep 09, 2011 9:52 pm

I should imagine that the supply voltage will remain NBV but the ground will be PWM which will be able to control the brightness of the bulb :)
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Re: VW Rear Bulbs

Postby Alan » Mon Sep 12, 2011 5:09 am

The reason for the change to the single filament bulbs is mostly to save weight and cost by running one less wire from the front of the car to the lights at the back. Its something we are going to see more of (although the single filament bulbs are on their way out as LEDs take over).

I am not sure which approach is used in your VW as there are three I know of.

1. To switch the brake light on full battery voltage is used, for just the rear light the body computer outputs a lower voltage (about 8V from memory?).

2. In some multiplexed systems there will be a single power feed to the light cluster and either a CAN bus or LIN bus node to switch lights. This typically can also feedback diagnostic messages - I have an Audi with LED rear lights which decided to fill with rain water and gave lots of error messages. A tiny hole drilled to drain the water worked as a temporary fix (actually that was two years ago and I still have not got round to replacing the cluster so that rather stretches the definition of temporary).

3. A PWM signal is used. If 12V is applied all the time then the bulb is at max brightness. If its switched on / off rapidly then the bulb is dimmer. The switching needs to be done fast enough that the filament in the bulb does not have time to cool down too much otherwise the on/off effect would shorten the life of the bulb. If a capacitor is used to smooth the voltage this would avoid the stress on the bulb and reduce the possibility of electrical noise being generated.

The best thing to do on your VW is to look at the signals with the PicoScope. Please report back if you find anything interesting.
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Re: VW Rear Bulbs

Postby dgmotors » Fri Nov 25, 2011 4:57 pm

As a mk5 golf was to hand when I had my scope setup I took some readings for you.

It uses a fixed earth by the rear lamp and switches the 12v supply.
I used a back probe into the plug on the right rear lamp and used the earth point by the lamp,
Here are the screen shots:
Going from brake light to sidelight
G5Stoptotail.JPG
VW Golf mk5 taillight, Going from brake lights to sidelight


and a full screen shot of the sidelight output.
G5sidelight.JPG
G5 tail light, ow they dim a 21w bulb to look like a 5w bulb
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Re: VW Rear Bulbs

Postby dgmotors » Fri Nov 25, 2011 5:16 pm

We had a mk5 golf in today so while my scope was setup i took some readings,

It uses a perment earth and switched live,
The scope was backprobed into the rear of the right rear tail lamp and the used the earth by the right rear lamp.

A picture of the trace going from brakelight to sidelight:
G5Stoptotail.JPG
G5 tail light feed, Going from brakelight to sidelight


And a trace off just the sidelight feed:
G5sidelight.JPG
G5 tailight, Making a 21w bulb dim to used as a sidelight
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