The other day, someone gave me an autotransformer – a hefty 10 kg of metal and wires:

Made by Philips, probably well over half a century ago (even before Philips had a logo?):

AC mains did not include grounding at the time, just 2 banana jacks spaced 19 mm apart:

So what does it do? Well, an autotransformer (a.k.a. Variac) allows you to generate an adjustable AC voltage from the fixed AC mains voltage. At the time, AC mains was 220V – nowadays, it’s 230V in Europe, so the output should now reach 260/220*230 ≈ 272 VAC.

Here’s the schematic, similar to the one printed on the side of this device:

(this isn’t fully variable, like the unit above, but the taps are a first approximation)

One way to explain what’s going on – at least as first approximation – is that it works like a transformer, but with a variable number of turns on the secondary side. Think of the incoming voltage as generating an alternating magnetic field of a certain strength, with X Volts per turn. The “tap” (which is a mechanical wiper) makes contact with one of the turns of all the turns laid out in a circular fashion, creating a circuit with a variable number of turns. The more turns, the higher the output voltage.

The intriguing bit is that the output voltage can actually exceed the input voltage, by adding a few more spare turns at the top – or equivalently: by placing the input voltage on a tap and not entirely at the end of the coil.

Note that the output of such an autotransformer is not isolated from the input, unlike regular transformers with separate primary and secondary coils.

The other difference is that part of the energy is not transferred as magnetic flux, but directly through the shared windings. It merely acts “more or less” like a regular transformer, in practical use.

I’m very pleased with this gift, which will allow me to explore the effects of a varying AC mains voltage on all sorts of appliances, power supplies, etc. – from very low voltages to somewhat over the normal 230 VAC.