Variac and Auto Transformer Fundamentals

A variac is a vary simple device to use.  It’s purpose is to raise or lower the voltage that enters it.  Example: If your input voltage is too low – say 105 volts AC, you can just dial it up to the desired voltage, like 120 volts.  If your input voltage is too high (130 volts AC) then you can lower it to your desired voltage (120 volts AC).
It does not automatically adjust voltages.  That would be a line conditioner and much more expensive.
One application for a variac is for use with coffee roasters (you need a good thermometer for this).  By raising or lowering the output voltage of the variac into the coffee roaster you can manually regulate the temperature of the coffee roaster.  This allows an inexpensive coffee roaster to rival the roasting qualities of much more expensive coffee roasters.
Another use is for cutting styrofoam.  When a variacs output voltage passes through a nichrome wire it becomes heated and cuts the styrofoam like a hot knife through warm butter.
Almost everyone is familiar with a dimmer switch in their home to adjust the light output of a bulb.  A auto transformer variac does the same thing by varying the output voltage to the device you connect into it.
Old stereo amplifiers and old guitar amplifiers are a good candidate for a variac.  Think of an old piece of tube equipment and compare it with a person who has not exercised in a very long time.  With a variac you can slowly bring up the voltage applied to the amplifier over 24 or 48 hours.  This allows the capacitors and other components to gradually be brought back up to full operating efficiency without blowing up the parts that have not been used in years.
One of the things a variac may often not be used for is in conjunction with an electric motor.  Unlike a light bulb an electric motor may require a strong surge of power to activate the motor and get it spinning.  Always check with a professional before using a variac with an electric motor.
You should always match the inputs and outputs of a auto transformer variac.  For example – it is not a good idea to think you can use a variac to change the input voltage from 120 volts AC  to 220 volts AC.
Also be sure that the number of amps of electricity that will be used by the item connected to the variac.  A guitar amplifier that requires 10 amps at 120 volts CANNOT be used with a variac that is designed for a 5 amp output.  This could cause damage to the variac and possibly even start a fire.


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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