Electro magnetic ballast for a gas discharge lamp

An electro magnetic ballast (11 for a gas discharge lamp (2), comprising:

• input terminals (3), for receiving a mains voltage;
• lamp connector terminals (4), for receiving a lamp;
• a controllable semiconductor switch (26) coupled in parallel to the lamp connector terminals;
• a current sensor (127) connected in series with the controllable switch (26);
• and a control circuit (28) for controlling the controllable switch (26) and responsive to the current sensor (127);
• wherein when operating in a normal mode, the control circuit (28) is responsive to a current sense signal received from the current sensor to switch the controllable switch (26) ON if said current sense signal indicates a current flowing in the controllable switch (26) and to switch the controllable switch (26) OFF if said current sense signal indicates that no current is flowing in the controllable switch (26);
• an impedance connected in series with the lamp connector terminals, the impedance comprising at least an inductor (L) and preferably comprising a series arrangement of a capacitor (C) and an inductor (L);
• an electronic switching circuit (120) having input terminals (22, 23) connected in parallel to the lamp connector terminals;
• wherein the electronic switching circuit (120) comprises:
• the controllable switch (26) coupled in parallel to the input terminals (22, 23);
• and the control circuit (28) having an output (28a) coupled to a control input of the controllable switch (26) and having a sense input (28b) coupled to an output of the current sensor (127);
• wherein the control circuit (28) is capable of operating in a preheat mode in which it generates a first control signal for the controllable switch (26) such as to render the controllable switch (26) conductive;
• wherein the control circuit (28) is capable of operating in an ignition mode after said preheat mode in which it generates a second control signal for the controllable switch (26) such as to render the controllable switch (26) non-conductive;
• and wherein the control circuit (28) is capable of operating in a normal mode in which a lamp current is flowing and the control circuit (28) normally maintains its second control signal for the controllable switch (26) such as to keep the controllable switch (26) non-conductive;
• and wherein the control circuit (28), while operating in said normal mode, is responsive to the current sense signal received at its sense input (28b) to generate its first control signal for the controllable switch (26) if said current sense signal indicates a current flowing in the controllable switch (26) and to generate its second control signal for the controllable switch (26) if said current sense signal indicates that no current is flowing in the controllable switch (26).