PWM signal for DRL in VW/Audi

[eddie_1]

I was wondering if any one had a good idea on what the waveform looks like on newer cars (but before LEDs) for driving the std. DRL bulbs? My understanding is that the DRL is driven by PWM (Pulse Width Modulation) on my 2008 Audi for example i.e. on/off signalling and the duty cycle is used to control the average power provided to control the intensity, for example 80% light. The issue is that there are many after market LED based bulbs now like ba15s that can be inserted in the same bulb socket. I tried a set of these but they died after a couple of months. I've seen many many people having the same problem depending on car type. The problem seems to be that the DRL signal is not readily usable for the LED array. The LEDs require a constant current source whereas the PWM is a switching voltage. When my LED failed I opened one up and saw there is a circuit board in front of the LED array. From the internet I found the datasheet for it. Basically it was a DC to DC converter which is not ideal because the PWM signal is switching and the DC/DC circuit seems to have died due to possbile high power switching effects from the PWM signal, whereas the LED Array itself was fine. In the lab I noticed the LEDs need max 350mA to give off the maximum light. The DC to DC Converter could take in any voltage and provide the constant current, but continous switching at input it is not desgined for imo. This comes back to my original question if someone really knows the characteristics of this PWM signal for DRLs. For example the amplitude, frequency and duty cycle %? Also what the driver stage of the DRL driver in the car looks like? If this info is known, a circuit could be easily designed to filter the PWM signal or use it to switch a relay etc, otherwise it would be more a trial and error approach. In fact if I cant get more info on this signal I was thinking of measuring it with a scope.

 

[Lug_Nut]

On the '03 it's using an additional resistance in series with the lamp. There is no 'duty cycle' on that year's DRL. There are 12 volts across the string, but the roughly 2 volt drop through the added DRL resistance means the lamp is running at about 10 volts. The 15% or so voltage drop across the lamp means a squared drop in wattage resulting in about 75% of the light output.
However, the voltage drop across the DRL resistor also produces wattage 'heat' that needs to be dissipated. The net result is that the power consumed on DRL is the same as on full intensity.

LED's are far more susceptible to voltage variances than incandescent lamps. I won't vouch that the 12 volt LEDs might be damaged by operation at 10 or so volts.

 

[Powder Hound]

...many after market LED based bulbs now like ba15s that can be inserted in the same bulb socket. I tried a set of these but they died after a couple of months....

The method of dimming LEDs is to use PWM, as the nature of the LED makes dimming difficult if not impossible otherwise. If you graph input voltage vs. light output you see a large non linear jump, which shows that PWM will work very well, since on/off on an LED usually can happen in a few microseconds.

The reason most aftermarket LEDs burn out after just a few hours of operation (while they advertised a lifetime of thousands of hours...) is that they are terribly overdriven and there is no way the package can get rid of the excess heat.

The way to make such things work is to 1) engineer a package that can reject the excess heat from bright LEDs, 2) buy LEDs that can handle that much power as long as you get rid of the heat, and 3) engineer a constant amperage power supply at the right amperage for the LED setup you are using.

I do not have much faith that the current aftermarket offerings do this. The usual stuff you see is too cheap and not well made. The photos you include are a case in point. You will not get much life out of a package that has no heat sinking for the LEDs or the power supply. Something had to go, and it looks like it was those tiny chips, doing whatever they tried to do, then acting as a fuse to save the LEDs.

 

[Powder Hound]

Here is a shot of city bulbs replaced by amber LEDs on my Y2k 4-dr Golf. You should be able to click on the thumb to get a larger image:



In this case, each city bulb was replaced with 3 amber LEDs. The power supply is inside the headlamp assembly and is separate from the LEDs. To keep from having an irritating point light, all 3 were aimed up into the high beam reflector. This set up was installed more than 6 years ago (June, 2007) and is still working very well. I have disabled the DRLs on this car, but use the parking lamps only if I feel the need since I have a euro switch. I think the amber city lamp done this way provides excellent use, a pleasing effect, and the performance so far shows that the likelihood of having to replace them in the foreseeable future is very low.