18 ohms for the rheostat on a '79 Porsche 928. The panel lamps draw about 1 amp at full voltage (13.5V).
This is the second one I've burned out, so I'm installing a PWM dimmer in its place.
-- Paul Hovnanian mailto:Paul@Hovnanian.com ------------------------------------------------------------------ >> Insert witty message here
Does anyone know the resistance of a car dashboard light dimmer? The wattage would be bonus.. Thanks.
Older cars often had a fuse on the dash light circuit in the 2 A to 5 A range, so that sets an upper bound on the wattage at around 30 to 75 W. In reality it's probably much less than this. These were usually wirewound rheostats with a ceramic insulator. Common dash light bulbs in American cars draw anywhere from about 2.7 W to 4.9 W at 14.0 V. Four of these bulbs in parallel would be from around 10 to 18 ohms, so your rheostat would need to be at least that much to cut the voltage approximately in half. At 10 ohms you'd need a 20 W resistor and at
18 ohms you'd need an 11 W resistor.Matt Roberds
I guess will try a 1 ohm, 10 watt wirewound resistor from Radio Shack then. I just want to cut it down a little to extend bulb life and cut brightness a bit. If that dont work, will add one more in series or just wire it direct. I will see how warm it gets. This might be hard to figure out cause the resistance of lightbulb is not linear, goes up with heat (brightness). This I know because I worked in telephone biz and 20 years ago anyway we used lightbulbs as current limiters on some 2 wire loops. Thanks
That will probably work. Another option would be to wire a rectifier diode in series; it will drop somewhere between 0.6 and 1.1 volts. Put the striped end of the diode towards the lamps and the plain end toward the switch. You can put more in series to drop the voltage some more. Common rectifier diodes are 1 amp; you probably really ought to use 3 amp diodes for this. Radio Shack sells (sold?) a pack of 4 3-amp diodes for $3 or so which should work fine in this application.
Given the numbers I had, the 1 ohm resistor will dissipate around 2 W max, so it shouldn't get screamingly hot. But it is probably a good idea to stand it off from the wiring harness and switch a bit, instead of tying it down directly to some surface.
This is true. The filaments of an 1157 tail light bulb are about 6.1 ohms and 24 ohms under load; cold, they're about 0.5 ohms and 2.1 ohms.
Matt Roberds
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