Will a DC supply be ok ?

[John Burgess]

Will a quartz lamp work ok with a DC power supply? Before I try it out I wanted to ask members who maybe have already done this
Thanks John

[Brian Fretwell]

As AC voltage is measured as RMS (Root Mean Square giving power equivalent of DC of the measured voltage) I would think that plain rectified DC or smoothed would work fine. After all car headlights now use halogen bulbs off the 12v accumulator which is DC even those that replace non-halogen ones.

[John Burgess]

Thanks Brian, I will be giving it a go next week , I feel happier having read your quick response.
John

[Paul Adsett]

Hi John, running a Halogen lamp from a DC power supply will work just fine. As with AC powered projector lamps, you may wish to incorporate a low voltage setting for preheating the lamp. This will extend your lamp life very significantly. Last September I built a 24vdc power supply for my GS1200, using a switching dc module, so that I could use the 250watt ELC lamp, which provides a very significant boost in brightness over the existing 2OOw ESC lamp.
Using DC to power your projector lamp also has other advantages (at least theoretically). First DC power totally eliminates any induced lamp circuit hum in the amplifier. Second, it takes nearly all the load off the projectors transformer, probably extending the life of the transformer by a factor of 100 or more, Third, and highly theoretical but I've seen it happen, the presence of a magnetic field in the neighborhood of an AC powered incandescent lamp can introduce a mechanical resonance of the lamp filament which will produce very early lamp failure.
(When I was working with magnetrons at a company in England we experienced rapid filament failure on the magnetrons cathode heater. We cut a hole in the side of the cathode so we could see the filament and focused a stroboscopic light on the filament. When the 20v 5O Hz heater power was applied you could see that filament vibrating like crazy, literally shaking itself to death due to the axial magnetic field of the magnetron. The chances of this happening on a film projector are very close to zero but I thought it was an interesting story!)
Anyway good luck with your project.

[John Burgess]

Thanks Paul
Interesting reading, I had a feeling it would be fine, but better safe than sorry!
I have an early Elf president projector with mains lighting. Using one of these switched modules I intend to use 24 v 250 watt lamps.
John

[Paul Adsett]

John I have found that these switching modules will not start up into a cold ELC lamp, which has a very low resistance. The power supply sees thIs load as a short circuit and shuts itself off. You need to start up into a power resistive load of about 8 ohms, and capable of dissipating about 100 watts, and then switch the resistive load out of the circuit. This is actually a good thing as the resistive load will still provide enough voltage to preheat the lamp to a nice orange glow. You may also wish to provide a finned heat sink with forced air cooling (see my photo above) to keep the temperature rise on the resistors to below about 60C. This will enable you to leave your lamp indefinately in preheat mode. For maximum safety also add an overtemp thermostat on the heat sink to shut off 24v power in the event of an overheat event.

[John Burgess]

Thanks for that Paul, I will bear your reply in mind when I start the mod. Glad I joined the forum , there’s always an expert on hand to help with these issues when they crop up !
Cheers John