Dimmable electronic transformer circuit

Claim 1

Electronic transformer circuit for operation of a low-voltage load (L), and especially an incandescent lamp, having terminals (L1, L2), from an a.c. power network having a substantially higher supply voltage, said circuit being capable of connection to said power network through a dimmer circuit utilizing phase control, said circuit having a rectifier system adapted for connection to the power network, said rectifier system including system output terminals (Ts), a rectifier (GLR) having rectifier output terminals (Tr) and a rectifier output capacitor (C3) of low capacity value connected across the system output terminals; a self-starting half-bridge oscillator circuit connected in parallel across the rectifier system output terminals (Ts) including two alternately conducting transistors (T1, T2) having their main current paths serially connected across the rectifier system output terminals and defining, between said transistors, a common middle junction (M), a feedback circuit (TR1.1, TR1.2; TR1.3) coupled to said transistors to operate the transistors as a freely oscillating half bridge oscillator; and a starting circuit for the transistors, including a starting resistor (R1), a starting capacitor (C4), a starting diac (DC) and a starting diode (D1) connected to provide for triggering one of the transistors to operate the transistors as the freely oscillating half-bridge oscillator; a symmetry circuit including two symmetry capacitors (C6, C7) of equal capacity value, each having one terminal connected across the main current paths of the respective transistors and each having a second terminal connected together and defining a junction (J), said junction being coupled to said common middle junction (M); and a lamp supply transformer (TR2) having a primary winding (TR2.1) connected between the common middle junction (M) of the transistors and the junction (J) of the second terminals of the symmetry capacitors (C6, C7), said transformer (TR2) forming the coupling to the common middle juntion (M) and having a secondary winding (TR2.2) to which the terminals (L1, L2) of the load (L) are connected, and comprising, in accordance with the invention, a non-current-compensated choke (L1) having an inductance of at least 50 mH connected in series between the rectifier output terminals (Tr) and the system output terminals (Ts) for storing energy from said rectifier and providing for substantially sinusoidal power to said rectifier; an oscillation start-accelerating circuit (C8, R8) comprising a series circuit including an accelerating capacitor (C8) and an accelerating resistor (R8) connected in parallel to the starting resistor (R1) of the starting circuit; and a pair of symmetry resistors (R6, R7) of equal resistance value, each symmetry resistor of the pair being connected in parallel to a respective symmetry capacitor (C6, C7) and hence to said junction (J), thereby providing for gradual transistor voltage adjustment and substantially flicker-free brightness of the lamp upon operation of the circuit at less than full wave from said a.c. power network upon phase control by said dimmer.