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?
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.
Producer, Director, Creative Director, Cinematographer
HellGate Pictures, Inc.
BlueSky, LLC www.film-and-video.com
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
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.
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.
VFX & Photography for Motion Pictures
LA based but not there now:-)
(once a gaffer, always a tech-head)
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?
Variac’s are transformers, with a "variable"
secondary--meaning you can "dial" in the voltage
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
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.
...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?
Variac : Against - expensive, big, heavy, fragile For - smooth power output (sine wave at any level hence no
against - generates lots of RFI (which varies as one dims!)
sometimes mechanical noise from filament
for - cheap, readily obtainable, compact, lightweight
BTW, "Variac" is a trade mark of General Electric,
the generic term is
Perth, Western Australia.
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?
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
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
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
Woodward, Clive writes :
>BTW, "Variac" is a
trade mark of General Electric, the generic term is >'variable
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.