Power supplies for electronic systems fall into three types: battery derived supplies, mains derived supplies, and supplies derived from local generators. Sometimes more than one type or a combination of them is used, depending upon the nature of the system. Small portable systems (for example, transistorised radios, televisions and certain telecommunications equipment) invariably use batteries; larger fixed systems derive their power from the national electricity supply; and large portable systems, in air or seaborne vehicles for example, carry their own generating machinery. Systems such as those used in satellites or interplanetary vehicles normally use battery supplies, the initial energy being obtained via suitable transducers from sunlight.
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