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Some adapters have the option of varying the voltage of the output i.e. for example i have an adapter which gives a 1.5V,3V,4V,6V,9V output. How are these variable output voltages obtained? Changing the no. of turns of the transformer is not practical. One method could be having a series of resistances and then taking the voltages from each of the resistors to the ground. But i opened the adapter and this isn't what is done. There are numerous taps from the transformer but unfortunately i couldnt observe them as the wires had come off! So how is the voltage varied?
Also some adaptors have a polarity switch. What is this switch for??


This type of adapter has some basic properties that should be considered:
...the outputs are poorly regulated if they are regulated at all;
...the outputs are poorly filtered if they are filtered at all; and
...the rated voltage is at best an approximation of the actual output voltage.

Generally, all that they are doing here is using a multi-tapped transformer and then passing the output (secondary) current through a simple rectifier. By switch-selecting which transformer tap is placed in circuit, the user can choose from the various offered output voltages.

Some of these adapters will of course filter the output afer rectification, but many don't even bother!
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