The mains voltage is supplied to the standard terminals of the transformer 220 V. Undervoltage 127 V is removed from the corresponding terminal of the transformer. Such a switching circuit is reversible, that is, by applying a voltage of 127 V to the 127 V terminals, the mains voltage can be removed from the 220 V terminals.
A transformer with a 110 V tap is useful in trains to obtain a voltage of 127 or 220 V to power low-power devices - an electric shaver, radio tape recorder, video player, etc.
In some models of tube TVs and radios, power transformers with a "237 V" tap were used. Such a transformer can be used to power household appliances in the case when the mains voltage is overestimated for a longer time of the day (230 ... 240 V), which is quite common. To do this, it is enough to turn on the transformer according to the diagram shown in Fig. 2. This method will significantly extend the service life of expensive household appliances and appliances.
The secondary windings of the transformers are not used in these cases, so they can be removed altogether. If you make taps for the required voltages at the primary winding (for example, 6.3 V, 36 V, 42 V, etc.), you will get a good laboratory autotransformer.
When working with the above circuits, remember that the autotransformer is galvanically connected to the mains, therefore, appropriate safety measures must be observed.