Mike,

OK - I think I know what's going on.

I hooked up a bulb and a dimmer. The bulb was in a fixture with a separate on/off switch.

When I switched the bulb on and then checked the voltage at the base of the fixture, it dropped as the bulb dimmed. Exactly what one would expect.

But when I switched the bulb off, and checked the voltage at the base of the fixture, the voltmeter dropped slightly, but not much as I turned the voltage down.

Put the bulb in the circuit and you get what you'd expect, take the bulb out of the circuit and you get pretty much what you've described.

In other words, because of the way these dimmers work (ie not really reducing the peak voltage, just cutting off the current for an instant every cycle), you can't just measure the voltage without sufficient load in the circuit. Hook up a bulb and your voltmeter should be able to see the "effective" voltage flowing through the circuit and you ought to be able to match it to 115 or whatever pretty closely.

I think you're in good shape after all.

Hope this helps. I was kind of suspecting this, but it was fun to actually see it happen.

Hook up a bulb and your voltmeter should be able to see the "effective" voltage flowing through the circuit and you ought to be able to match it to 115 or whatever pretty closely.

I'm not sure I follow... how would measure with a voltmeter if I have a fixture plugged in to the dimmer?

BTW... Jim, you're awesome! I can't possibly express enough my gratitude for your help!

Well, the easiest way would probably be to use a reasonably sturdy extension cord with a double or triple outlet on the end and plug the lighting unit into it and stick the voltmeter probes into one of the unused outlets.

I just used a standard household fixture and left the wires bare on the botom so I could get the meter probes on them.

BTW - thanks for the compliment. I actually found myself wondering what was going on here and got curious and decided that I wanted to know the answer myself. I like mysteries!

Let me know how it all works out in the end.

Well, the easiest way would probably be to use a reasonably sturdy extension cord with a double or triple outlet on the end and plug the lighting unit into it and stick the voltmeter probes into one of the unused outlets.

Ah, got it! I'll give it a whirl and let you know what are the results.

use a reasonably sturdy extension cord with a double or triple outlet on the end and plug the lighting unit into it and stick the voltmeter probes into one of the unused outlets.

IT WORKED!

Just as you said it would, adding a load to the circuit made the difference.

I also noticed a buzz coming from the dimmer! Should that worry me?

Also, something just occurred to me. I grounded the dimmer and the cord to the metal enclosure. Is this necessary, if the cord is already grounded at the outlet (source)?

I'm not sure about the buzzing - if it isn't too loud or overheating I'd probably not worry about it. I think one of the speed controls would still be a better idea than a hardware store dimmer, though.

Re grounding, without seeing the way you've wired it I can't say much. The principle is that the ground wire should run in an unbroken line all the way from the source to the destination. The ground wire is usually joined to the white wire at the service entrance where the wires enter the building or maybe some other point in a commercial building - it just provides an additional "just in case" return path and also protects agains inadvertent miswiring of the hot wire. Usually you can find a book on basic wiring at Home Depot or similar stores and one of these might be a worthwhile investment.

And I'm glad it seems to be working!

If Canadian electrical codes are like the US, they don't tell you what you have to do if there is a plug or other "disconnecting device" between your wiring and the mains. Neverhteless, it's still a very good idea to stay consistent with the code.

IT WORKED!
Also, something just occurred to me. I grounded the dimmer and the cord to the metal enclosure. Is this necessary, if the cord is already grounded at the outlet (source)?

Yes, you did the right thing by grounding the metal box. Should a hot conductor come off in the box, it will trip your breaker (or blow your fuse) before creating a shock hazard to anyone touching the otherwise energized metal box.

Lot's of nice help here, folks. What a great forum.

Brian Brown
BrownCow Productions

Yes, if the box is metal it should be grounded. Not necessary if it's plastic!

I think usually something that mounts into a box will ground it through the metal screws that attach it to the box, but is's best to check for sure using the continuity test funcion on your voltmeter.

Better safe than sorry!

Hi guys:

I too was a physics major and I did some electrical construction for a little while. Some of it was residential and some high voltage pole line work. I also did a lot of work as a theatrical electrician so I might be able to shed a little light here.

1. Thyrister or SCR (Silicon Controlled Rectifiers) work by turning on the voltage at controlled points in the waveform. The leading edge of the waveform looks like a square wave; it goes straight up or down (positive or negative) until it reaches the level that the sign wave is at when it turns on. The first experimental examples of these were very crude and use only one half of the wave form. Latter a full wave circuit was developed. The steep leading edge has a lot of harmonics in it so it can cause the filament to vibrate and sing or buzz. The best commercial products for professional use include at least an inductor to smooth out the leading edge and suppress the buzz.

2. If you have a florescent fixture with a "dimming" ballast, a circuit the was derived from the SCR dimmer circuit can dim those. The dimming ballast are expensive and hard to find. One manufacture modified the SCR dimmer circuit to turn a florescent into a strobe. The unit would drive 40 40watt tubes. I once used this for a black light effect. Yes I said 40 40watt tubes on one controller.

3. Contrary to "common knowledge", the voltage delivered by your local electrical utility is not always at the specified voltage. They work hard to do it but you must take into account the resistance of the lines from the transformer substation to the local transformer and then to your house or shop. I live about a block from a major substation and routinely see 135volts at my outlets. This is why in large industrial, commercial and specialized instillations you will find what are called "buck and boost" transformers. These are somewhat special autotransformers that have taps on them to select the amount of change and the direction of change desired.

4. A historical note: Well into the 1980's many the older theaters on Broadway in New York still had DC current supplied for most of the lighting so the only dimmers that could be used were resistance dimmers that came in 750W, 3000W and 6000W sizes and had to have a matching load on them. Because of this you could find old instruments with no lenses or even radiant heater coils as dummy loads. The dimmers were mounted in metal lined wood cases. The 3000W ones with 13 to a case and the 6000W ones with 12 to a case. These cases, when they had their shipping covers on, looked like a crate for an upright piano so they were called "piano boards".

The 750W dimmers came in rectangular boxes of 8, they were hooked up in groups of four so that each of these "auxiliary" boards had two leads that were each plugged into one 3000W dimmer. All the boards were manfully operated, the "piano boards had a mechanical linkage for a master handle that could move all the dimmers in that board together as long as they were all at the same level. The individual dimmers could have their handles disengaged from the master or could be set to engage the master when it reached the dimmers level. There was a switch for each dimmer and a maser switch for each light (3000W per dimmer) or heavy (6000w per dimmer) board.

Besides being heavy to move into place these boards put out a lot of heat. Each operator ran two boards, usually one light and one heavy. Most setups had 4 auxiliary boards fed from the light board; they were stacked 2 on each board. A big complex musical would have six or more boards, usualy 3 light and 3 heavy. With the dimmers throwing off heat and the dummy loads adding to the heat, the operators often striped to their underwear to survive. A complex move could involve an operator standing on one foot while sitting on both masters, moving one dimmer with his knee, moving one auxiliary dimmer each with each had and elbow and pulling one down with a string in his teeth. Lighting Designers were know to give an operator as many as four rehearsals to work out the choreography!


Grayson

I remember the old theater light controls as well - not professionally, but when I was in High School I remember pulling the levers for some production or other.

I have a friend who's a lighting designer in NY and he tells me everything is digitally controlled these days. Heck, when I was in HS I don't think the "digit" had been invented yet. I actually remember clearly the first time I ever heard anyone around me say the word "computer" in general conversation - I think it was in the early 60's. I know I was getting on an electric bus at Harvard Square in Cambridge Mass when I heard some other passenger say it. I remember thinking that it must mean that the world was changing and maybe someday I could retire my slide rule.

I have a friend who's a lighting designer in NY and he tells me everything is digitally controlled these days.

Heck, you don't even need to touch the sliders... just punch the data into the automation software and watch 'em slide on their own!

No problem after hours and hours with the Omnis and the FTK 500 WATTS .
how ever in this part of the planet FTK are very difficult to find.
Have you tryed the DYS 600 watts ? How well do the perform ?
Thankyou

Yes Fillipe, I use the USHIO DYS 600 watt bulbs in my omnis - no problems, work great.

If anyone's still watching this, here's one, and a link to one vendor's page. I have checked a couple of vendors, and the price is about the same--much higher than the FTK, but if it solves the problem...

JCD120V-500WC

http://www.topbulb.com/search/results.asp

There is also a 300-watt version, JCD120V-300WC.