Photo's and info supplied by Alan Nadon:

I'm guessing that's a variable auto-transformer (a.k.a. a Variac) and not a variable resistor. It's definitely not a "dimmer" in the triac-type sense of the word.
http://www.superiorelectric.com/LUXTROL.htm

Larry I do not understand.


It is certainly a dimmer, it is just not a solid state dimmer.

I would say the device in the picture has been called a 'dimmer' longer than solid state dimmers have been around.

IMO that is a 'dimmer' and a solid state device is more of a 'pulser'.

Bob

It appears that Luxtrol variacs are still made in a very similar style:
http://www.superiorelectric.com/LUXTROL.htm

Ahh!!
I actually have a few of those kicking around, but of a different style and operating mechanism, same manufacturer... The ones I have came out of my old highschool and were mounted on massive racks and used to control all the stage lighting.. They were operated with mechanical linkages that ran from the variac to the front of the rack where someone stood and operated the handles...
They also had motor driven variacs up in the loft that controlled the auditorium lights and were operated via a pair of pushbuttons in the projection room.
All that was ripped out in favour of 2
"black boxes" that did the job of that big rack, and was all controlled from a table -top control board that could be plugged in several locations in the autitoriom... Gotta love modern technology eh!!
A.D

This is indeed a variable autotransformer (a D-2000) manufactured by the Superior Electric Co. - in wiring, it's coil is connected between a hot leg and neutral, and a sliding contact taps off the desired load voltage. It can handle any load up to its maximum rating of about 2000 watts. I've also seen these in use in many locations - theater stage lighting switchboards with a rack and pinion gear operating mechanism, single dimmers such a this one with a large operating knob, and single and multiple (stacked) models operated with a gear-reduction electric motor from a remote location. Our community theater here uses two of these D-2000's to control the auditorium lights, operated by a pair of momentary contact SPDT switches backstage and in the lighting/sound control room.

Mike (mamills)

[This message has been edited by mamills (edited 05-07-2006).]

Check out this thread:
https://www.electrical-contractor.net/ubb/Forum17/HTML/000148.html

..and the others of the same titles for more fun dimmer stuff!!

Yes, this is an autotransformer and does a true voltage reduction.

A solid state SCR dimmer does not really lower the voltage. It is a simple RC delay circuit that delays the SCR turn on somewhere into the sinewave cycle. This lowers the effective energy available to the lamp filament but doesn't lower the voltage per se. Of course turning the SCR on during the falling curve of the sine wave does in fact present an effectivily lower voltage.

The efficiency here is that the SCR is a switch and is either full on or off so no energy is wasted as would be in a resistive dimmer. There is a 0.7v drop across an SCR and that's why the need heatsinking. Do the math - 0.7v drop at several amps is still a bit of heat generated.

Autotransformers are somewhat less efficient buy very electrically quite. Solid state dimmers throw out both RFI and EMI. In recording studios, TV control rooms, etc you will often find Variacs on the lights. Solid state dimmers are still verboten wherever high quality audio is recorded.

*Note for purposes of this discussion an SCR and TRIAC are the same thing.


[This message has been edited by Gus99 (edited 05-08-2006).]

You can still rent old Luxtrol units like this one (although probably "newer", from the 50s or 60s) from a lot of entertainment/stage lighting companies. We have two of them in the booth on our mainstage at Portland State.

I've also seen a few that allow you to adjust the output voltage, up to 150 volts or so. They are commonly used on followspots to give the spot an extra "kick", even though it results in significantly decreased lamp life.

I always understood that they were only to be used with the control knob in the "up" positon.

We had things like that in our school labs.
The tables had a trunk with special plugs and you could plug in whatever box you wanted. Three boxes I particularly remember: 220V AC 50 Hz, 16A (i.e. 6 standard Schuko receptacles and a circuit breaker, and IIRC banana jacks), 0...220V DC 10A and the biggest one, 0...220/380V 3ph AC with a variable transformer. Once we used that beast to recreate the old 127/220V 3 phase system... well more or less accidentally. We got 3 capacitors and were told to adjust the voltage to get the nominal current of 1A... and they were built for 220V line voltage, so whe we just watched the amp meter we accidentally got the right voltage.

Good solid VARIAC i call it.

Works as an auto transformer with a variable output.
I have a couple 10 Amperes 2.2 kVA at home for testing power transformers etc, these were dumped by our local POCO from obsolete metering test equipment, happened to be at the right place at the right time.

They usually go to the normal mains voltage and a bit more.

For 110 they go up to 130 Volts.
For 220 they go up to 260 volts.

I always turn the knob about 10% from zero point.
Sometimes the inrush current can trip the MCB at switch on.

Fro the "school" wiring, see: https://www.electrical-contractor.net/ubb/Forum5/HTML/001308.html

I have a box that has in it a tapped auto trasnformer to go slightly above or below 120V, in about a dozen steps.