Voltage Converter

Converting between volts is trivial; the trouble is knowing which voltage. The 380 V and 220 V of three-phase power differ by √3 (about 1.732), not by a factor of two: 220 V is the phase voltage (one live wire to neutral) and 380 V is the line voltage (between two live wires). Industrial equipment and central air conditioners are rated by line voltage, household sockets by phase voltage, so the same machine can look contradictory in two places.

Mains voltage and frequency vary by country, and Japan is split. China is 220 V / 50 Hz, most of Europe 230 V / 50 Hz, the US 120 V / 60 Hz. Eastern Japan (Tokyo) runs 100 V / 50 Hz while western Japan (Osaka) runs 100 V / 60 Hz — two frequencies in one country, a historical leftover. That is why chargers are printed 100–240 V: they genuinely work in all of these. High-power appliances such as hair dryers and rice cookers are a different matter.

A handful of DC voltages cover most cases: 1.5 V dry cells, 1.2 V NiMH, 3.7 V nominal lithium-ion (4.2 V full, ~3.0 V cutoff), USB 5 V, PD fast charging negotiating 9/12/15/20 V, and automotive 12 V systems (13.8–14.4 V while the alternator runs).

Millivolts and microvolts are for signals: thermocouples, strain gauges and microphones output in the millivolt range, which is why instrumentation amplifiers and shielded cable exist — the signal really is that small.