View Full Version : What's a Variac and what can I do with it?

Doc Nickel
03-12-2010, 03:36 PM
Okay, okay, I know what a Variac is. :D

Well, sort of. If I understand it correctly, it's basically a big variable resistor. The usual versions I see have an enclosure and can be used for a rough speed control on- I'm guessing a bit- things like routers or drills.

This one was in a box-o'-junk I just got:


No charge, so if it's useless, I'm not out anything. But if it IS usable, what's a typical application for it? I'm assuming it'd only work on certain kinds of motors (induction only?) and other limitations, but still, what might I use it for? Or what should I save it for? :D

If it matters, I haven't seen the dual-knob version on the standalone types. On this one, the top knob appears to be a fine control, and turns between stops (or only turns a short distance.) The larger outer knob turns what appears to be the full swing.


03-12-2010, 03:41 PM
It's good for dimming the stage lights :D

Also good for testing brownout tolerance of electronic (and other) equipment.

With a small universal type motor, you could use it as a speed control and at low speeds you would have a safe "clutch slippage" effect as you load a polish wheel, brush, etc., since it has no torque maintaining abilities per se like a triac or scr control.

Bruce Griffing
03-12-2010, 03:41 PM
No not a variable resistor - a variable autotransformer. So - 60Hz 120V AC in and 0-135V out - or something like that depending on model. It does not isolate since it is an autotransformer (primary and secondary share winding).

03-12-2010, 03:54 PM
if you find yourself turning on an old piece of tube type electronics for the first time in years...

pull all of the tubes
plug the electronics into the variac
run at 10 volts for two hours
run at 20 volts for two hours
run at 30 volts for two hours...

it's called forming the capacitors. if you don't do this...


John Stevenson
03-12-2010, 04:38 PM
I have one on my welding rotator / positioner which has a 240 volt DC motor on it.

Together with a double reduction box I can run from nothing to about 10 rpm which is ideal for building up shafts with weld.



03-12-2010, 04:38 PM
Yes, it's not a variable resistor, it's a transformer where you get to choose the output voltage. It can do a similar job to a resistor depending on the load, but that's where the similarity ends.

It's actually a very useful device. As a light dimmer, it is fully setable to any level, including increasing the brightness, and it doesn't have the interference problems than plague electronic dimmers. If you were cutting styrofoam with a hot wire cutter, this is one of the best ways to control the current in the wire (through a second transformer). It's good as a soft-start device for many mechanical applications, though it could play havoc if you tried to control the input voltage to a switch mode power supply for example.

Used with an ammeter, you can safely run up the voltage to a motor or something which you suspect to have a short. Testing amplifiers is a similar use, or powering up some old electronics gear, as AD5MB said.

I use two of them in the shop to control the speed of dc motors- one on the lathe, another the toolpost grinder, another the dc drill press accessory. I have about five of them around- the big one being a 1.5 kilowatt version.

So far I've been able to avoid the scenario in that picture-

Looks like what you have is about a 5 amp model, and with that many connectors on it, looks like it might be useful at 110 or 220. I'd have to look that up though. One of mine was similar to that, and it could be wired up as either supplied by 110 and capable of delivering a voltage from 0 to 240, or supplied by 220 and capable of giving 0 to 240 or so. Mine was originally made to operate on 230 and would go from 0 to volts output. Many of the 110 volt models will go up to 140 or so if they don't have the option to run on 220 as well.

03-12-2010, 04:40 PM
Variacs can be used for varing certain electric heaters. Do not use a variac to change the speed of an AC motor.

Variacs are also great for making a variable DC power supply. All you need is a bridge rectifier $6 and a capacitor. You vary the AC in so the DC out is also varied.


03-12-2010, 04:46 PM
I've had one for years and used it a lot for hot-wire cutting styrofoam model airplane wings and floats. No telling how many miles of the stuff I cut with it.

I guess I'm lucky I didn't kill myself with it due to it not being isolated. Just dumb & lucky cuz I had no idea about this isolation thing. It did puzzle me when the nichrome wire vaporized in a flash of light when I screwed up and let it touch the iron on my table saw as I laid the cutting bow down.;) I don't use it anymore now.

Come to think of it, AD5MB's trick may help safely resurrect my 1966 Silvertone Twin 12 guitar amp. Thanks!

Weston Bye
03-12-2010, 04:57 PM
My plating power supply has a variac that takes the incoming line voltage and feeds 0-120VAC to a big honkin' transformer that steps down to 0-25VAC. That in turn goes to a rectifier to make DC out at 0-50 Amps. As it is a plating supply (anodizing, too) there is no capacitor. The supply is useful for occasional engine cranking in cold weather too.

03-12-2010, 05:59 PM
I had a guy tell me that a variac with a 1 ohm high wattage resistor can be very useful in detecting you have a winding in a motor that has turn to turn shorting.

The drop across the resistor or winding is easier to measure since inductance causes the winding to look like a resistor in series.

Ohming out a fairly high HP motor is futile with normal dvm's. Resistance is too low.


03-12-2010, 06:14 PM
A variac is actually a variable transformer. I see yours has 2 taps. There are 2 different ways to hook it up. One way is to attach power to one tap and take it out at the center tap which is the variable tap.

The other way is to attach the center tap to one end tap and put the power in on that tap, take the power out on the other tap. The advantage of this type connection is it will double the power rating of the variac. If your variac is rated 1500 watts it is probably intended to be wired like this.

If you connect it to 120 volts AC with the knob turned all the way to the left voltage out should be Zero. If you turn the knob all the way in the other direction voltage out should be 140 volts. You can connect it for counter clockwise rotation or clockwise rotation.

Your variac has taps so it is for a special application. Those extra taps do not have to be used.

Check the tag to see how many watts it is rated for. Do not over power it because it will over heat and burn up.

03-12-2010, 06:30 PM
my dad has a similer thing to that used it for dimming lights worked really well for that

03-12-2010, 07:07 PM
That is one of the really good old variacs. Its worth about $50 on ebay. If you take that variac, wire up a circuit that includes a bridge rectifier, a dc voltmeter, and a couple jacks for leads, and you've got a world-class titanium anodizer.

Ted Coffey
03-12-2010, 07:14 PM
I use one to control the voltage to heater elements of a heat treatment oven.
Allows me to adjust the temperature.

03-12-2010, 07:16 PM
When I was in Africa doing sound and lights for a rock band, we had the use of some pretty large variacs for the lighting. You didn't move them very easily. I don't recall the name of the theatre, but the one that we worked in was THE predominant place to watch a show, and it was well equipped. I won't soon forget that night- new years eve, huge place where the band had to be told to turn the volume down for some reason, even though we didn't have enough power to come even close to saturating the place. Outside on the main street in this city of 5 million at the time, they were drag racing up and down the street, full of broken glass. It was wild.

Our band didn't use variacs as part of our own gear- I had a number of triac circuits made up, but they were noisy- no zero crossing stuff in those days. It was funny- the boxes I had made up for the circuitry were dwarfed by the three huge knobs they each had on them. You had to use what you could get-

When I returned to Canada, I had to carry my own variac with me on the flights so I wouldn't have to pay for extra baggage weight. Here I am, suitcase in one hand, and this heavy box in the other, lugging it through all the customs and baggage checks on the way back. And you can just imagine- nobody knows what those things are, or what they're for-

03-12-2010, 07:17 PM
You can emulate Van Halen:


'bout halfway down the page.

apparently Eddie ran his Marshall stack at 90 volts.

03-12-2010, 08:19 PM
Built up into a bench along with some volt meters and some other goodies they make a nice test stand capable of making pretty much whatever voltage you need.

Lew Hartswick
03-12-2010, 09:15 PM
First off. Variac is a registered trade mark not a generic term.
So lets be accurate. Turns out the OP does have a "General Radio" one
but there are several other manufacturers that make variable autotransformers
with other names. :-)

03-12-2010, 09:44 PM
what's a typical application for it?


Used to calibrate the "Phase Failure" relays at sub-stations with a 3 phase Variac fixture

03-12-2010, 11:12 PM
I use Variacs for all sorts of uses , such as controlling the voltage, and therefore wattage, to my floating pond heater. When used as a light bulb dimmer, a Variac does not cause the irritating high pitched noise that electronic " chopper" dimmers cause. When used with a rectifier, Variacs make good variable charge rate battery chargers, and the same charger can charge batteries of any voltage you choose. I usually plug my Variacs into a " Kill-a Watt" digital watt meter ( about $30 ) which gives me a convenient readout of voltage, amperage, wattage, frequency, power factor, and cumulative watt hours. Unlike a variable resistor, the Variac does not dissipate power needlessly . Variacs and other similar autotransformers are seldom actually purchased retail by home shop tinkerers like me, since they are expensive. The challenge is to accumulate variable autotransformers on the cheap at auctions and yard sales, and from people who don't know what they are .

J Tiers
03-13-2010, 07:59 AM
A variac is actually a variable transformer. I see yours has 2 taps. There are 2 different ways to hook it up. One way is to attach power to one tap and take it out at the center tap which is the variable tap.

The other way is to attach the center tap to one end tap and put the power in on that tap, take the power out on the other tap. The advantage of this type connection is it will double the power rating of the variac. If your variac is rated 1500 watts it is probably intended to be wired like this.

Not a good plan. Very bad plan.

The "variac" (Staco variable transformer) http://www.variac.com/ or other variable transformer should ALWAYS have the power input on one of the 'fixed" taps, and the output power from the variable tap.

Any connection which changes the total number of turns across the source voltage will inevitably lead to a burned-up unit. Ditto for shorting any turns. The indicated section in the quoted post seems to suggest doing that.

There are generally 4 taps, plus the variable. Sometimes a center tap also. Two are the ends, and two are tapped a bit down from each end.

if you connect voltage across the entire winding, the output may be varied from zero to full.

if you connect to one end, and the other end on the tap, you may vary from zero to about 15% over the input voltage.

Some other winding and tap type exist.

The current rating is the limit, and that depends on the variable tap, which uses a brush to make the connection. The brush can only carry so much, according to the rating. Excess current will burn the brush and potentially damage the windings from overheating.

A Variac etc is the best way to "bring up" many forms of electronic equipment after fixing.... and evaluate before fixing. If they are going to draw big current,. they often will do it at a low voltage, you can see that with a meter and shut off before more damage.

David Powell
03-13-2010, 09:20 AM
I built an electrically heated boiler to drive a Stuart 10 steam engine for demonstrations where we cannot light fires. I use a variac to drop the voltage for the heater element so that the engine can run slowly for hours without any attention.The element is 1500 watts, with 60 volts on it we get steady running at about 150 revs.Regards David Powell.