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Author Topic: Dimmer as power distribution?  (Read 1688 times)

Kurt Hansen

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Dimmer as power distribution?
« on: November 20, 2004, 12:02:04 am »

Our High School theatre has 12 2kw dimmer circuits feeding the FOH catwalk.  Our fixtures are 500 watt units. To add more individually controlled fixtures, why can't I park 1 (or more) of the 12 circuits on high, and put a small bar-mounted 4 channel dmx dimmer pack on the circuits?  I could potentially turn 12 2kw channels into 48 600 watt channels.  This wouldn't necessarily be a permanent install, but would be done as needed for a particular show or event.

What's the problem with this?
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Brian Ship

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Re: Dimmer as power distribution?
« Reply #1 on: November 20, 2004, 02:16:15 am »

Good theory on your part.  About works except that you still have some form of dimmer curve both using power and efecting the dimmer under loading by way of it eating voltage.  You thus after the primary dimmer are say at 118v, perhaps after the second dimmer are at 115v and this is given a proper trimming of the primary dimmer pack so that whay seems like full to you actally is.  Unless the first dimmer and output take into consideration this full loading not to mention sinewave chopping effects, on it even at full, for the same reason you don't want to run drills and motors off a dimmer, I expect it's not a good idea to run dimmers off a dimmer.

Could work, but is it healthy for your primary dimmer however is the question.  Don't know but my assumption is that it is not going to be electronically.  Better to feed power up to the grid than to dim the dimmer pack.

Hopefully others more wise than I into the dimmers can answer better.
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rick

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Re: Dimmer as power distribution?
« Reply #2 on: November 20, 2004, 02:57:17 am »

In order to bypass the potentially weird/wrong situation of  dimmed power feeding a dimmer pack I offer you a few options.

If your dimmers are not hardwired to your circuits (i.e. if theres a patch bay or something), or if you can somehow get access to where the circuits attach to your dimmers you could directly attach a 20A circuit to a circuit out there, basically creating a hot plug on your catwalk where you could plug in your dimmers. Failing that if your dimmers are a sort that have removable dimmer modules (Sensor, CD80, etc) you could buy a contactor module (designed for motor and fluorescent loads) which instead of dimming is basically a relay and put it in the slot for one or more of your FOH circuits. Or if your a little more daring you could do some surgery on your dimmers to bypass the dimming circutry to make one of your dimmers a hot patch. The TD at a theatre I worked at did that with a couple of CD80 modules.

You could also put a scope on the output of one of your 2k dimmers at full and see how dirty the power is, if its nice and sinusoidal then perhaps you don't need to worry about it.

Rick
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Kurt Hansen

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Re: Dimmer as power distribution?
« Reply #3 on: November 20, 2004, 09:16:54 am »

More info:
1 - No patch bay.  Everything's hardwired.
2 - Since it's a high school and there would be students involved, I want to keep it as simple as possible.  The tech students can understand the concept and mechanics of plugging things in, but I don't want them opening the back of the dimmer rack, or swapping modules.  I also want it to be quick and easy to do/undo.
3 - We  use an ETC/Express 48/96 console, so the TD can park those dimmer circuits on high so they stay at full power.  He can also configure the board in such a way that the new 'subdimmers' are ganged with the main channel.
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rick

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Re: Dimmer as power distribution?
« Reply #4 on: November 20, 2004, 02:27:45 pm »

I would say that I understand not wanting to rewire the back of your dimmer rack if you want this to be easily changable, rewiring is pretty permenent.

However I really don't think that swapping dimmer modules is beyond the grasp of high school students, I know I was swapping modules when I was in high school (for budget reasons we had more circuts than dimmers and had planned to buy more dimmers at a later date, which never happened).

Now I'm not an EE so I don;t understand completly but I know that the SCRs that dimmers use to dim thier loads do weird things to the shape of AC power (its what causes filiment buzz), and that dimmers use thier imput signal as a reference (something about when it crosses from pos to neg), so that I'm pretty sure that a dimmer pack with a dimmed signal as an imput would behave weirdly. So again if you have access to one put a scope on the output of one of your dimmers at full and make sure the signal isn't all weird and hacked up. Or maybe put a power condiditoner before the portable dimmer pack.

Rick
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