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Can a variac be used to maintain constant voltage?

Hi: I amusing a variac to control the power going to our Threshold S/500 II. Over the past year that I have had the amplifier connected I have only had to adjust the variac voltage within +/- 1 volts perhaps thrice in order to maintain a constant 120 volts. I simply wonder if in fact the variac is designed to maintain a consistent voltage despite incoming voltage swings. I read one forum thread in which a member stated that a variac can not compensate for swings in incoming voltage, for instance if the incoming voltage is 125 volts, the variac will swing to 125 volts despite having the set the dial to a constant 120 volts. Thank you for your assistance.
somut
almarg's avatar
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almarg
9,670 posts
 
On a related point what is the difference between a variac and an autotransformer?
The Wikipedia writeup I linked to earlier provides a good answer:
From 1934 to 2002, Variac was a U.S. trademark of General Radio for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. In 2004, Instrument Service Equipment applied for and obtained the Variac trademark for the same type of product.
So a Variac is a kind of autotransformer. Other kinds of autotransformers may not be variable, and may be designed for purposes other than controlling AC power. The "autoformers" used in the output circuits of some McIntosh amplifiers are an example of an autotransformer that is not a Variac.

Regards,
-- Al
somut OP
82 posts
 
Thanks all for your informative input. I guess a variac is simply not suitable for my purposes so I am selling it.
dhl93449
455 posts
 
Kind of late to the party here, but I have used a product built by Furman called the AR-15 series II, which regulates AC output voltage. It works like some of the products mentioned above, where it has a multi-tapped transformer tapped every volt or so between 90 and 130 volts. It is electronically switched (as opposed to mechanical contacts). Regulates quite well, although I have stopped using it for my audio equipment. The dynamic output impedance of this device fights with the power supplies of the Cambridge Audio 840 CD deck. Still use for computers and other devices. Really works to minimize power brown outs or over voltages.
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