I have a travel transformer that I use when visiting countries with 220/240 voltage. What would happen if I plugged it into a 110 outlet? Would it output 60V? Explode? Open a rift in the space time continuum?
I won’t, of course, but I am curious.
Basically it just won’t work. I’m assuming this is a device that you plug into a 220v outlet to get 110v for your portable devices?
Also of course, your plug shouldn’t fit any 110 outlets so you would have to build something to do this in the first place.
My best answer is:
1: You would get 55 VAC
2: The transformer would not care and be cool with it.
That is not how transformers generally work.
I’m very rusty, but the voltage output would be closer to 110v if other circuits didn’t prevent operation.
@engineer_comp_geek will know the science.
It would put out approx half the voltage, but at 60hz.
Correct on both counts.
It depends on how its wound. The primaries are for 230, secondaries for 120. You could plug a 120-12 v transformer backwards with 12v and get 120 on the primary side.
If it’s an old-fashioned dumb transformer, then it’s not actually a 220 → 110 transformer, it’s a half-voltage transformer. It’ll output half of whatever the input is. Or if you swap which end is input and which is output, it’ll double. It might be less efficient at some other voltage, but it’ll still work, and still halve/double it.
But a lot of modern devices that do the job of a transformer are smart devices, designed to output the same thing no matter what their input is (within some limits). If it’s one of those, then probably any input from 110 to 300-ish will output 110 (though higher than 240 would probably cut down its lifespan considerably).
I’ve been paged. 
There are three types of “travel transfomers”.
Mechanical plug adapter
This is technically not a “transformer”, it’s just a physical plug adapter. It doesn’t affect the voltage at all. But it works fine for things like laptops and phone chargers that are designed to work over a wide voltage range (typically 100 to 240 volts). You would not be able to physically plug this into a 110/120 outlet, but if you did, it wouldn’t affect the voltage.
Voltage Transfomer
This is literally two coils of wire wrapped around a hunk of iron. This is the same technology the power company uses to get electricity to your home, only the travel transformer is a lot smaller than the power transformer outside of your house. Typically the iron core is made out of laminated sheets to cut down on eddy currents and not a solid piece of iron. It’s simple and effective, and the voltage output is basically the ratio of the turns of wire in each coil. In other words, if the primary side has a 2:1 ratio to the secondary side, then the voltage also follows a 2:1 ratio. In other words, if you plug it into a 240v outlet it outputs 120v. If you plug the primary side into a 120v outlet, you’ll get 60v out of the secondary side. Transformers don’t defy physics. They don’t create energy out of nothing. You have the same amount of power on the primary side as on the secondary side. So if the secondary side is drawing 2 amps in the above transformer, the primary side is drawing 1 amp. Secondary is 120x2=240 watts, primary side is 240x1=240 watts (if you want to get really nitpicky, a small amount of power is turned into waste heat since the wires aren’t superconductors and some energy is transferred into the iron plates). These types of transformers only work on AC, which is why Edison’s DC lost out to Westinghouse’s AC.
The downside of a physical transformer is that if you need a lot of current, they are big and heavy, and a big hunk of iron plus some thick wires is relatively expensive.
Voltage Converter
You can also do the voltage conversion with semiconductor circuits instead of big coils of wire and a heavy iron core. Designs for these vary. There are simple ones which use either what is called a half wave rectifier or they use a full wave rectifier. The half wave rectifier simply chops off the “negative” part of the sine wave, and the full wave rectifier essentially takes the “negative” swings of the AC sine wave and flips them over to the “positive” side. Both of these effectively cut the voltage in half, but they also makes a mess of the sine wave.
The advantage of these types are that they are very cheap, very small, and very light. The disadvantage is that some devices will not be too happy with the massively distorted sine wave that results from them. Because of this, I don’t think they are very common these days (though I really haven’t checked to see what technologies are commonly used these days). The half wave converter is particularly bad since it completely chops off the bottom half of the sine wave, effectively turning the output “off” for half of the AC cycle.
If you plug 120 volts into this type of converter, the output is going to basically be half of the input, or about 60 volts, with a massively distorted sine wave.
You can also use a fancier semiconductor circuit to “cut off” the top and bottom of the sine wave when the voltage gets too high. This also distorts the sine wave, but not as much as a half wave or full wave rectifier. It’s not quite as cheap as a half/full wave rectifier, but it’s still a lot less expensive than a physical wire coil and iron based transformer. Depending on how the circuit is triggered, this type will probably put out close to 120 volts for anything ranging from 120 up to 240 volts. It’s only when you get below the triggering voltage that the output voltage will drop.
This type is what @Chronos is referring to as a “smart device”. I think most travel converters these days are this type, but like I said, I haven’t really checked to confirm that.
Except that a lot of the voltage converters nowadays also include electronics to either clean up the waveform (if the desired output is AC) or flatten out the voltage (if the desired output is DC), and they also have some way of changing the ratio (this is ultimately how modern laptops and other devices are able to tolerate a wide range of inputs). I think that, in detail, the usual method is to create an entirely new, very high-frequency AC waveform, then to feed that into a basically conventional transformer (which, since it’s high frequency, can be very small and lightweight), and then to remove that high-frequency component.
The devices that do all of this in a tiny package are marvels of engineering, which is why a high-quality phone charger (like a genuine Apple one), which produces a nearly perfectly clean output, costs $20. The ones that just chop off part of the input are much cruder and simpler, which is why the cheap knock-off chargers can cost only $5.
A switching power supply is just that, AC to DC, high frequency to small transformer and thence rectify and filter. That gets you the garden variety DC power supply.
Where it gets a lot messier is when you want reasonably nice AC output. That adds a whole extra step of regenerating AC at the desired voltage and low frequency. Nasty devices just create a square wave or modified square wave (ie a couple of steps) and add rudimentary filtering to remove the worst of the RF hash. Better and more expensive devices will PWM modulate the output and add more sophisticated filtering. None of this is cheap. But it does get you close to clean AC.
Whilst it is possible to cut out the middle and PWM rectified input AC, such a device gives me the willies. The isolation provided by the transformer isolates you from a host of nasty failure modes that you would really prefer not to experience. Those trivial half wave rectifier designs above even worse. Maybe it is because I live in a land of 240 volts. Mains doesn’t just bite here. It will strike you down where you stand.
There’s a canonical interpretation of transformers that the voltage ratio will be the ratio of the number of windings. We see that implicit in a few posts here. It’s very close to correct when there’s no load.
However, real transformers are often designed to output rated voltage at rated current, or some nonzero fraction thereof. Since both windings are imperfect conductors, there will be losses to resistance, which real transformers often compensate by shifting the ratio of windings.
Just adding that the transformer based ones are often called linear power supplies or ‘linears’ after their proportional output. was at a ham radio festival this weekend and saw literal tons of ‘Astron linears,’ the big 50 pounds+ lunkers.
Upon further review and having read this thread, I’m not sure what I have. Here’s an Amazon link (which unfortunately is shortened) to it. It says that it takes an input voltage range of 100 - 240 and outputs 100 - 120. At least that’s what’s stamped on it. The Amazon verbiage is a little different. It also says not to plug hair dryers, curling irons and the like into it. It says not to do it “regardless of the wattage” but I’d guess that it’s actually because of the wattage. It has a max output of 250 watts.
Everything that I can think of that I’ve used in foreign countries takes 110 or 220, which kind of defeats the necessity of having one of these things. I’m struggling to think of what I could take on my next trip to a 220 country that only ran on 110.
Electronics are usually pretty tolerant of different inputs, but dumb motors often aren’t. An American electric shaver, for instance, is likely to burn out on European voltage.
Motors might also run at the wrong speed when supplied with the wrong frequency. Might not be an issue for a shaver, but might be for a clock (though clocks calibrating themselves by the power supply frequency is less common now than it used to be).
One of the reviews stated the device refused to power a 35W hair iron, so it’s not wattage.
If the converter claims to have a max output of 250 watts, but it didn’t work with a 35 W resistive heater, then someone’s just plain lying.
Without the burden of actual knowledge, my WAG would be that they didn’t actually know the wattage of their hair iron. 35 watts wouldn’t get very hot.