system
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human
[ELI5] Conversion of 12V to 230V

I never quite got the concept of voltage converters. It seems mind boggling to me that you can have two 230v outlets in a car which just has a 12(or24)V battery.

Can someone explain to me how it’s done and what the downsides to a real outlet in your house are?
gpt
There are two common ways to change voltages: using a transformer or using a switched mode converter.

Let's start with a transformer. If current passes through a loop of wire, it creates a magnetic field. The reverse is also true: a loop of wire in a changing magnetic field will induce a current. It's important that the magnetic field is changing: a wire won't see any current in a static magnetic field.

A transformer basically consists of two coils of wire. The wires are coiled so you get a lot of wire loops. One coil is powered by the input, let's say the 12 volts of a car. The other coil is unpowered. The 12V input has to be alternating current (the current constantly changes direction), so the magnetic field is also constantly changing. Since a car battery produces direct current, a circuit is needed to change it to alternating current. In essence the circuit switches the plus and minus of the car battery at a fixed rate (50 times per second). This changing magnetic field induces currents in the second output coil.

And here's where the real magic happens: by varying the turns of wire for each coil, you can vary the output voltage. So if the input coil and the output coil have the same number of turns of wire, the output voltage would also be 12 volts (assuming no losses). But if you give the output coil more turns than the input coil, the output voltage becomes higher by the same ratio. So to get 230 volts from 12 volts, you'd need 230/12 times as many turns on the output coil as the input coil, so approximately 19 times as many turns on the output coil.

Transformers are great at what they do, but they're quite expensive and bulky. The coils are connected by a structure of laminated iron to increase the efficiency. This, combined with the coils themselves make transformers really heavy and quite expensive, especially for high power applications. So transformers are being replaced more with switched mode transformers.

The operation of a switched mode converter is a bit more complicated, but to simplify: a switched mode converter uses a coil of wire that is constantly being connected and disconnected from the input power. Since coils create magnetic fields, as explained above, and the magnetic field can't collapse instantly, this makes the coil 'want' to keep the current flowing. Thus a large spike in voltage is generated. These large spikes in voltage are tuned and filtered in additional circuitry to match the wanted output voltage.

Switched mode converters are much lighter and cheaper. Nearly all modern adapters (think phone chargers and the like) use switched mode converters, and I'd assume most car converters do too. 

As for downsides compared to a regular outlet: besides the obvious downside of having a battery that can run out, another downside is that the output isn't as well regulated as the power in your home. In converters that use transformers there is a circuit needed to change the direct current of the battery into alternating current, and if they skimped on the design of this circuit, it might not produce a true sine wave like a regular power outlet. This won't matter much for simple devices, but more sensitieve electronics may behave erratically or even be damaged by an input that isn't a true sine wave.

Similarly, a switched mode converter can deliver a true sine wave with the right design, but cheaper models will often have significant high frequency noise that can disrupt or even damage sensitive electronics.