What does a variac look like?

Two types are tyically seen; bench models and open frame types. The bench model (typical example to the left) contains the winding in a ventilated metal shield, with the control knob on top. The shield is usually cylindrical, and the variac sits on the bench top like a soda can sits on a counter top. The bench model typically has an on/off switch, a fuse, one or more AC outlets, a line cord and sometimes a current or voltage meter.

Open frame types, like the one shown on the right, are intended for panel mounting in equipment racks or as a part of some larger piece of equipment. Electrical connections and the inductor are exposed, but the operating principles are the same as the bench model.

What does a variac do?

Variacs provide a voltage-adjustable source of alternating current (AC) electricity. Usually plugged into a wall outlet (120 volts AC at 60 Hertz in the USA), they have a knob-controlled output that can range from 0 volts AC (VAC) to about 140VAC, depending on how the winding is connected (see below). Variacs do not provide isolation from the wall outlet (the line voltage) as does a true transformer; thus are not suitable for directly powering low-voltage devices. They are typically used in tube audio for powering experimental circuits and for equipment testing and repair, not as a permanent source of voltage-shifted power.

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