Protected dimming control interface for an electronic ballast

A dimming control interface having input terminals and output terminals, comprising:

• an isolated coupling means in communication with the input terminals and the output terminals operative to generate a control output voltage between the output terminals in response to a control input voltage generated at the input terminals by an external dimming control device, the output terminals electrically isolated from the input terminals;
• a current source means in communication with the input terminals operative to provide a current to the external dimming control device when the external dimming control device is connected across the input terminals, the current source means including a first high voltage resistance means for limiting a current flowing into the input terminals when an external AC power source is connected to the input terminals;
• a voltage clamping means in communication with the isolated coupling circuit and the input terminals operative to limit a voltage developed between the input terminals to a predetermined level when the external dimming control device is disconnected from the input terminals and operative to limit a voltage developed at an intermediate terminal to a predetermined level when an external AC power source is connected to the input terminals;
• and
• a second high voltage resistance means connected between the input terminals and the isolated coupling circuit for limiting the current flowing between the input terminals when the external ac power source is connected to the input terminals.

An isolated control interface, comprising:

• first (J201) and second (J203) dc input terminals;
• a first resistance means (R218) connected between the first dc input terminal and an intermediate terminal (J214);
• a second resistance means (R208 and R217) connected between the intermediate terminal and a first control input terminal (J205);
• a second control input terminal (J204) coupled to the second dc input terminal;
• the isolated control interface operative to develop a current-limited control voltage between the first and second control input terminals, the control voltage having an open-circuit voltage level when no current is drawn from the first and second control input terminals;
• voltage clamping means (220) having a common terminal (J222), a sense terminal (J223), and a clamping terminal (J221), the sense terminal coupled to the first control input terminal through a third resistance means (R209), the common terminal coupled to the second control input terminal, and the clamping terminal coupled to the intermediate terminal;
• the voltage clamping means operative to provide a conductive path between the clamp terminal and the common terminal when the control voltage rises to a pre-determined level such that the open-circuit value of the control voltage is limited to the pre-determined level;
• and
• isolated coupling means for coupling a signal from the control input terminals to first (J215) and second (J216) control output terminals, the control output terminals being electrically isolated from the control input terminals.