I've been working on a circuit to allow the use of LEDs for dashboard instrument lighting and give complete dimming from 100% down to 1-5%. The circuit will provide a PWM output and will work with LEDs and incandescent bulbs.

Circuit simulations show very good functionality and linearity over the full range of dimmer adjustment.

Since the light switch potentiometer is relatively low resistance, when changing to LED dash lights, the LEDs tend to dim just a bit then turn off. This circuit would provide a ballast resistor to sense the change in resistance of the pot.

One thing I need though is the range of resistance of the potentiometer of the light switch. If anyone has a light switch laying around, I would appreciate a measurement of the resistance from bright to off. It looks like the pins for the pot are 58b (12V power) and 58a (dimmed output to the bulbs).

quick chk with my cheap dvm 200ohm range is from 3.5-27

Ugh. I was hoping the resistance would be a bit higher. If I want a 3V drop with the maximum resistance (27 ohm), then I'll need a ballast resistor of 99 ohm and current of 111ma. The maximum power dissipation with 99 ohm ballast resistor is 2W. That is massive for a small board. Plus it's just wasted power as all it's doing is dropping voltage across the pot.

Given the power dissipation, if I divide up the 2W across, say 6 resistors, they would each drop 0.33W. I could use 2512 sized surface mount resistors which are capable of 1W power dissipation.

Jerry,
Don't forget to take into account the almost 50 years of crud buildup on the dimmer windings. That raises the resistance in them.

Me personally, I just bypass them.

Clay

Might help... not once have I turned the dash lights in any car below maybe 50%. Usually its 100% to 75%.

Maybe just a switch that turns off half of the LEDs? I'm assuming you are working with your strings and not the replacement bulbs.