Response to MMc and others who have made statements regarding dimming voltages:

Continuous dimming of incandescent and halogen lamps.

Continuous incandescent and halogen lamp dimming involves controlling the voltage delivered to the lamp. As the voltage is reduced, the lamp current and resulting lamp power are reduced. This can be achieved by using an adjustable transformer to decrease the amplitude of the 60-cycle waveform or by turning off the applied voltage with solid-state controls for a part of the cycle.

In either case a portion of the AC waveform received by the incandescent filament is chopped away. The longer the triac conducts electric power through each half of the voltage waveform, the brighter the light output (Fig. 1 below).



From: http://www.ecmweb.com/mag/electric_underst...g_lamp_dimming/

Thanks Brain, thats successfully un-confused me

Brain (Or simular electronicsy person),

Is there anything wrong with me using a conventional light dimmer on my OHP to 'soft start' the bulb to try and make it last longer (ordinary Halogen lamp).

I know it will be OK on the lamp itself, but will it damage the transformer or the fan in my OHP?

Come to think of it, I'm going to have trouble finding a dimmer that will dim 400W....

Thanks,

Matt

If you just use the dimmer on the lamp you won't have a problem. Don't run the fan etc through it. And many garden variety household lamp dimmers are rated at 15a, so no problems there. Check the amperage rating on the switch, to convert to watts simply multiply amps by volts. Also of note, running a halogen lamp just 5v under its rating can double or even triple the lamp life.

Well the lamp is only 36V, it has a whacking great big transformer next to it. Will the dimmer still work with such a low voltage (compared to the 240v they are designed for)?

Yes, the dimmer will work with 36v, 240v is simply the maximum rating for the dimmer. If you put less volts through it, it will most likely heat up less is all (which in my opinion is a benefit).