There is a concept in EE I don't understand.

In Europe we have a 230 V supply voltage and in the USA e.g. 120 V. Now the thing I don't understand, how can I use my charger( or a light bulb) in different voltage supply regions, wouldn't the current not be way smaller in North America (as I = V / R). So if the voltage is smaller, so should be the charging current or am I wrong?

  • 1
    \$\begingroup\$ A charger yes, an old fashioned tungsten light-bulb NO! The charger has electronics to adapt to a lower voltage, the light-bulb has not. \$\endgroup\$
    – Oldfart
    Sep 23, 2019 at 17:05
  • \$\begingroup\$ Light bulb and a charger are very different things. Light bulb (incandescent) is pretty much a resistor - which may or may not burn at a power level it is not rated for. Chargers have circuitry which may or may not to be designed for dual voltage. \$\endgroup\$
    – Eugene Sh.
    Sep 23, 2019 at 17:05
  • \$\begingroup\$ Thanks, so if I would connect a 120 V rated light bulb in Europe, it would draw way more current and probably burn off? \$\endgroup\$
    – Verman
    Sep 23, 2019 at 17:08
  • \$\begingroup\$ It will draw more current and might burn, yes. \$\endgroup\$
    – Eugene Sh.
    Sep 23, 2019 at 17:09
  • \$\begingroup\$ Yes, and a 230 V bulb in the US would only dimly light. \$\endgroup\$ Sep 23, 2019 at 17:09

1 Answer 1


For a purely resistive light bulb (old style wire element) the difference between 110V and 220V on power is a factor of 4:

110^2/R * 4 = 220^2/R

The current will also change, not a good idea as 110V lightbulbs would probably burn out at 220V (the current would double, power would quadruple). 220V bulbs at 110V will be much more dim.

Other lightbulbs that are rated for both voltages (CFL's or LED's) might have circuitry that switches between voltages.

Many other power supplies are rated for both and automatically switch between the two voltages. Some products have a selector switch between the two voltages.

The load determines how much current flows.

  • \$\begingroup\$ Don't SMPSes (like ATX power supplies) simply have a wide input voltage rather than switching? \$\endgroup\$
    – jaskij
    Sep 23, 2019 at 18:14
  • \$\begingroup\$ Some do simply have a wide input range, however it is challenging to design for such a wide range and so efficiency drops. Some SMPS's have a voltage selector switch, to optimize efficiency at that input. All SMPS's internally switch (thousands of times per second) to create the desired output voltage. \$\endgroup\$
    – rdtsc
    Sep 23, 2019 at 18:32
  • \$\begingroup\$ Most have a selector switch, some can auto switch and some can handle the full range. \$\endgroup\$
    – Voltage Spike
    Sep 23, 2019 at 18:39

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