Power-supply circuit having circuitry for switching from a battery

Abstract

A self-oscillating power-supply circuit includes between two power-supply terminals (1, 2), the series arrangement of the primary winding (n1) of a transformer, a first transistor (T1), a first resistor (R1), and a second series arrangement of the secondary winding (n2) and a first diode (D1), with terminals (2, 4) for the connection of a battery (B) and, via a switch (S1), a motor (M). The secondary winding (n2) is connected to the base of the first transistor (T1) via the series arrangement of a second resistor (R2), a first capacitor (C1) and a third resistor (R3), a first zener diode (D2) being arranged between the third resistor (R3) and the first resistor (R1). The ends of the first resistor (R1) are coupled to the inputs (16, 17) of a switching amplifier (15) for turning off the first transistor (T1) at a specific voltage across the first resistor (R1). In order to achieve change-over from a normal charging current to a trickle-charging current at a specific battery voltage and in order to power the motor (M) with a constant voltage which is independent of the load, the circuit further has a switching device including the series arrangement of a fourth resistor (R4), a second transistor (T2) and a fifth resistor (R5), the base of the second transistor (T2) being coupled to the node (6) between the first capacitor (C1) and the second resistor (R2), to which a second diode (D3) is arranged in parallel.