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class="Paragraph" Variac’s vs Dimmers

class="Paragraph" Published : 11th Feb. 2004

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I'm embarrassed to ask this question since I've been in the industry for 12 years, but what exactly is the difference between a variac and a 1K or 2K commercial grade dimmer that one can buy at Home Depot?

What makes them better?

Tom Burke
Gaffer
Atlanta, GA


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Don't be embarrassed, it's nice to see that you are secure enough with yourself to ask. And guess what, now others who don't have the nerve to ask will know too.

OK Variac's are big windings of wire and the variable part of the output voltage comes from moving a conductive slider along the windings. In doing so, the slider moves through various parts the winding and the output voltage varies. The key here is that variac's use inductance and not resistance in reducing voltage. That is the reason why you can have higher output voltage than the primary voltage on a variac just like a transformer that has the ability to output more voltage than put into it.

A dimmer chops the wave of the AC voltage in order to cut the current time to the filament creating a dimming effect. Don't confuse it with an old fashion rheostat which uses resistance to dim transferring the resistance into a lot of heat. AC voltage reverses instantaneously every second (US). Dimmers trick the timing of an AC cycle offering a shorter part of the wave to the filament. So dimming a circuit doesn't cut the voltage but the time of the AC sine wave. Even at 100 percent with a dimmer in the circuit you are already cutting the wave a bit so most store bought dimmers are actually 80%-90% of full sine wave even in the highest position and some have the ability at deciding how much they cut at the lowest position.

Walter Graff
Producer, Director, Creative Director, Cinematographer
HellGate Pictures, Inc.
BlueSky, LLC
www.film-and-video.com


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>I'm embarrassed to ask this question since I've been in the industry for >12 years, but what exactly is the difference between a variac and a 1K or >2K commercial grade dimmer ...

Nothing to be embarrassed about at all!

Variac is a trade name for an autotransformer. Huh?

A variable autotransformer is a transformer that allows you to reduce (or boost) the output voltage compared to the input voltage. Unlike other kinds of transformers that have two sets of windings that are not electrically connected but are connected by the magnetic field that the line side creates in the load side, autotransformers do not isolate the load and line sides from each other.

The household dimmers (often incorrectly called rheostats) are usually triac dimmers - they are electronic switching devices.

The functional difference is this :

Variac’s take the 60 cycle sine wave at line voltage and reduce or boost the voltage without changing the sine wave - if you look at the wave on an oscilloscope, it will look the same as the stuff coming out of the wall or the generator but the voltage at the peaks will be higher or lower depending on where you spin the knob (and how the thing is hooked up - if you open the cover of the variac, you can see that there are several ways to wire it depending on what you want to do with it)

Electronic dimmers, such as the household type which use a triac, or theatrical dimmers that usually use Silicon Controlled Rectifiers (or SCR's) adjust the voltage on their load side by taking that line voltage sine wave and very rapidly switching it on and off at various points during the cycle...thereby taking a smooth wave and chopping it into shorter spikes which average out to a lower voltage.

So what are the differences in use?

Because the triac dimmers are rapidly switching the current on and off, they can generate radio frequency interference which sometimes affects the sound department. They can also cause certain lamps to buzz if the switch points at a given setting resonate with the filament's coils. This is especially prevalent with certain types of bulbs where the filaments run in a zig-zag pattern - the adjacent zigs take turns attracting each other magnetically and then ignoring each other as the dimmer switches the current on and off rapidly.

Variac’s are likely to be less noisy in RF interference and possibly acoustically as well, which is why you will find them built into the walls of many sound recording facilities.

Since variac’s are transformers, they can be used with lights that have transformers in them, like some of the MR 16 track light systems that are intolerant of the electronic dimmers. A lot of the little triac dimmers will get unhappy if you plug a light with a transformer on them and may fail.

Also, you can use variac’s to feed lower voltages to low voltage lights when doing prop gags and etc using 12 volt peanut bulbs or MR 16's, where the electronic dimmers might blow the bulbs.

Hint :

When using variac’s to feed 12volt lights, take two and plug one into the other - set the first one down at 50 per cent or lower and use the second one to make the final voltage adjustment...this has the effect of expanding the scale of the second one so that a slight adjustment of the knob does not zing the voltage up too high to fast (which will blow your little low voltage bulbs)

The home depot type triac dimmers are a lot lighter and easier to hide under coffee tables or couches when adjusting practicals, so having some of them around is still a really good idea.

Mark Weingartner
VFX & Photography for Motion Pictures
LA based but not there now:-)
(once a gaffer, always a tech-head)


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>So dimming a circuit doesn't cut the voltage but the time of the AC sine >wave.

One thing I always wondered but never got around to testing : when you dim down with an electronic dimmer — are you drawing less total current? For example – if you dimmed them way down with electronic dimmers (squeezers), could you run three 1Ks on a 15 amp circuit, for example?

Blain Brown
DP
LA


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Variac’s are transformers, with a "variable" secondary--meaning you can "dial" in the voltage you want.

The waveform remains AC sine wave....while dimmers are electronic "circuits," usually an SCR (silicon controlled rectifier) or Triac type circuit, which changes the waveform into a square wave, which is where/how you sometimes get hash or noise introduced on the line....

I was hoping I could summarize that more, but alas...

Don Daso K4ZA
ACME Film & Video
Charlotte NC

I sometimes use the little dimmers on practicals; I use variac’s on "real lights" as/if needed. Heavy iron to lug around but quiet & reliable.


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>...what exactly is the difference between a variac and a 1K or 2K >commercial grade dimmer that one can buy at Home Depot? What >makes them better?

Tom,

Variac :
Against - expensive, big, heavy, fragile
For - smooth power output (sine wave at any level hence no RFI)

Triac dimmer:
against - generates lots of RFI (which varies as one dims!) and
sometimes mechanical noise from filament
for - cheap, readily obtainable, compact, lightweight

BTW, "Variac" is a trade mark of General Electric, the generic term is
'variable transformer'

HTH...

Clive Woodward,
electronics techie
Perth, Western Australia.


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Thanks for the info Guys!

Does that mean that a $15 300W slide dimmer is just as good as a $60 1500W dimmer as far as RF and lamp buzz is concerned? Also, how much does a variac ( variable transformer ) cost?

Tom Burke
Gaffer
Atlanta, GA


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>A lot of the little triac dimmers will get unhappy if you plug a light with a >transformer on them and may fail.

I also recall that you are not supposed to put an electronic dimmer in line after a Variac, but you can put one in line before a Variac. I believe this also applies to some flicker boxes.

Blain Brown
DP
LA


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>Does that mean that a $15 300W slide dimmer is just as good as a $60 >1500W dimmer as far as RF and lamp buzz is concerned?

Lamp buzz is mainly a factor of filament design and the relationship of the dimmers rating and the lamp wattage. Dimmers are designed with chokes that help mask the problem when the correct dimmer is applied to the correct wattage of fixtures. Over rated dimmers help eliminate noise. I have three 2k dimmers in my kit that I use with smaller fixtures and the noise created when you find the right frequency with certain filaments is reduced. In industrial applications we actually install boxes that take on all the noise. Of course you don't put them anywhere near the room your dimming or you hear a mighty nice buzz. I always suggest folks use Lutron dimmers because they are built to exacting specifications.

If interested in learning how to build one I have an article on my site under the articles/instruction page called something like how to build a 2k dimmer for your light kit or something like that.

Walter Graff
NYC


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My preference is for a variac since they are built like tanks and take mechanical and electrical abuse for decades. Most SCR dimmers I encounter are home made and lack proper insulation. Eventually, the fader gets wonky from the heat. Also with fussy DoP’s, the longer, smoother 'slider' distance makes for better fine-tuning (Don't even think...)

My favourite trick with a Variac is 'fixing' low voltage from long runs or low house power - Common Variac’s will boost 120V to 140V which often allows that pesky HMI to stay on for the whole scene!

A variac however will melt gel and blow bulbs when the new kid on the set pushes it to 'full'!

Simon Crawshaw - Grip/Electric etc. - Canada


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Woodward, Clive writes :

>BTW, "Variac" is a trade mark of General Electric, the generic term is >'variable transformer'

Hmm, didn't know that.

Visually it looks like a torroidal transformer with the varnish buffed off the wires on one face, and a carbon brush that can wipe round the windings like a variable tap.

These are surprisingly common for controlling the musicians lights in theatres, since the absence of switching pulses means that they pose less of a noise problem given the number of microphone cables littering the orchestra pits.

They are heavy brutes, since they are sized according to the anticipated load like a conventional torroidal transformer.

One thing I've never quite twigged though....The carbon brush tap often seems to bridge a few turns of the windings simultaneously.

Doesn't this cause a closed loop in the windings?

It's probably worth mentioning that these things are basically variable auto-transformers, so they don't isolate the load from the mains.

Clive Mitchell
http://www.bigclive.com

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>BTW, "Variac" is a trade mark of General Electric, the generic term is >'variable transformer'

Was! GE cancelled the patent on my birthday (July 6) in 2002. So now if you want the trademark, it can be yours.

Walter Graff
Producer, Director, Creative Director, Cinematographer
HellGate Pictures, Inc.
BlueSky, LLC