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Old April 22nd, 2005, 08:25 AM   #1
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DIY Dimmer Box

Hi all,

I'm thinking of building a DIY dimmer box to handle perhaps 4 channels with a maximum of 1kw per channel. I've got a good understanding of basic electrics, but nothing too advanced and I have a few questions - some theoretical, some practical, if any of you knowledgeable types can spare a few moments...

1) Method of dimming. The dimmer will be used with standard inkies - open face and fresnel - can I simply dim them by putting a potentiometer in line and dropping the power supply or will this (a) damage the lamp or (b) change the colour temp?

2) If not, do I need to do something clever with knocking out certain frequencies, and if so is this an easily explainable technique?

3) Where can I find high wattage rated pots? I assume I want something rated fairly well over 1k for each channel, to allow a bit of headroom?

4) Is it even worthwhile?? Are there any cheap but useable dimmers that I could simply buy instead?!

Bear in mind that I'm in the UK where everything is (a) more expensive and (b) more difficult to find!!!

Thanks a ton in advance guys - looking forward to the info!... :)
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Old April 22nd, 2005, 12:17 PM   #2
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Hi Dominic
I've built several 1k dimmers recently using Leviton 1K's I got from a guy on Ebay (do a search for Leviton and you'll see he sells tons of the stuff). They were New old stock and were around $18 each. They work perfectly well. No problem handling 1K lights. Dimming a tungsten lamp will reduce the color temp (but may not matter depending upon what you are after). It shold not introduce any flicker. But, you can get some audible hum if you get too near audio.

I built single units and I think total cost per unit was around $25. You could easily thorugh several in a box and have a panel that way. Wiring is about as easy as it gets, nothing special.

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Old April 22nd, 2005, 12:39 PM   #3
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Not likely that you are going to do this with a potentiometer unless you want to scour the antique stores ;-) That would be a resistance dimmer, and while they were the mainstay of theatrical lighting for many years they have some issues. First, they convert the energy that isn't going to the lamp into heat, and they will get very toasty. Second, the dimmer capacity must match the lamp wattage. So if you use a 1000w rated dimmer with a 500w lamp it won't dim completely out. "Back in the day" we had to connect "ghost loads" offstage to deal with this problem.

But you can buy solid-state dimmers at any hardware/home improvement store and also pickup junction boxes and other supplies to mount them. Just be sure to follow safe wiring practices and use the right size wire. These can still get very warm. But the difference is the rheostat knob is just controlling a small trigger voltage that sets the output level of a solid-state device like an SCR or triac.

There's a whole quality range here again; some of them won't work very smoothly and may not offer the full dimming range. Others will be noisy - both in terms of RFI and "lamp sing" and some may produce objectionable flicker from the lamps. Generally speaking - like everything else - you get what you pay for.

Now you were talking about 4 - 1000w dimmers. I think you'll find the typical household variety tops out around 600w. Be sure to check the rating. Also realize that 2000w is about all you are going to get from any single household 120v outlet; household breakers are usually either 15 or 20 amps (remember the "West Virginia Law" W = VA. So with a 120volt system you would have 15 x 120 = 1800watts on a 15 amp breaker or 20 x 120 = 2400 watts on a 20 amp breaker. It's a bad idea to push this to the limit also.

Anyway, my point is that you will only be able to run about 2000 watts from a single outlet, so if you plan to load up each 1000w dimmer to full capacity you should make two units with two dimmers each and plug them into separate circuits. Your only other option would be to go with a 220v outlet, or wire something directly into the power panel (not a good idea unless you REALLY know what you're doing). BTW, I think the current issue of DV Magazine has a big article on electric wiring for video shoots, haven't read it yet though.

Having said all this... unless you really understand electricity, proper wiring techniques, and especially the safety issues, then going the homebrew route might not be such a great idea...
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Old April 22nd, 2005, 06:02 PM   #4
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I agree with Boyd, unless you really know what you are doing, don't do it. I make hand dimmers, for China Balls, from the 660W dimmers available at most hardware stores, but I never put more than 500W through them, lest disaster occur. That's about the extent of my derring do with electricity. Even then, I check the dimmers for heat and damage often.

Unless you are a certified electrician, or have access to one, you are better off purchasing a manufactured dimmer. They are expensive, but expense it relative to safety and peace of mind. I'd rather pay for a UL approved device that I know will work under the conditions I will be working under, rather than praying my wiring technique will hold up. That doesn't mean you can be careless with manufactured products, but you don't have to worry quite so much about safety. You still have to follow proper safety precautions no matter what and a healthy dose of safety paranoia is a good thing here.

Spending a few hundred dollars on a commercially manufactured dimmer, is better than having to buy a new house, or buy someone else a new house because of an electrical fire.
Mark Sasahara
Director of Photography
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Old April 23rd, 2005, 07:44 AM   #5
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I dissagree. A dimmer is about as simple a wiring job as it gets. If your average homeowner can stick one in a wall I don't think it is too much for you.

Go to the bookstore and have a look at a DIY home wiring book. Read a bit and if you still feel uncomfortable, then don't do it. I think you will find it is quite easy. Here are some things to watch for though:

The box. Put your dimmer in a metal box. Most hardware stores will have metal boxes designed for one, two or three outlets/switches/dimmers. They have little metal disks you knock out for the wire going in and out. My favorite is to get a two gang box and put the dimmer in one slot and an outlet in the other. You then knock out one disk for your extension cord. Make sure you get a metal cover for the box.

Get a beefy extension cord (rated for 15 amps) and cut off the socket end. run that through the hole in the box and clamp it in place. Use the metal clamp type deals (I can't think of what you call them) to secure the cord.

Wire the dimmer to the outlet. You now have a dimmable stinger with two outlets. BE VERY CARFUL HERE. I is easy to now plug two 1K's into this thing and start making smoke. I stick one of those child proofing things in one socket with a label on it. I like the two outlets for when I run two chinese laterns (250 w each).

If you want pictures, let me know as I am about to build four of these for a special project.

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Old April 23rd, 2005, 08:09 AM   #6
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Thanks guys...

I shouldn't have any problem with the wiring, it's the method of dimming that I could do with more info on - inline vs triacs etc - and availability/component codes for relevant parts.

I appreciate your caution - I'd advise the same - but have run my own mains rings through my attic as well as turning my hand to a fair bit of circuitry and circuit bending et al. So I'm not a complete muppet when it comes to electrical jobs. That said, I'm not an electrical engineer either!

Mike - I'd love to see some photos and/or circuit diagrams, if you have the time. Many thanks!

Also, the rating issue is less of a problem for me as I live in the UK where we have 240v 13A mains (delivering a technical wattage of 3,120w although in practice somewhere around 3.4Kw is the actual operating limit). The dimmers will be used solely by myself, and I'm sensible enough not to put 4 1K fresnels on the thing - so no worries there!!! I may also build a 2 channel version to allow me to split the load when we have the option of multiple mains rings...
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Old April 23rd, 2005, 08:26 AM   #7
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I'm going to come down somewhere in the middle on this. Sure, it can be done and it ain't rocket science. However just a few things to keep in mind WRT Mike's post.

First, the hardware store items like dimmers, utilitiy boxes and other wiring supplies were designed for permanent installation in a wall, not portable use. The boxes will have a variety of holes in them that will allow moisture, dirt and foreign objects to get inside. Probably not a big deal, but they aren't intended for a project like this.

A bigger concern is that these boxes will be designed accept solid conductor wiring, either romex, conduit, or metallic sheilded cable. They are not designed for flexible cords that will be wiggled and pulled. When you put an "extension cord" into them you are definitely kludging something together. Have I done it myself?... sure. But it really isn't very elegant. You need to get some sort of strain relief fitting to anchor the cord where it enters the box. This would probably be a romex fitting designed for a different shape and type of cable. It works - sort of - but it will crimp your cord. And since it isn't designed for mobile use, it will loosen up in the field. Maybe not a big deal again, but you are using something in a way it was never designed to be used.

The same goes for the dimmer itself. It was intended to sit safely in a wall box, not get banged around in the trunk of your car, etc. And you will connect it with wire nuts which again aren't intended for applications where things will get bumped, twisted and pulled.

This is all fine if you're comfortable with these risks and mainly if you UNDERSTAND them. If the information in my post is news to you then you probably don't have any electrical experience; proceed with caution.

Realize, I was the nerdy kid who built all kinds of home-made dimmers and stage lights in my basement 40 years ago :-) I've been there, done that. I also learned some things the hard way, like getting a serious burn across all my fingers from one homebrew project that ended me up in the emergency room when I was fresh out of college (and knew everything). In a previous job, we paid our master electrician to build a control box for some lightning flashes that is very similar to the sort of home made dimmer box your describe. It looked good and worked fine. It sat on the stage manager's console backstage so they could push the buttons. But evidently there was a grounding problem inside. At the second rehearsal the metal outlet box came in contact with the metal console and sent line voltage through the entire theatre complex's audio system. The result was that it burned out every intercom and monitor system panel in the whole building (we're talking a 2,300 seat multi-purpose building built around 1976). This was a huge problem that cost thousands of dollars to fix and took a couple months. In the meantime we had to scramble to rent portable intercoms and monitors and get them all wired and working just to do our show.

I also remember running the lights for community theatre show at age 13, and watching as my home-made dimmer board got hotter and hotter, the plastic began to melt, smoke started coming out, and wondered if it would make it to the end of the act! Somewhat later in life I renovated an old house and rewired it myself. It still hasn't burned down, but every now and then I wonder. Last summer I kept smelling something burning in my present home but never could find it. Several days later I noticed a breaker had tripped. Looking around I discovered that a wire inside an outlet box had obviously been loose for awhile and had started arcing. The heat from this literally incinerated the wiring inside that box, and the outlet itself was burned to a crisp. Now that really scared me, and it's another example of how dangerous electricity can be.

I have pleny of other stories but I'll spare you (if you've even read this far). Hopefully you get the point however. Stepping down off my soapbox now...
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