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Steve Toner · 06-30-03, 12:19 PM

GT40s got dome lights???
[img]/ubbthreads/images/graemlins/confused.gif[/img]

This is a variation of a standard voltage regulator circuit. In the voltage regulator, the car door switch is replaced by a resistor and the capacitor and resistors are replaced by a zener diode. Also, the sense line (pin 2 of the 741) is taken from the load. Basically, the capacitor and resistors (10K + VR1) provide a variable voltage reference, and the voltage across the lamp will track this. VR1 sets the charge rate of the capacitor. VR2's kind of a kludge...

Some things to consider:
The 741 is always drawing a small amount of power (probably not more than a couple of milliamps) from the battery. It's so small that I wouldn't worry too much about it draining the battery.

Automobile environments are subject to high temperatures and vibration. I would replace the variable resistors with fixed values once you find out where you want them set.
Also, the voltage rating on the capacitor is a little marginal for this environment - I would use at least a 25V rated cap. You typically have to derate the voltage rating on capacitors at elevated temperatures, so this provides an additional margin of safety. Also, use a high-temp capacitor (105 degree C). DO NOT use a tantalum capacitor here. Failure modes of tantalum capacitors are such that it could, if it failed, produce a direct short across your +12V supply. Electrolytic capacitors typically fail by leaking and messing up your circuit board, but they end up open circuit - that would just mean that the lights would come on full brightness immediately, which you can probably live with better than a blown fuse...

The 2N3055 is probably overkill, depending on how much current the lamps draw. It comes in a big honking TO3 case. You could probably replace it with another NPN transistor in a TO5 or TO220 case, as long as the transistor can handle the max supply voltage & current that the lamps require.

The prices you quoted for the parts look like quantity prices. I didn't look at the link you provided, but for small quantities of parts like this, try Mouser or DigiKey.

06-30-03, 12:19 PM	#3 (permalink)
Steve Toner toner 8 Tenths Join Date: Apr 2002 Location: Sundance, UT GT40: ex-CAV owner (stolen) Posts: 895 Rep Power: 15	Re: Dome Light Dimmer GT40s got dome lights??? [img]/ubbthreads/images/graemlins/confused.gif[/img] This is a variation of a standard voltage regulator circuit. In the voltage regulator, the car door switch is replaced by a resistor and the capacitor and resistors are replaced by a zener diode. Also, the sense line (pin 2 of the 741) is taken from the load. Basically, the capacitor and resistors (10K + VR1) provide a variable voltage reference, and the voltage across the lamp will track this. VR1 sets the charge rate of the capacitor. VR2's kind of a kludge... Some things to consider: The 741 is always drawing a small amount of power (probably not more than a couple of milliamps) from the battery. It's so small that I wouldn't worry too much about it draining the battery. Automobile environments are subject to high temperatures and vibration. I would replace the variable resistors with fixed values once you find out where you want them set. Also, the voltage rating on the capacitor is a little marginal for this environment - I would use at least a 25V rated cap. You typically have to derate the voltage rating on capacitors at elevated temperatures, so this provides an additional margin of safety. Also, use a high-temp capacitor (105 degree C). DO NOT use a tantalum capacitor here. Failure modes of tantalum capacitors are such that it could, if it failed, produce a direct short across your +12V supply. Electrolytic capacitors typically fail by leaking and messing up your circuit board, but they end up open circuit - that would just mean that the lights would come on full brightness immediately, which you can probably live with better than a blown fuse... The 2N3055 is probably overkill, depending on how much current the lamps draw. It comes in a big honking TO3 case. You could probably replace it with another NPN transistor in a TO5 or TO220 case, as long as the transistor can handle the max supply voltage & current that the lamps require. The prices you quoted for the parts look like quantity prices. I didn't look at the link you provided, but for small quantities of parts like this, try Mouser or DigiKey.