----- Original Message ----- From: "Jinx" To: "Microcontroller discussion list - Public." Sent: Monday, November 28, 2005 9:16 AM Subject: Re: [EE] means of dimming auto tail light? > > > But what about the filament life time ? One of the biggest > > problem of the halogen lamps appear in the cold-warm > > transient of the filament > > I know halogens have their problems with dimmers Vasile, but > Danny was asking about tail lights. Are tail lights usually halogens ? In general car bulbs (normal ones and halogen bulbs) do have a big difference between cold and hot resistance. For instance a 12V/5W bulb (often used in European cars for taillight) has a "hot" filament resistance of around 28 Ohms and a "cold" filament resistance of < 2 Ohms. One solution of the car industry is to switch between around 3V and 12V, then the difference will not kill the driving mosfet and the intensity of the 3V "preheating" is nearly not visible. The driving Mosfets are dimensioned for maximum peak sourcecurrent of up to 100A. In the meantime many (new) cars have now LED tail- and braking lights which are easier to dim between tail- and braking lights. There are up to 10mm high intensity LEDs available I have used on my motorbike for tail- and braking light together with a constant current regulator (LM317) switched between 28mA for taillight and 200mA for braking lights. I used 16 diodes in groups of 4 (4 LED in serial connection) and 4 of this groups in parallel connection, driven by one LM317. The switch between taillight and braking light is done by a small relay operated from the brake switch - this relay switches between a 7.5 Ohm/1 W resistor for braking lights and 39 Ohms/ 0.5W for taillights. Regards Enrico -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist