OVERVOLTAGE PROTECTION CIRCUIT

Abstract: A 0-10V dimming interface protection circuit generating an indication that line voltage is applied across first and second leads of the dimming interface controlling light output by a lamp, includes a first device for limiting current serially connected between a line voltage source and the first lead of the dimming interface, a second device connected across the first and second leads for detecting application of line voltage to the dimming interface and for generating a control signal, and a third device for switching, operated in response to the control signal, for causing an increase in resistance of the first device to thereby limit current to the dimmer interface and limit light output by the lamp. According to one aspect of the invention, the protection circuit also includes a fourth device for preventing spurious operation of the second device.

Description: OVERVOLTAGE PROTECTION CIRCUIT The present invention relates generally to a protection circuit. More specifically, the present invention relates to a 0-lOV interface protection circuit. As illustrated in Fig. 1, the output of many ballast-driven lighting systems can be controlled by employing an industry standard 0-10V dimmer. During the installation of 0- 10V dimming ballasts, one of the commonly made mistakes is that the dimming input wires, together with the line input wires, are connected to the mains, which predictably results in the damage to the dimming interface circuit inside the ballast. A protection circuit which protects the dimming interface circuit from being damaged when the dimming wires are wrongly connected is known. However, the known protective circuit sets the ballast at full light output. Thus, although the interface circuit is protected from excess current/power, the user is not alerted to the fact that the system has been incorrectly wired. Thus, when the dimming input wires, together with the line input wires, are connected to the mains, full light output does not provide enough warning of the incorrect wiring. What is needed is a device which clamps the ballast at the lowest light output level when the dimming wires and the line input wires are incorrectly connected to the mains to thereby provide a positive indication of incorrect wiring. What is also needed is a protective circuit which provides the above-described protective function at low cost. It would be beneficial if the postulated protective circuit would include one or more current limiting elements. Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a 0-lOV dimming interface protection circuit which overcomes the above-described deficiencies. The present invention was motivated by a desire to overcome the drawbacks and shortcomings of the presently available technology, and thereby fulfill this need in the art. One object of the present invention is to provide a dimming interface protective circuit which clamps the ballast at the lowest light output level when the dimming wires and the line input wires are incorrectly connected to the mains to thereby provide a positive indication of incorrect wiring. Another object according to the present invention is to produce a dimming interface protective circuit which provides the above-described protective function at low cost. Still another object according to the present invention is provide a dimming interface protective circuit which provides the above-described protective function at low cost by utilizing one or more current limiting elements. These and other objects, features and advantages according to the present invention are provided by a circuit protecting a low voltage component from application of high voltages. Advantageously, the circuit includes a positive temperature coefficient (PTC) thermistor connected to one lead of the low voltage component, a voltage detector which detects application of the high voltages to the low voltage component and generates a control signal, and a switch operated responsive to the control signal. According to an aspect of the present invention, operation of the switch increases resistance of the PTC thermistor to thereby limit current to the low voltage component. These and other objects, features and advantages according to the present invention are provided by a 0-lOV dimming interface protection circuit generating an indication that line voltage is applied across first and second leads of the dimming interface controlling light output by a lamp. Preferably, the protection circuit includes a positive temperature coefficient (PTC) thermistor serially connected between a line voltage source and the first lead of the dimming interface, a voltage detector connected across the first and second leads to detect application of line voltage to the dimming interface and to generate a control signal, and a switch operated responsive to the control signal, where operation of the switch increases resistance of the PTC thermistor to thereby limit current to the dimmer interface and limit light output by the lamp. These and other objects, features and advantages according to the present invention are provided by a 0-lOV dimming interface protection circuit generating an indication that line voltage is applied across first and second leads of the dimming interface controlling light output by a lamp, including a first device for limiting current serially connected between a line voltage source and the first lead of the dimming interface, a second device connected across the first and second leads for detecting application of line voltage to the dimming interface and for generating a control signal, and a third device for switching, operated in response to the control signal, for causing an increase in resistance of the first device to thereby limit current to the dimmer interface and limit light output by the lamp. According to one aspect of the invention, the protection circuit also includes a fourth device for preventing spurious operation of the second device. These and various other features and aspects of the present invention will be readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, in which like or similar numbers are used throughout, and in which: Fig. 1 is a functional block diagram of a conventional lighting system which includes a plurality of ballasts controlled by a 0-1 OV dimmer; Fig. 2 illustrates a first preferred embodiment of a 0-1 OV dimmer interface circuit according to the present invention; and Fig. 3 illustrates a second preferred embodiment of a 0-10V dimmer interface circuit according to the present invention. Fig. 2 illustrates a first preferred embodiment of a 0-10V dimmer interface protection circuit according to the present invention, which includes a current limiting device, a peak voltage detector, and a switch for causing the current limiting device to operate. Moreover, the 0-10V dimmer interface protection circuit advantageously may include a device to prevent spurious operation of the protection circuitry. As illustrated in Fig. 2, a current limiting device 10, which in an exemplary case advantageously can be a positive temperature coefficient (PTC) thermistor, is installed in one lead, i.e., the Violet lead between the Violet bus and the 0-10V dimmer. It will be appreciated that a thermistor is a thermally sensitive resistor, i.e., a device whose resistance changes significantly and predictably with temperature. The resistance of a PTC thermistor increases as temperature increases. This increase in resistance advantageously is so abrupt and predictable that the PTC thermistor can function as an electronic switch. It will also be appreciated that the increase in resistance advantageously may be a several orders of magnitude increase. Still referring to Fig. 2, the protection circuit, in addition to the current limiting device 10, includes a switch Ql, which preferably can be a transistor switch and which most preferably can be a MOSFET transistor, operated by a voltage detector 12. In an exemplary case, the voltage detector 12 can be constructed from series connected diode D2 and capacitor Cl. It will be appreciated that the zener diode Dl prevents operation of the protection circuit according to the present invention due to spurious voltage transients. From inspection of Fig. 2, it will be appreciated that the switch Ql advantageously can be turned on when the common node of the voltage divider formed from resistors Rl and R2 is above the gate operating voltage. The diode D3 insures that the voltage applied to the gate is within the operating voltage. Capacitor C2 filters the noise applied to the gate of switch Ql; moreover, the time constant dictated by the RC circuit C2 -R2 advantageously controls the rate of operation of switch Ql . It should be mentioned that the PTC thermistor is selected such that at low voltage condition, e.g., less than 20V across the PTC thermistor, the element 10 advantageously will remain at a low resistance, e.g., approximately IK Ohms (Ω), However, when high voltage across element 10, e.g., 120 Vrms or 277 Vrms, a high current will result in a dramatic increase in the resistance, e.g., the resistance will increase to 100 KΩ, which thereby limits the current through both the current limiter 10 and, ultimately, the 0-10V dimming interface. It should also be mentioned that the nominal voltage for the Dl zener diode advantageously can be in the range of 50V to 180V for 277V applications, and in the range of 50 to 80V for 120V applications. During the normal operation, a low DC voltage (0-10V) will be applied across the Violet and Grey wires, i.e., be applied to the 0-10V dimming interface. In that case, the PTC thermistor 10 will remain in a low resistance state, due to the low current. The zener diode Dl will prevent the peak voltage detector from activating and, thus, the switch Ql, e.g., the MOSFET transistor will remain off. In contrast, when the dimming input Violet and Grey wires are connected to the mains, i.e., to a 120V or 277V source, instead of a low voltage source, the peak detection circuit (D2 and Cl) will detect the peak voltage of the mains, and turn MOSFET Ql on. Since R3 is a low value resistor, e.g., approximately 10 ohms, the Violet and Grey leads are effectively shorted, which will force the ballasts 201, 20N into the lowest dimming level. Meanwhile, turning on the switch Ql results in application of the over voltage across both PTC thermistor 10 and resistor R3. Since the value of the resistor R3 is much less than the steady state resistance value of PTC thermistor 10, e.g., 25-100 Ω, the current through resistor R3 is much higher than the current through PTC thermistor 10, so that the current to the dimmer interface is thereby limited. The current through PTC thermistor 10 will increase the temperature of the PTC thermistor 10, which, in turn, will increase the resistance of PTC thermistor 10 dramatically, as mentioned above. Therefore, the PTC thermistor 10 operates to further limit the current applied to the ballasts 201, 20N at the respective dimmer interfaces. Fig. 3 illustrates a second preferred embodiment of the protection circuit according to the present invention, which advantageously includes parallel PTC thermistors 10' and 10". In this exemplary configuration, resistors Rl, R2 and the zener diode Dl, which are disposed across the mains, prevent the protection circuit from operating responsive to voltage spikes and other line transients. The voltage detector 12 advantageously includes diode D2 and capacitor Cl, with resistor R4 providing a discharge path for capacitor Cl. The zener diode D3 insures that the detected voltage is greater than the gate operating voltage of switch QV, while resistor R3 and capacitor C2 determine the switching time of switch Ql'. It will be appreciated that during an over voltage condition, i.e., when the 0-1 OV dimmer interface is connected to the mains, closure of switch Ql' will result in high current through PTC thermistor 10' and 10", and, ultimately, initiation of a current limiting function.

Claims: CLAIMS: 1. A dimming interface equipped with a protection circuit (100) generating an indication that line voltage is applied to first and second leads of the dimming interface controlling light output by a lamp; comprising: means for limiting current (10) serially connected between a line voltage source terminal and the first lead of the dimming interface; means (Rl, Cl) connected across the line voltage source terminal and the second lead for detecting application of line voltage to the dimming interface and for generating a control signal; and means for switching (Ql) being part of a switching circuit connected across the line voltage source terminal and the second lead and operated responsive to the control signal to thereby limit current to the dimmer interface and limit light output by the lamp. 2. The dimming interface as recited in claim 1, further comprising means (Dl) for preventing spurious operation of the detecting means. 3. The dimming interface as recited in claim 1 or 2, wherein the switching means (Ql) comprises a transistor (Ql). 4. The dimming interface as recited in claim 3, wherein the transistor (Ql) is a MOSFET transistor. 5. The dimming interface as recited in claim 1, wherein the current limiting means (10) comprises a PTC thermistor (10). 6. The dimming interface as recited in claim 1, wherein the current limiting means comprises parallel connected first and second PTC thermistors (10', 10"). 7. The dimming interface as recited in claim 1 , wherein the indication is low light output from the lamp during operation. 8. The dimming interface as recited in claim 1, wherein the switching circuit comprises a series arrangement of an ohmic resistor and the means for switching. 9. The dimming interface as recited in claim 1, wherein the switching circuit comprises a series arrangement of the means for limiting current and the means for switching. 10. The dimming interface as recited in one or more of the previous claims, wherein the dimming interface is a 0- 10V dimming interface. 11. A dimmable ballast circuit comprising a dimming interface as recited in claim 1.

Inventors: HERNANDEZ ADAN, VENKITASUBRAHMANIAN SREERAMAN, ZHANG MICHAEL Y, HERNANDEZ, ADAN, VENKITASUBRAHMANIAN, SREERAMAN, ZHANG, MICHAEL, Y.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H02H9
IPC codes: H02H3, H02H9, H05B41