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Most consumer electronic devices these days use switched-mode power supplies, which typically specify they can take input power between 100–240 V, at frequencies of 50–60 Hz AC. All my typical travel devices (phone, camera, laptop chargers) have these ranges, and with a universal plug/adapter (i.e. just a physical adaptor, no transformer) you can then plug them in pretty much anywhere.

But are there any countries with mains power outside this range? Is there anywhere that I need to be careful about or avoid using such devices? Or does the range completely cover all countries' mains power supplies?

I would hope (and am assuming) that the range is designed to cover everywhere in the world with mains power, but can imagine if one or two small countries were outside this range (e.g. 50V or 300V, or 100 Hz), it may not be worth the effort of all devices supporting them.

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All "normal" mains power supplies should be OK. Most 'universal' supplies will work down to 90 VAC. Most switch mode supplies convert the AC to DC and then deal with that.

You can find exotic systems - but not in normal use. Maybe shipboard or aircraft in extreme cases - but nothing that they would supply to members of the public.

Rarely in "out of the way" places you may encounter non-standard supplies that could cause problems - usually at remote or isolated locations. Usually where connection to a national grid is not available and power is produced locally. Examples might be diesel or LPG (usually) powered alternators, and low voltage to mains supplies using electronic inverters.

Even in such situations larger alternator systems usually produce properly regulated voltages. Frequency may wander somewhat during large loads or load variations. Voltages may be low under heavy loading.

"Bottom end" electronic inverters may produce quasi-sine-wave outputs - rather than providing a sine wave the output goes 0, +V, 0, -V, 0 ... with the on to off ratio adjusted to provide an approximation to mains AC for most equipment. Some power supplied (laptops, some PCs, others) may be damaged by applying these step waveforms due to input filter capacitors. (I have heard power supplies which buzz loudly in such cases. Some few systems may see these as too high or too low in voltage.)

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According to Wikipedia, the minimal voltage is 100 V in Japan and the maximum voltage 240 V in several countries. As for the frequency, they all lie between 50-60 Hz. So there isn't any country with such an exotic voltage or frequency that wouldn't fit the range you mentioned.

If you think of it, it wouldn't make sense for a small country to go too far outside the two common voltages, as that would mean that you'd have to produce a range of electric devices just for that one country.

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Unless the small country made the decision way back in the early days of electricity and is very stubborn / closed off from the rest of the world. Something like a North Korea perhaps... – hippietrail Jan 25 '14 at 15:30

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