Analyzing a Broken Step Up/down Transformer

Our 220V/110V converter transformer failed. Below are some ideas what to look for and some facts about these devices that would have made my repair quicker.

Device details: by SIMRAN, 1500 W, 110V <-> 220V in both directions

1. multimeter
2. Soldering iron
3. a bit of wire (>1,5mm2)

Obligatory safety warning: At any point verify that the system is without power!

This transformer can be used to step down 220V -> 110V or vice versa. For setting the input voltage there is a double-throw switch on the back. In both states both outputs (110V and 220V) are functioning.

Additionally a fuse holder can be found. The rating is 1500W. The main power switch is a kind often mounted in breaker boxes and can be attached to a DIN rail. For indicating power a small lamp with resistor is connected.

As backround: The device was heavily overloaded when it was used to power a 110V pressure washer from 220V grid. Although it worked for some time eventually it presumably overheated.

As a result the 110V output was dead, while the 220V socket was still functioning. Measuring resistance across the coils led to about 1,2R for each half. I noticed the 110V output tap was floating.

Interestingly, the current fuse was still intact. It had a 15 A rating, way too high for this appliance. The maximum power flow would have been P = 220 V * 15 A = 3300 VA, obviously offering no protection at all.

The transformer (which turned out to be an autotransformer, see below) was rather encapsulated with several leads exiting. In the schematic the thermal fuse to the right of the transformer coil blew. It makes sense that it is placed on the lower voltage side, because the current there is higher for the same power. (Higher current -> more ohmic losses -> higher temperature)

The situation could be fixed by bridging the 110V "input" and "output".

Safety warning: Bridging the thermal fuse means there is no protection against overheating! This is a safety problem.

Usually this serves as second layer of protection, but in my case the current fuse was way oversized and thus the current not limited. With a proper sized fuse I am for now ok with not having extra thermal protection, nevertheless I will add a thermal switch later.

Further speculation:

There might still be additional thermal fuses encapsulated, which I cannot verify. Having at least one more on the "110V input" / center tap would make lots of sense.

Initially I was not aware of the concept of autotransformers. The "standard" transformer one might think of consists of two coils wound around an iron core, being galvanically isolated from each other, meaning there is no electrical connection. The interaction purely takes place through electromagnetic fields.

Autotransformer on the other hand only consist of one long coil with one (or more) center taps, where input and output current "use" the same physical wire.

Obviously, less material is used, which explains their German name "Spartransformator", where "spar" refers to saving (material).

These devices are often used when voltage levels are similar, as the savings effect is largest there. Example applications include ignition coils in the car, systems for powering

The main drawback is that galvanic isolation is lost, which is very relevant for electrical safety. In a DIY context this means: they cannot be used as lab isolation transformers.

In order to evaluate if a transformer is "standard" or an autotransformer, a multimeter can be used to measure resistance between input and output. If there is no pair with infinite resistance then it is an autotransformer (or is seriously damaged)

(schematics from wikipedia articles on autotransformers)