There are many threads discussing how to fool VW's bulb out sensors (by using a parallel load resistor) when switching to LED replacement lights. An active thread right now is for MK V LED Fog lights, where the kit builder includes an 8 ohm, 50 watt power resistor - which seems like a pretty conventional method.
But does anyone really know how VW's bulb sensing circuit works?
I could imagine each sensed bulb's power wire could be monitored by a hall effect current sensor ; or they could monitor the voltage across a low ohmic value sense resistor in series with each bulb. But either one seems kinda expensive - so maybe they have some other way of doing it?
A normal incandescent bulb exhibits a large positive temperature coefficient, meaning the resistance of an operating (hot) filament is much higher (maybe 15x) versus when it's cold. So a simple parallel load resistor (when installing LED replacements) doesn't very accurately model the incandescent filament's behavior.
Also, I've read VW's do a bulb check when starting the car that might actually be different than the check made when each individual light is turned on (but they must use the same sensing method - right??). That difference was reported to have caused the car to fail the power up (starting) test, while passing the test made when the LED light (with parallel load resistor) was actually switched on. So maybe the start-up test is just a very short current pulse into the cold filament?
Anyway - does anyone actually know how those bulb-out sensing circuits really work?
But does anyone really know how VW's bulb sensing circuit works?
I could imagine each sensed bulb's power wire could be monitored by a hall effect current sensor ; or they could monitor the voltage across a low ohmic value sense resistor in series with each bulb. But either one seems kinda expensive - so maybe they have some other way of doing it?
A normal incandescent bulb exhibits a large positive temperature coefficient, meaning the resistance of an operating (hot) filament is much higher (maybe 15x) versus when it's cold. So a simple parallel load resistor (when installing LED replacements) doesn't very accurately model the incandescent filament's behavior.
Also, I've read VW's do a bulb check when starting the car that might actually be different than the check made when each individual light is turned on (but they must use the same sensing method - right??). That difference was reported to have caused the car to fail the power up (starting) test, while passing the test made when the LED light (with parallel load resistor) was actually switched on. So maybe the start-up test is just a very short current pulse into the cold filament?
Anyway - does anyone actually know how those bulb-out sensing circuits really work?