Control/Dimming - Dimmer profiling to avoid over-wattage?

[naharnahekim]

If I needed four 575W instruments on a single 1800W dimmer (such as on a shoebox dimmer), I know that I could assign a dimmer profile on my board to limit the output of the lamps ensuring that I wouldn't be pulling the full 2300W. I also know that I probably shouldn't do this, although I don't know why. Could anyone enlighten me?

[Footer]

You can do that all day long if you want, I have done it before. Now, it does kill your output. You would be better off lamping those fixtures down to a lower wattage, you will actually get more light out of it. Lamps only work at their full efficiency at full, when you lower their intensity they get dim on curve. So, a 300 watt lamp will burn brighter at full then a 575w at 60%

[icewolf08]

The other real issue is when someone else walks in and pushes a few buttons, or your show file gets corrupt and you for get to re-profile the dimmer (though you'll remember after the first time you trip the breaker). I am with Footer, just use a lower wattage lamp, you will be much happier.

[MNicolai]

Actually, that does not work. It's not a linear relationship that exists between the power consumed and the intensity of the light. Just because you're at 50% intensity, does not mean that you're at 50% total wattage, you'll actually be quite a bit higher than that.

If you lamp down to 375W, assuming you are using the HPL, which has that variation, then you can get all four fixtures on a dimmer and run them at 100% with no worries.

[Grog12]

Doesn't work. Its too early in the friggin morning for me to remember the exact math of why it doesn't work. Hopefully someone more awake and smarter will explain. Lamp to 375w.

[quarterfront]

Originally Posted by Grog12

Doesn't work. Its too early in the friggin morning for me to remember the exact math of why it doesn't work. Hopefully someone more awake and smarter will explain. Lamp to 375w.

Yeah, I used to get away with loading a 1.2k dimmer with 1200w of 150w R-40 lamps in striplights, and then running it at no more than 85% for no more than a few minutes - but I often ended up unscrewing one lamp when it kept throwing the breaker anyway because, surprise surprise, the scene went long in tech or I really needed the lamps on longer than I'd expected.

I don't know all the math and I'm with Grog about it being too early in the AM, but... [just taking a crack at it, somebody smarter tell me how and why I'm wrong] .... Okay, filament resistance increases as the filament heats up, which means that the resistance of a filament doesn't stay the same through the range of your dimmer, it's lower when the lamp is cooler (which is why you get a rush of current when you switch a lamp on cold). You can get away with pushing the load on the dimmer right up to the dimmer's capacity and then running the dimmer low to cheat (as with my R-40 strips example above) because loading a 1.2k dimmer at 1.2k your at a place where you only need a little help from the cheat to make up 5% of the dimmer's capacity. But in the case you describe, what you're trying to do is cheat by about 22% of the dimmer's capacity (and that's not even giving you a 5% cushion).

Again, I don't have the math; but just hazarding a guess, I bet if you test this you'd have to keep them running at >55% to keep from popping the breaker. Better to just relamp and save yourself the hassles.

[waynehoskins]

Huge nonlinear relationship between resistance (and therefore current) and brightness. I seem to remember hearing that much above half intensity (or maybe even a lower value like a quarter) the lamp pretty much draws full current. That means that to avoid drawing too much current through your dimmer in that case, you will have to run the handles below half, probably significantly lower.

Lamp them down or unplug some of the units. Much better that way.