U.S. Philips Corporation

Jouw Van Der Akker

Joseph A Plagge

A circuit for energizing a load (10) with different input voltages comprises a transformer (6), having a primary winding (n.sub.1) in series with a switching transistor (T.sub.1) the load (10), and a current sensing resistor (R.sub.2) between the input terminals. The secondary winding (n.sub.2), in series with a diode (D.sub.1) connected is in parallel with the load. A starting resistor (R.sub.1) starts the circuit oscillating via a positive-feedback loop comprising, in series, a first resistor (R.sub.4), a capacitor (C.sub.1) and a second resistor (R.sub.5). This feedback loop is coupled between one end of the secondary winding and the base of the switching transistor. A control transistor (T.sub.2), whose base is connected to the current-sensing resistor (R.sub.2), turns off the switching transistor as a function of the primary current. The energy stored in the transformer is then delivered to the load by an output current in the secondary winding. This output current increases as the input voltage increases as a result of an increasing saturation of the switching transistor via the positive feedback. A zener diode (Z.sub.1) is coupled to the switching transistor so that, above a specific input voltage, the zener diode turns on and drains away the base current of the switching transistor so as to limit the output current.

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Drachten, Netherlands

Jouw Van Der Akker