D
Dave
Guest
Does anyone know the resistance of a car dashboard light dimmer? The
wattage would be bonus..
Thanks.
wattage would be bonus..
Thanks.
P
Paul Hovnanian P.E.
Guest
Dave wrote:
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
aul@Hovnanian.com
------------------------------------------------------------------
>> Insert witty message here <<
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
aul@Hovnanian.com------------------------------------------------------------------
>> Insert witty message here <<
Guest
Dave <howierd@whoknows.com> wrote:
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
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
Guest
Dave <howierd@whoknows.com> wrote:
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.
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.
ohms and 24 ohms under load; cold, they're about 0.5 ohms and 2.1 ohms.
Matt Roberds
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