Abstract:
A circuit for protecting an electronic ballast for use with a remote controller for gas discharge lamps against miswiring to a power source. Depletion mode MOSFETS are employed in both sides of the circuit to the controller and the MOSFETS respond to overvoltage, including polarity reversal to create an open circuit condition in the circuit to the controller.

Description:
BACKGROUND OF THE DISCLOSURE 
     The present disclosure relates to circuitry for use with the ignition of gas discharge lamps and particularly to circuitry employing an electronic ballast with circuitry for interfacing with ballast with a remote controller operative for effecting dimming of the gas discharge lamps. Presently, it is common practice to house the ballast circuitry and interface circuitry in a common housing with a set of electrical leads extending therefrom for connection to a power line such as a 120 volt/240 volt single phase AC line and another set of electrical leads for outputting a low voltage low operage signal to the dimming controller. 
     In service, there has been encountered instances where, despite appropriate color coding of each set of electrical leads, the low voltage signal leads intended for connection to the controller have been miswired and connected to a line voltage power source. The connection of the low voltage interface circuit leads to the line power has resulted in an over current condition and burn out of the electronic ballast circuitry. 
     Heretofore, attempts to protect the ballast interface circuitry and ballast against over voltage have utilized a positive temperature coefficient or PTC thermistor which responds to the higher voltage by heating and increasing the resistance thereby reducing the current flow in the interface circuit. The PTC is operable to return to its lower or cold resistance state when the overvoltage is removed thereby enabling resumption of the low voltage operation with the remote controller. However, where the circuitry is encased in a potting compound, it is necessary to isolate the PTC from the potting compound to protect the potting compound from the elevated temperatures generated by the PTC in the event of miswiring. This has resulted in added cost due to the necessity of rearranging the circuitry to isolate the PTC. 
     Thus, it has been desired to find a simple and relatively low cost technique for protecting the controller interface circuitry employed with an electronic ballast for a gas discharge lamp from miswiring by connecting the low voltage controller leads to a high voltage power line. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure provides a solution to the above described problem by employing depletion mode MOSFETS in both sides of the circuit for connection to the remote controller for the gas discharge lamps in a manner that utilizes the switching properties of the depletion mode MOSFETS to respond to an overvoltage, including reversal of polarity, to create an open circuit condition in the circuit to the remote controller. 
     In the event of miswiring the leads for the low voltage controller to the high voltage power line, upon the voltage exceeding the voltage range of the controller in the positive direction one MOSFET goes open circuit; and, upon the voltage decreasing negatively outside a low voltage threshold, the MOSFET on the opposite side of the line switches to an open circuit condition thereby protecting the interface circuitry against mis-connection to a live voltage a.c. power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of the circuitry of the present disclosure; 
         FIG. 2  is an alternate arrangement of the interface circuitry portion of  FIG. 1 ; and, 
         FIG. 3  is a graph of the characteristic curves of the depletion mode MOSFETS of the type employed in the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , an electronic ballast circuit for gas discharge lamps is indicated generally in dashed outline by reference numeral  10  and may be of a conventional or well known design operative for connection at terminals  12 ,  14  to opposite sides of a power line indicated generally at LV. The ballast circuitry  10  is connected to an interface circuit indicated generally by reference numeral  16  which receives relatively low voltage D.C. power denoted, as for example 15 volts, from the electronic ballast and which is converted to a current level of about 500 microamps (500 μA) by transistor Q 1  which may be of the MOSFET variety and current limiting resistor R 1  which has a value of nominally 5 kilo ohms Zener diodes D 1 , D 2 , which may be 1N5242 devices, limit the voltage between lines  24  and  26  to about 12 Volts. The interface circuit  16  connects with the electronic ballast internally at junctions  20 ,  22 . 
     The interface circuit  16  provides a relatively low current signal of about 500 microamp along lines  24 ,  26  to the input of a protection circuit indicated generally at  30 . The circuit  30  transfers the 500 microamp current signal at output terminals  32 ,  34  which may be connected to respectively the positive and negative terminals of a low voltage controller indicated by dashed-dot line and reference numeral  38 . 
     Typically, the controller  38  is of the type which provides a variable impedance or resistance in response to a sensed condition such as, for example, the movement of a control element or member by the user, or by the signal from a sensor such as a motion, photoelectric, infrared, Doppler or other condition responsive sensor. The change in impedance (unshown) of the controller provides a variable or bi-level voltage signal for changing the voltage of the interface circuit for controlling dimming of the output of the electronic ballast to the gas discharge lamps. 
     The positive lead  24  of the interface circuit is connected to the source of switching device Q 3 ; and, the drain is connected to the connector terminal  32  for connection to controller  38 . The gate of Q 3  is connected to the opposite side  26  of the signal line. Referring to the enlarged view in  FIG. 1 , the device Q 3  has an intrinsic diode D 5  connected between the source and drain such that the device conducts so long as the voltage applied to the positive terminal in connector  32  is not greater than the 12 volt supply by virtue of the blocking diode D 5 . In the event a negative voltage greater than negative  12  is applied to the terminals, the upper transistor Q 3  will continue to conduct; and, if the gate to source voltage is greater than its threshold voltage which is typically around −2.5 Volts, the device will be in its ON state and the current source will supply 500 microamps to the controller. As the negative voltage increases in magnitude, i.e. gets more negative, current will continue to flow at the same level, but the drain to source voltage of the lower transistor Q 4  will increase; and, the device Q 4  matches the amplitude of the current source at the higher voltages to until the avalanche voltage of the device is reached, which in the present practice is about 600 volts. Thus, the circuit  30  of the arrangement of  FIG. 1  protects the interface circuit  16  from overvoltage, either positive or negative, in the event of miswiring a connection of terminals  32 ,  34  to a power line. 
     In the present practice, the electronic ballast circuit  10 , the interface circuit  16  and the protection circuitry  30  may be incorporated into a common housing with a pair of color coded leads extending to terminals  12 ,  14  for a connection to a power line and with another pair of differently color coded leads extending therefrom for connection to terminals  32 ,  34  for connection to a remote controller. 
     Referring to  FIG. 2 , an alternative arrangement of the interface circuit is indicated generally at  40  and has about 500 microamps of current supplied from the interface circuit  16  with diodes D 3 , D 4  connected across the line thereof. Protection circuitry  40  of the arrangement of  FIG. 2  has a switching device Q 5  connected in the negative side of the 12 volt signal line similar to the switching device Q 4  of  FIG. 1 , which device Q 5  may also be of the depletion mode MOSFET type. In the arrangement of  FIG. 2  the positive side of the line  42  has a diode D 5  connected therein; and, upon miswiring and application of a high voltage to the terminal  42 , the diode continues to conduct current flow in the line. The voltage on switching device Q 5  continues to increase until an avalanche condition occurs as in paragraph. 
     The drain to source voltage of the device Q 5  will be able to increase with the external voltage and prevent the device Q 5  from switching to open circuit. If the polarity of the external source is reversed, D 5  will block if this voltage exceeds about 12 Volts. The difference is that the version of  FIG. 1  will not have a diode drop because the channel of Q 3  will continue to conduct until the threshold voltage is reached. In this way, the version of  FIG. 1  has better linearity, especially at low controller voltages, i.e. voltages below 2.5 Volts. 
     Referring to  FIG. 3 , typical characteristics of the type BSS  126  switching devices employed are shown where it is noted that the current is maintained, substantially constant so long as the gate voltage is maintained irrespective of the source to drain voltage. 
     The present circuitry provides unique low cost and simple additions to an electronic ballast and interface circuitry for gas discharge lamps where it is desired to connect a controller for dimming the lamps to the circuitry and to provide protection in the event the ballast is miswired to the power line. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.