What is VAC?
VAC stands for volts alternating current, also written volts AC, V AC or V~. It's the unit for AC voltage, the kind of power that comes from a wall outlet, where the current reverses direction 50 or 60 times a second depending on the country. In PC work you see VAC on power supply labels, laptop power bricks, UPS units and surge protectors, describing the input side: what the device takes from the wall.
Wall voltage around the world
| Region | Nominal voltage | Frequency |
|---|---|---|
| North America | 110 to 120 VAC | 60 Hz |
| Europe, the UK, Australia, China, India and much of the world | 220 to 240 VAC | Usually 50 Hz |
| Japan | 100 VAC | 50 or 60 Hz, depending on the region |
Auto-switching vs. manual-switch power supplies
Most PC power supplies and laptop adapters are auto-switching (also called universal or full-range input): the label reads something like 100 to 240 VAC, 50/60 Hz, and the unit adjusts itself. Older or cheaper desktop PSUs may have a manual voltage switch on the back, marked 115 and 230.
- Switch set to 115 but plugged into a 230 V outlet: the PSU can be destroyed as soon as it's turned on.
- Switch set to 230 but plugged into a 115 V outlet: the PC usually won't power on or runs unstable.
When a PC or monitor travels to another country, check the label first. An auto-switching device only needs a plug adapter; a fixed-voltage device needs a voltage converter.
VAC in, VDC out
A power supply converts AC input to DC output. Its label lists the input in VAC and the outputs in VDC (volts direct current): +3.3 V, +5 V and +12 V for a desktop PSU, or a single DC voltage for a laptop adapter. On a multimeter, use the AC volts setting (V with a wavy line) for an outlet and the DC volts setting for PSU outputs. Mains voltage is dangerous, so use a properly rated meter with insulated probes, or a plug-in outlet tester.
Don't confuse VAC with VA (volt-amperes), the capacity rating on a UPS.
For the CompTIA A+ exam
Core 1 (220-1201) objective 3.6 contrasts 110 to 120 VAC input with 220 to 240 VAC input, and with the 3.3 V, 5 V and 12 V DC outputs. Expect questions about a PC moved between countries, a PSU voltage switch set wrong, or reading a power supply label to tell input from output.