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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

Showing 3 responses by almarg

almarg's avatar
almarg
9,670 posts
 
What you read is correct. For a given setting, an ordinary Variac will provide an output voltage that is proportional to input voltage.

There are some Variac-like devices, such as this one, which regulate output voltage by using a motor to automatically vary the setting as the input voltage changes. When the motor runs, however, this particular series of regulators, and I would imagine others that are like it, makes far too much noise to be usable near an audio system.

Regards,
-- Al
almarg's avatar
almarg
9,670 posts
 
Actually, a Variac is a variable Autotransformer, which can be, and often is, designed to step voltage up as well as down. That would be done as shown in the first figure of the linked writeup, where the input voltage is placed across a segment of the winding that encompasses fewer than the full number of turns. Variacs that are designed for 120 volt single-phase inputs are commonly designed to output up to 140 volts, and sometimes more.

One other benefit they might provide, in addition to those Kijanki mentioned, is some reduction of high frequency noise that may be present on the incoming AC. However, as Rodman indicated Variacs are not specifically designed for audio applications, and an audio-oriented power conditioner (which in most cases won't regulate voltage) or a power regenerator (which will regulate voltage) can be expected to be a better alternative, albeit at a considerably higher price.

Regards,
-- Al
almarg's avatar
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
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