Dropping AC voltage for custom lamps

[da na]

I recently acquired a large box of old high-wattage light bulbs from a theater converting their house lights, spots, and PAR to LED bulbs. The bulbs themselves are gorgeous, most being between 20 and 60 years old, consisting of up to twelve tungsten filaments in quite lovely patterns. Trouble is, they're really, really good at their job. They use 500-1,500 watts, which isn't that practical to continuously run. And naturally, the heat they radiate (especially the 1.5 kilowatt lamps) is enough to burn hands from the instant they're plugged in. 

I'd love to find a way to drop the power being fed to these but I'm really not sure how to go about this. I presume dropping the AC voltage with a transformer would be the most practical route, but I don't quite know where to start there. I'd rather not buy a nice transformer and, after hooking it up, realize that the voltage provided by the transformer is too low to even light the bulbs (or something like that). 

Additionally, I might want two brightness settings, one for actual illumination and a second less eye-searing level for ambient lighting. Would I do so with two separate transformers, or two transformers in series, one of which could be bypassed?

I'd love a little help in figuring out how to tame these bulbs.

Thanks!

[AbydosOne]

14 minutes ago, da na said:

I'd love a little help in figuring out how to tame these bulbs.

Thanks!

The "correct" way would be to use a dimmer rated for 1.5kW (this might be an interesting place to start). As these bulbs are essentially "industrial" equipment, I'm not too certain what exists for driving them individually (as opposed to a huge bank of dimmers from a auditorium lighting setup).

 

Using a transformer would require some calculations because the resistance of the filaments will go up dramatically between cold and (normal) operating temperatures. Running it at 24VAC, say, may not get the filaments hot enough to decrease the resistance enough to use a transformer with a capacity scaled quadratically (as power does with voltage).

 

Example: P = V²/R -> 1500 = 120²/R -> R = 9.6Ω... however the filament resistance may be closer to 4Ω at a cooler temp meaning at 24VAC it will draw 24²/4 = 144W. Scale factor is ~10 W/W instead of the expected (120/24)² = 25 W/W.

 

IMO, I wouldn't get a transformer rated for any less than the lamp is rated for, regardless of output voltage, but that puts you into some very industrial sized transformers, e.g. https://www.zoro.com/dayton-transformer-in-120v-out-1224v-500va-4mtt9/i/G2435456/ (for reference: VA = W for resistive loads).

 

Ensure your wiring is good for the amperages these will pull.

[OhioYJ]

Well they make Variacs in all sizes and ratings, but that might be overkill for this. Wouldn't a simple dimmer switch suffice for something like this? 

 

1500 watts ~ is approaching 13 amps at 120 volts meaning this would be at the edge of most 15 amps circuits and 15 amp switchs in US houses. You likely wouldn't want much else on with one of these lights. 

[da na]

17 minutes ago, AbydosOne said:

Ensure your wiring is good for the amperages these will pull.

16 minutes ago, OhioYJ said:

this would be at the edge of most 15 amps circuits and 15 amp switchs in US houses

So what you're saying is, when I stripped an AC cord and taped the ends to the terminals of the bulb, it was in fact the ridiculous power draw and not my piss-poor wiring which immediately popped the breaker? Makes me feel a little better!

Thank you both for the suggestions regarding dimmers. It seems I greatly misunderstood how a dimmer functioned, assuming it was just a large potentiometer. I didn't want to use any sort of resistor-based solution since, well, that'd just be putting a kilowatt of heat a couple inches down the wire... Really should've looked into dimmers more.

[Kilrah]

Yep a simple dimmer is the solution, that's would have been behind these lamps in their original setting.

 

Note that efficiency goes down a lot when underpowering, so... you'll end up with a space heater that happens to make a bit of light. Wouldn't be surprised if you need a good 200-300W to get a 1500W one to even usefully glow...

[Heats with Nvidia]

The easiest and cheapest way is, you just connect them in series to each other. Two in series means half the voltage, three means a third...

You can even switch between these modes with the right switch.