What is a variac?

[caddisgeek]

Is a variac the same thing as a variable power supply?
How do you use one and why?

sorry if this is in the wrong place/ is a stupid question

[ablethevoice]

Not a stupid question. A variac is basically a variable AC power supply. Absolutely necessary when determining the condition of old and questionable tube gear. All the antique radio restorers use one. It consists of a 120 primary and a secondary with a wiper which is controlled by a knob that connects to the secondary windings enabling one to take the voltage from zero to about 130 volts. No one in their right mind would just jam the plug to a dusty 1930s vintage RCA cathedral radio just found in a barn after 50 years in the elements into an AC socket unless they want to watch "the magic smoke" - and a lot of it! - come out of the radio. Most restorers start at about 30VACfor 10 or 20 minutes. If nothing starts to misbehave, the voltage is increased to maybe 50VAC and so on until faulty components start to manifest themselves - almost always these components will be capacitors.

http://www.cathedralstone.net/Pics/TenmaVariac.jpg

[mhardy6647]

not a power supply per se, a Variac is a variable autoformer.
http://en.wikipedia.org/wiki/Variac

[RetroHacker]

A Variac is a handy tool for the "vintage" electronic enthusiast - but it's not strictly necessary. As others have said, they are variable autotransformers, that can reduce (or in some cases, slighty increase) the AC line voltage.

I restore old radios and TV's, and I don't have a variac. If I found one cheap, I'd pick it up, as it's handy. But it's not a requirement. You can do pretty much the same sort of thing with a much simpler, cheaper homemade device - a dim bulb tester. A dim bulb tester is just a light bulb socket wired in series with an electrical outlet. It serves the same basic function - reducing line voltage. It also limits current to the device, and works as a quick "short detector". I reccomend everyone build one, they come in handy.

A word of warning. A variac is NOT an isolation transformer. It's an autotransformer, which is NOT isolated from the mains. You'll want a separate isolation transformer in addition to a variac when working on "hot chassis" equipment on the bench.

-Ian

[Fred Longworth]

A variac is an adjustable autoformer. It allows a relatively "clean" AC voltage from 0-130v or so to be applied to the unit under test by the simple setting of a dial. This is important when testing amps which have just had work done in the power amps, or in "reforming" power-supply caps on older tube units.

The light bulb tester can be thought of as a "poor man's variac," in that it also allows a clean but reduced AC voltage to be applied to a line cord for test purposes.

There's an amateur brain surgeon here at AK who may come into this thread arguing that a cheap lamp dimmer works just as well as a variac, and that variacs are a waste of money -- yadda, yadda. This is silly nonsense. Typical residential-grade lamp dimmers slice the AC line into ugly pieces using the variable-duty-cycle method, which generates a dirty, transient-laden, high-order-harmonic-rich AC waveform that, to put it bluntly, SUCKS. This power voltage can plough its way into portions of the machine-under-test which don't run on well-regulated supplies. It can make AM radio unlistenable, and it can directly-radiate into sensitive low-level circuits such as phono preamps.

Best,

Fred

[ccheath]

If anyone is new to using variac, I advise extreme caution. I have learned a lesson from one of my variac rated 3 amps. It's labeling is from 0-100 percent. Anyone with normal mind would think 100 percent would mean exactly what was put in will be put out. My case it actually put out 140 volts while the input is 120. I was fortunate that I hadn't blown neither of the 3 receivers I tested with it turned to it's 100 percent position feeding 140 volts to my receivers. That would have been an expensive lesson for me. I was just also given another variac that's rated 7 1/2 amps and it's label is pretty accurate within a few volts and it also labels up to 140 volts. Make sure you know your variac before using it.

[Fred Longworth]

ccheath has a good point. Variacs are often poorly calibrated. I always like to take the variac and hook up a test line cord to the variac and an AC voltmeter, and then find the point on the dial that corresponds to 120VAC. I will then either "remember" the point on the dial or mark it with a Sharpie pen.

Fred