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Old 03-06-2008, 01:14 AM   #1
VPTurner
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Buzzing Lights with Dimmers

So now that I have some more lights to play with I am discovering what I've read about, and that's buzzing when dimming. It seems the higher the wattage, the more noise the bulb makes. The 100W incandescent bulbs aren't too bad, but I got some 300W bulbs for my newly aquired paper lanterns, and these are loud! Interestingly, my Smith Victor Q60 (the only semi-pro light in my kit so far - I've had it for years) also buzzes when dimmed. That surprised me.

I'm just doing lighting experiments right now so sound is a "don't care", but when I start playing on an actual set and I need quality sound, the buzzing will drive me nuts. I know my ME66 will pick up every bit of it, too.

I suspect what I will have to do is rely on the inverse square law and back the lights away from the subject and save the dimmers for practicals.

Thoughts?

Thanks!
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Old 03-06-2008, 07:21 AM   #2
oakstreetphotovideo
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Vince, I'm curious about this. I haven't used a dimmer in a long time. We had them for stage productions, but I haven't worked a stage production in decades. What I'm wondering is if the dimmer is working by controlling the duty cycle (solid state), rather than a purely resistive system? Can you put an oscilloscope on the output to check if it's cycling off/on or if the voltage is being reduced without affecting the power sine wave?
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Old 03-06-2008, 11:23 AM   #3
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Don't have a scope at home, unfortunately, and I don't want to leverage work equipment for personal use. But in the office I deal primarily with milliwatts and nanoseconds.

Had my right-brain engaged last night. I'll switch to the left-brain and dissect one in the name of science.

They are not resistive. They have an inductor choke and an interference capacitor (to cut down on EMI). A quick Google search revealed that these are "triacs". Triacs function by varying the point that a load is turned on during each alternating current (AC) cycle (in the United States, AC current has 60 cycles per second). That is, triacs vary the time at which the load is switched on after zero-cross during each AC cycle. This rapid "switching" serves to reduce the total current being delivered to the lights. But this rapid switching can also cause a "buzzing" sound in the light, as well as electromagnetic interference.

From your stage days, those were probably resistive, wasting a lot of power and generating significant heat.

That explains the buzzing sound, so I guess I answered my own question. I'll check my 1000W dimmer with the worklights, but it looks like I won't be using these where good sound recording is a requirement. I'll need to resort to scrims and controlled lighting placement. My 300W paper lantern needed to be fairly close to my subject (still life), so I will need to add more wattage and back it up a bit, then check the audio recording. The directionality of the ME66 may adequately cut the noise if it's far enough away. This paper lantern's output falls off very rapidly relative to my 600W open-face.

Side note, the reason I'm using lower wattages is to play with the depth of field. I had the aperture wide-open (F1.6) with no ND (although I had the gain up a bit: +6dB). Looked very nice!
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Old 03-06-2008, 02:09 PM   #4
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I keep a wide range of wattages on hand. I find I can generally find a bulb and light-to-subject distance that yields the luminance I want.

Yes, in the old days, we had resistive dimmers, but they did not get hot, they were just bulky. I'm sure they wasted some energy, but we had a stage with 30K of lights, nobody was counting watts. I have constructed my own, computer controlled dimmer circuits with triacs, I am well aware of what they are and how they work, which is why I asked the question.

My aversion to dimmers in videography is mostly from a color balance perspective. Most lights change their color temperature when not running at their rated wattage, and of course you cannot dim fluorescents.

You've already suggested the solution to your own question, so I'll stop rambling.

Warm regards,
Doug
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Old 03-06-2008, 03:32 PM   #5
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Quote:
Originally Posted by oakstreetphotovideo View Post
I keep a wide range of wattages on hand. I find I can generally find a bulb and light-to-subject distance that yields the luminance I want.

Yes, in the old days, we had resistive dimmers, but they did not get hot, they were just bulky. I'm sure they wasted some energy, but we had a stage with 30K of lights, nobody was counting watts. I have constructed my own, computer controlled dimmer circuits with triacs, I am well aware of what they are and how they work, which is why I asked the question.

My aversion to dimmers in videography is mostly from a color balance perspective. Most lights change their color temperature when not running at their rated wattage, and of course you cannot dim fluorescents.

You've already suggested the solution to your own question, so I'll stop rambling.

Warm regards,
Doug
Ah, but now the Peanut Gallery knows, too (not that they cared).

Yes, I noticed the color temperature change. Another good reason to avoid them. But I will still use them, just not with my primary lights. It's interesting that most recommended lighting kits I've read about in DIY discussions always mention keeping a range of dimmers in your kit. My 1000W dimmer came from a well respected and exceptionally talented lighting technician from whom I've learned a lot, and I would have thought he'd mention (given his professional lighting technician status) about the "buzz". I was planning on having it handy for the construction lights. That was an expensive lesson.

EDIT: Wasn't fair to name names because I've learned a lot from the man and his generous donations to the independent filmmaking community.
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