I have a few questions and not enough time to research them myself as I have a long weekend ahead... so hopefully you fine lads can help.

Firstly some background. I have a set of 250 watt PAR lamps that i wish to use as auxillary headlamps on my truck. However 250 watt is wayyyy too much. I dont want to melt peoples eyes out peaking hills when I inevitably turn off the lights too slowly.

I would like to dim them down to around 50 percent brightness for "normal high beam use" as this would fall within both reasonable and legal road use guidelines here in my area. When offroad or on extended long flat roads with no hills i could switch them to full power mode.

I am aware that running them in series will dim them roughly by half and would likely require the least amount of components for switching between parallel and series.

But I am also curious about the resistor route. What wattage of resistor would i need to dissipate roughly half of the lamps wattage... 125W? What ohm rating would I need to dim it by half if the lamp is on a 14 volt supply and pulls 250 watts... how much heat would I need to dissipate away from said resistor to prevent failure...


Also would running halogen capsules at half of their rated wattage disrupt the halogen cycle and shorten their life? Or would the filaments still be sufficiently hot to prevent such failure??

Half the power means supplying them by about 10V, that is rather impractical (you need either some DCDC or waste a lot of power on a resistor).
Or by a 50% duty ratio PWM.
The later method sounds to me simpler - "just" a power FET controlling each of the lamp, driven alternate manner, so to minimize current ripple into the rest of the electrical system (the current will stay constant, just alternate between left and right lamp).
And because of the large thermal inertia, the switching does not need to be that fast (50..100Hz), so neither the switching edges, so you should get away with rather simple and not that much demanding controller, include the required FET gate driver strength (few mA would be enough, so simple push-pull complementary stage).
The drawback would be both of these lamps will be on a common fuse, so in case of a fault, both will get dark. But because we are talking a bout a supplementary light, which when dies does not pose hazard, so I think it would be acceptable.
And you would need to use a bank of really low RdsON FETS, total in the 1mOhm range. But because of the operating frequencies, it should not be that big problem to drive, even when the gate capacitance would be huge (easily in the 100's nF)...

I can help design the circuit Medved suggested and choose the component values if you ask

Well, you may try parallel/serial connection essentially for free. Though, running incandescent lamp at 1/2 voltage is not 1/2 brightness, sure.

Serial connection reduces the power not to 50%, but to 25% (a bit more in the reality because resistances get lower, but not that much to make for the 25 vs 50 difference). And that would mean barely 5..10% light output. I don't think HomeBrewLamps wanted to go that far...