Symmetrical power supply

Electronic circuits that draw a current of more than a few milliamperes should preferably not be powered by a battery: most are, therefore, powered by an integral power supply or have provision for connection to a mains adaptor. In fact, a mains adaptor is the most often used power source. On such a unit all that needs to be done is for the wanted vonlatge lebel and polarity to be selected with a switch.