Abstract: The invention relates to a power supply system for a device designed to cooperate with a portable electronic medium, e.g., a credit or identification card having integrated electronic circuits therein. This system comprises a main power supply circuit (5a) that applies at least one voltage (V) to a processing unit (3) by means of a main supply source (6a) and an auxiliary backup supply circuit (5b) by means of an auxiliary supply source (6b). A detector circuit (7) detects any anomaly of the main source (6a) and automatically connects the auxiliary source (6b) in order to apply the voltage (V) to the unit (3).The invention is more particularly applicable to the supply of a transaction terminal or unit which cooperates with portable electronic mediums, e.g., credit or identification cards.