What's the difference between a voltage converter and a voltage transformer?

Voltage converters and voltage transformers perform the same general function (change the voltage from one standard to another) but they do it in different ways, and it's an important difference. Each has its advantages and disadvantages, but ultimately what it comes down to is what kind of device or appliance you're plugging in.

First things first, if your device (or, more likely, its charger) is already compatible with 100-240 volt electrical input on its own, and most chargers for smartphones/iPhones, tablets/iPads, laptops and digital cameras are multi-voltage compatible, you don't need a converter or a transformer. On the other hand, if your appliance isn't compatible with the voltage at your destination, then you'll need one or the other. So what's the difference?

  • Pro: Converters are lightweight, compact and inexpensive
  • Pro: Converters support high wattages
  • Con: Converters can't be used with electronics, only things like hair dryers and irons
  • Pro: Transformers can power just about any kind of appliance
  • Pro: Transformers can be safely used with electronics
  • Con: Transformers are larger, heavier, more expensive, especially at high wattages

Voltage converters and voltage transformers do basically the same job. They allow appliances and electronic devices to be used in countries where the voltage supplied by the wall outlet is different from the voltage for which the appliance was built. But they accomplish this in different ways. Alternating current (AC) electricity is supplied by a wall outlet in what's known as a sine wave. The difference between a voltage converter and a voltage transformer is the way they handle the sine wave when converting the voltage.


A Voltage Converter's Chopped Sine Wave

Voltage Converters

A voltage converter performs the voltage conversion by simply chopping the sine wave in half, much like a light dimmer does. It's a fairly simple operation that doesn't require a lot of materials. This is why converters are smaller, lighter and less expensive.

Electric appliances or "resistive loads" like heating elements in hair dryers, clothes irons, etc. can handle a chopped sine wave just fine. But electronic devices that use chips or integrated circuits, like computer equipment, audio/video equipment, etc., need a full sine wave.

Voltage converters can only decrease (or "step down") the voltage, not increase it. So voltage converters are only useful when the electricity being supplied by the outlet is higher (220-240 volts) than the voltage required by the appliance (110-120 volts) like, for example, bringing a US or Canadian hair dryer to Europe.

Voltage converters deal with the excess power coming from the wall outlet by disippating it in the form of heat. And a lot of it. A voltage converter will heat up fairly quickly, especially at high wattages. This is a perfectly normal and expected side-effect of the conversion, but it's for this reason that voltage converters should not be used for long periods of time. Some people say an hour or two, but we'd advise no longer than about 15-30 minutes. And you should always unplug a voltage converter from the wall outlet when you're not actively using it.

Bottom line? A voltage converter is perfectly suitable for powering a 110-120 volt North American hair dryer or curling iron or steam iron, and other small non-electronic bathroom appliances from a 220-240 volt foreign outlet, for relatively short periods of time. But just about any other application is going to require a voltage transformer instead.


A Voltage Converter's Modified/Stepped Sine Wave

Voltage Transformers

A voltage transformer outputs what's called a "modified sine wave" or "stepped wave" rather than just chopping the sine wave in half. This is a more complicated operation, and requires more materials.

This is why voltage transformers are much larger, heavier and more expensive than voltage converters, especially as the wattage increases. On the inside, a voltage transformer is basically a big hunk of iron wrapped in copper wire. And the more power (expressed as wattage) it needs to handle, the more metal the job requires. A 3000 watt voltage transformer can weigh upwards of 35 pounds! Not exactly the most convenient thing to carry around in your luggage during a short trip abroad.

But voltage transformers have significant advantages as well. In fact, they're objectively "better" than voltage converters for all applications, if you disregard the size and weight factors. Because they output something far closer to a "pure" sine wave, voltage transformers can be used with just about any kind of appliance or device, including electronics which have chips and integrated circuits. Voltage transformers can also be used continuously to supply power to appliances that are used for hours or days at a time. Generally speaking, they can be left plugged in and turned on indefinitely.

So if you're going somewhere for an extended stay, or moving somewhere permanently, and you're going to need your appliances and devices to work on a different voltage system for a long period of time, you're going to need a voltage transformer.

Note: You may have seen or heard modified sine wave or stepped-wave power referred to as "dirty power." And, yeah, that's fair enough. Most appliances operate perfectly fine with a modified sine wave, but it's definitely a downgrade from the native "pure sine wave" electricity supplied by a wall outlet. Unfortunately, that's just part of the deal when it comes to changing voltage. Some more sensitive equipment, particularly audio/video equipment, medical devices, etc. may be especially sensitive to the difference and may require a pure sine wave to operate as intended. That would require a complete regeneration of the power, from AC to DC and then back to AC.


Step Up or Step Down?

To Step-up or to Step-down? Go with the flow!

A voltage transformer can either "step up" (increase) the voltage, or "step down" (decrease) the voltage. Some models can do both, by way of an input voltage selector on the back of the transformer. So which way do you need to go? Just follow the flow of the electricity.

The electricity is coming from the wall outlet, and going to the appliance's plug. So if the outlet's voltage is higher (like the 220-240 volt European outlet) and the appliance's voltage is lower (like the 110-120 volt US plug) then you need to step down. Conversely if the outlet's voltage is lower (like the 110-120 volt US outlet) and the appliance's voltage is higher (like the 220-240 volt UK plug) then you need to step up.

Another thing that can be confusing is what input and output mean when it comes to a voltage transformer. Again, just follow the flow of the electricity. Outlet voltage transformer appliance. The electricity coming from the wall outlet is going into the transformer, and is therefore the input. Then the transformer sends the electricity at the converted voltage out to the appliance's plug, and is therefore the output. So a step down transformer takes a higher input and provides a lower output, while a step up transformer takes a lower input and provides a higher output.


Japanese 100 Volt Electricity

And then there's Japan: A unique situation

Japan is the only country on the planet where the electrical systems supply 100 volts of electricity, as opposed to 120 volts in North America and 230 volts in most of the rest of the world. Most North American appliances have a voltage tolerance of roughly +/-10%, though some equipment is more sensitive, and may have a tolerance more along the lines of 5% or even 3%. In any case, 100 volts is slightly outside the range of tolerance of most appliances. How big a deal is it? Well, it depends on the appliance, and how it's being used.

Make no mistake, even a slight undervoltage condition is not at all ideal, and not good for the health of an appliance. But undervoltage is generally considered less serious than overvoltage. You're not going to "fry" your appliance, and it's not going to catch fire or anything. In fact, it'll probably work, though less-effectively, and electronics may "act weird." And over time, undervoltage absolutely will damage and eventually destroy an appliance. That's why US/Japan "booster" transformers exist. Manufacturers wouldn't produce them if there wasn't a need for them.

If you're bringing North American appliances to Japan for an extended stay, or a permanent move, you'll want to use a booster transformer to change the voltage from 100 volts at the outlet to 120 volts at the appliance's plug, the voltage the appliance was built for. And if the appliance is particularly sensitive, or if the appliance "acting weird" is unacceptable (think medical equipment) then you need to make sure it's getting the voltage it requires.

On the other hand, if you're bringing a Japanese appliance built for 100 volts to the US or Canada, then you do have an overvoltage condition. A slight one, but an overvoltage nonetheless, and overvoltages are a lot more serious. Again, your appliance may actually work, but it won't for very long. And in the meantime, you'll probably notice performance glitches, overheating and other symptoms. You might be able to get away with it for a little while, but if you need your Japanese 100 volt appliances to work properly over long periods of time in the US or Canada, then you need to make sure they're getting the 100 volts they require. Use a Japan/US "reducer" transformer to step the voltage down from 120 volts at the outlet to 100 volts at the appliance's plug.