By John Papiewski
Updated Mar 24, 2022
Variable autotransformers—commonly called variacs—are inexpensive, reliable tools that let you fine‑tune AC voltage for lamps, instruments, lighting rigs, and other sensitive equipment. They are indispensable for technicians, musicians, cinematographers, and anyone who needs to ensure optimal power delivery.
Multiply the unit’s amp rating by 84.8 to obtain its maximum wattage. For instance, a 10‑amp variac can safely handle up to 848 W.
Choose a lamp whose power rating is below the wattage calculated in Step 1. This guarantees the variac won’t be overloaded.
Ensure the variac’s switch is off and the dial is set to zero before connecting any devices.
Plug the lamp’s power cord into the variac’s outlet.
Connect the variac’s power cord to a grounded wall outlet.
Turn on both the lamp and the variac’s power switches.
Turn the dial clockwise to raise voltage; counter‑clockwise to lower it. Notice how the lamp’s brightness changes accordingly.
Use RMS power (0.707 × peak) to assess the variac’s safe load: 120 V × current × 0.707 ≈ 84.8 × current. Keep your lamp’s rating below this figure to avoid overload.
Variacs typically include a fuse. If the unit fails, inspect the fuse first. A lower‑rated fuse (e.g., 5 A) can serve as a protective test load, revealing hidden wiring issues without stressing connected equipment.
Voltage markings on the variac reflect the incoming line voltage. If your local supply is 90 V, the variac’s output will proportionally match, not the nominal dial value.
A variac is not a motor speed controller; do not use it on AC motors.
At maximum dial setting, a variac can exceed 120 V. Exercise caution when adjusting, as high voltages can damage equipment.
Older models may lack a grounded outlet. Verify grounding before connecting sensitive devices.
