Abstract:
A secondary ground fault protection for a high voltage power supply has a high voltage transformer with a center tapped secondary coil. The primary coil of a monitoring transformer is connected to the secondary coil at the center tap, which is approximately the midpoint of the secondary coil. The power supply load is connected across the end terminals of the secondary coil. The monitoring transformer is connected between the center tap and an earth ground on the primary coil side and between sensing circuitry and a digital ground on the secondary side. The sensing circuitry includes sub-circuits to generate various outputs which indicate the presence of faults, including a floating ground, excessive fault current or an open sensor transformer. The circuit outputs can be combined using a logical OR gate to cause specific actions in response to each detected fault, including terminating the high voltage generation in response to an excessive fault current. The fault detection circuit includes binary inputs for indicating what load is being powered by the power supply so that the ground fault sensing is more accurate and effective.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of ground fault interrupt protection for electrical power supplies and in particular to a new and useful ground fault protection for the secondary coil of a high voltage transformer. The ground fault protection is especially useful for power supplies used to power lighting applications through a high voltage transformer, such as a neon lighting display. 
     Ground fault protection circuits for lighting display power supplies are generally known in the art. U.S. Pat. No. 5,751,523, for example, discloses a power supply for neon lamps having a transformer with a return path that is separate from the earth ground, which permits detection of a fault current. The primary coil is connected to a power source. A load, such as gas discharge tubes, is connected across the secondary coil end terminals. The mid-point of the secondary coil of the power supply is connected to one side of a secondary ground fault protection circuit and to ground. The secondary ground fault protection circuit is also connected to the primary coil terminals. The secondary ground fault protection circuit includes a relay for breaking the connection to the AC power source connected to the primary coil when a ground fault is detected. 
     Other patents disclosing power supplies having fault protection include U.S. Pat. Nos. 5,387,845, 5,841,239, 5,241,443, 4,507,698 and 3,666,993. 
     In certain cases, different gas pressures and types of gas discharge tubes, such as neon gas tubes, can present different amplitude loads to power supplies. In the case of a system where the color generated by a gas discharge tube may be changed by changing the voltage amplitude, frequency and/or duty cycle supplied to the tube, a power supply which is operating safely when driving tubes generating a yellow color may be subject to faults when a blue color is generated using the same power supply instead. 
     There is a need for a ground fault protection circuit for a power supply having a sensing circuit which can detect ground faults, a bad sensing circuit and floating grounds while distinguishing between different amplitude loads. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a secondary ground fault protection for a power supply transformer having fault sensing circuitry capable of indicating and reacting to different fault conditions. 
     It is a further object of the invention to provide a secondary ground fault protection which can verify the function of the sensing transformer and the security of the earth ground connection. 
     It is yet another object of the invention to provide a secondary ground fault protection for terminating both hardware high voltage generation and power supply software power generation instructions. 
     A further object of the invention is to provide a ground fault protection for a power supply used to power a changing load. 
     Accordingly, a secondary ground fault protection for a high voltage power supply is provided having a high voltage transformer with a center tapped secondary coil. The primary coil of a monitoring transformer is connected to the secondary coil at the center tap, which is approximately the midpoint of the secondary coil. The power supply load is connected across the end terminals of the secondary coil. 
     The monitoring transformer is connected between the center tap and an earth ground on the primary coil side and between sensing circuitry and a digital ground on the secondary side. The sensing circuitry includes sub-circuits that can generate outputs indicating the presence of faults, including a floating ground, excessive fault current or a defective sensor circuit. The sub-circuit outputs can be connected to cause specific actions in response to a particular fault, such as terminating the high voltage generation in response to an excessive fault current. The ground fault detection circuit includes inputs for indicating what load is being powered by the power supply. The ground fault sensing is made more accurate and effective by using a threshold comparison voltage corresponding to the load being powered. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic circuit drawing of the connection between the power transformer and a monitoring transformer according to the invention; 
     FIG. 2 is a schematic circuit drawing of a ground fault situation in the circuit of FIG. 1; 
     FIG. 3 is a schematic circuit drawing showing an equivalent circuit to the one in FIG. 2; 
     FIGS. 4A and 4B are two halves of a schematic circuit drawing of a sensor circuit according to the invention; and 
     FIG. 5 is a schematic circuit drawing of a portion of the sensor circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, in which like reference numerals are used to refer to the same or similar elements, FIG. 1 shows a schematic drawing of a power supply high voltage transformer T 1  connected to a series of loads, such as neon tubes N 1 , N 2  . . . Ni, and the primary coil TW 21  of a ground path sensing transformer T 2 . The loads N 1  . . . Ni are connected across the end terminals V 1 , V 2  of the secondary coil TW 12  of high voltage transformer T 1 . The primary coil TW 21  of sensing transformer T 2  is connected between a center tap at the midpoint M of high voltage transformer T 1  secondary coil TW 12  and earth ground  5 . The primary coil TW 11  of the high voltage transformer T 1  is connected to a power source  20 . 
     The secondary coil TW 22  of the sensing transformer T 2  is connected between a sensing circuit  10  for detecting different fault conditions, and a digital ground  15 . 
     Sensing transformer T 2  has a coil turn ratio of 1:1, in order to isolate the sensing circuitry  10  from the secondary coil TW 12 . Due to the coil turn ratio, any changes in the ground path in the high voltage transformer T 1  will be represented nearly identical in ground path sensing transformer T 2 . 
     In normal operation (no fault conditions), the neon tube loads N 1  . . . Ni will generate light. Since the loads N 1  . . . Ni are isolated from ground  5 , there will be no current flowing through the center tap M or to the primary coil TW 21  of sensing transformer T 2 . The voltage V 1 -V 2  is preferably about twice V 1 -M and V 2 -M. 
     Referring now to FIG. 2, if a ground path is closed, such as the secondary ground fault  6 , through a resistance R G , the resulting circuit can be modeled by the equivalent circuit of FIG.  3 . 
     As shown in FIG. 3, a current I G  is created through the sensing transformer T 2 . A voltage source  25  equivalent to V 2 -M is created on the high terminal of primary coil TW 21  of the sensing transformer T 2 . Assuming there are no losses in sensing transformer T 2 , the current is determined by the equation: I G =(V 2 -M)/R G ( 1) Again, because of the coil turn ratio, the current IG will be represented substantially identical in the secondary coil TW 22  of sensing transformer T 2 . This permits monitoring of the center tap M of the high voltage transformer T 1 . If a current IG, or other secondary ground fault current is developed, a reaction can be produced depending on the current value. A sensing circuit  10  connected to the secondary coil TW 22  of the sensing transformer T 2  can be used to both sense and react to the presence of a ground fault in the high voltage transformer T 1 . 
     Referring to FIG. 4, a sensing circuit  10  is shown having a different output indicators  100 ,  102 ,  104 ,  106 ,  108  for displaying different fault conditions connected to the sensing transformer T 2 . A series of inputs D 0 , D 1 , D 2  on analog switch  50  are connected to a microprocessor  300  are used to indicate to the sensing circuit  10  what load is present on the high voltage transformer T 1 . 
     The components of the sensing circuit  10  shown in FIG. 4 are connected as follows. 
     A center tap M 2  on the secondary coil TW 22  of sensing transformer T 2  is connected to ground  15 . Each terminal of the secondary coil TW 22  is connected to an anode of one of rectifying diodes D 11 , D 12 . The cathodes of rectifying diodes D 11 , D 12  are connected together to provide full wave rectification of the complex power waveform present on the secondary coil TW 22  when a fault occurs. Filter capacitors C 38 , C 39  are connected in parallel between the cathode of diodes D 11 , D 12  and ground  15  as filters for completing the rectification circuit  200 , while parallel connected limiting resistor R 25  determines the maximum DC amplitude that the fault current will generate for a given current value. 
     A peak hold circuit  210  has the anode of peak hold diode D 13  connected to the cathodes of rectifying diodes D 11 , D 12 . Peak hold capacitor C 45  and resistor R 12  are connected in parallel between the cathode of peak hold diode D 13  and ground  15 . The cathode of peak hold diode D 13  provides an input voltage to the non-inverting terminal of comparator  70 , which is used as a voltage follower  220 . 
     The output of voltage follower comparator  70  is connected in a feedback loop to the inverting terminal. Comparator  70  also has power connections to Vcc and ground  15 . It should be noted that comparators  70 ,  75 ,  80 ,  85  can all be contained on the same chip, and so power connections are only shown for voltage follower/comparator  70 . The voltage follower  220  is used to couple the high impedance circuitry of the rectifier  200  and peak hold  210  circuits to the remainder of the sensor circuit  10 . 
     The output of voltage follower  70  is connected directly to the non-inverting terminal of comparator  75 . The inverting terminal of comparator  75  is connected to a reference voltage output  55  generated by reference voltage generator circuit  270 . 
     The reference voltage generator circuit  270  provides a reference voltage output  55  from analog switch  50  based on the binary inputs D 0 , D 1 , D 2 . When three inputs D 0 , D 1 , D 2  are used, a total of eight combinations are possible, and, therefore, eight different reference voltages  55  can be generated. The ability to selectively choose different reference voltages  55  permits a microprocessor controller  300  which is used to select different known loads to send a binary code input signal using binary inputs D 0 , D 1 , D 2  corresponding to a particular one of the known loads. In this way, the reference voltage  55  can be adjusted to the known load being supplied power, and in effect, tuned to the particular load. 
     The analog switch  50  can be one such as a 74HC4051 made by Motorola, having three channel selector inputs A,B,C and a non-inverted output X. Alternatively, a digital-to-analog (D/A) converter can be used for the analog switch  50 . 
     As shown, the inputs D 0 , D 1 , D 2  are connected to channel selector inputs A, B, C, respectively. One of eight different resistances, R 33  through R 40 , are connected between a fixed reference voltage Vcc and each of eight channel inputs X 0 -X 7 . The output reference voltage  55  is determined by voltage division of the fixed reference voltage Vcc across the resistance R 33 -R 40  on the selected channel X 0 -X 7  and series-connected division resistor R 41 . Capacitor C 46  is connected in parallel with division resistor R 41  to filter noise components from the output reference voltage  55 . 
     When the non-inverting terminal input voltage of comparator  75  is greater than the applied reference voltage  55 , a fault is indicated and the output of comparator  75  is high, or a digital  1 . This causes indicator SGFAULT  100  to activate, such as a signal lamp or tone, thereby providing notification that the power supply is producing an RMS ground fault current and generating a corresponding ground fault voltage that is higher than the selected preset output reference voltage  55 . The indicator SGFAULT  100  can also be connected to a switch on the power supply controller, such as a CPU (not shown), to stop generating power at the power source  20  connected to the high voltage transformer T 1 . 
     As a further result of comparator  75  producing a digital high output, latching circuit  230  is activated by logic diode D 16  having its anode connected to the output of comparator  75  conducting the high signal to the non-inverting input of latch comparator  80 . In normal (non-fault) operation, the non-inverting input of latch comparator  80  is a digital low. The inverting terminal of comparator is set at Vcc/2, as a result of voltage division of Vcc across matched resistors R 27  and R 28 , which have the same resistance value. Thus, until the voltage applied to the non-inverting terminal of comparator  80  is a digital high, the output will be a digital low. 
     The high input signal from diode D 16  causes the output of latch comparator  80  to also go high. FAULT indicator  104  is connected to latch comparator  80  and is activated by the high output. Latch diode D 10  is connected in a feedback loop from the output to the non-inverting input of latch comparator  80 . Conducting resistor R 29  is connected between the non-inverting input of latch comparator  80  and ground to generate a voltage at the input when any of the diodes D 10 , D 14 , D 15 , D 16  are conducting. Latch diode D 10  conducts as well when a high signal is present, preventing the sensor circuit  10  from leaving the fault condition, even if the fault is removed, until it is reset by turning the power to the circuit  10  off and back on. 
     The latching circuit  230  output also controls power switch circuit  240 . The power switch circuit  240  has transistor Q 6  with the emitter connected to Vcc, the base connected to the output of latch comparator  80  through resistor R 23 , and the collector connected to SAFEVCC indicator  106 . Resistor R 24  is connected between the emitter and base of transistor Q 6 . Transistor Q 6  is normally conducting, so that SAFEVCC indicator  106  is at voltage Vcc When the output from the latching circuit  230  is a digital high, the voltage across resistor R 23  cause transistor Q 6  to stop conducting, and reduces SAFEVCC indicator  106  to zero. The SAFEVCC indicator  106  can also be connected to a relay for enabling (no fault) or disabling (fault condition) the high voltage power supply to the high voltage transformer T 1 . The power switch circuit  240  provides hardware safety control for the power supply having the sensor circuit  10 . 
     Thus, the operation of the sensing circuit  10  to detect ground faults and take corrective action to prevent damage to the power supply or loads has been described. 
     The sensing circuit  10  further includes sub-circuits for detecting a defective sensing transformer  250  and detecting a floating ground  260 . Logic diodes D 14 , D 15  and D 16 , which have their cathodes connected at a common node, create a logical OR gate  280 . Thus, if the output of any one of the fault sensing comparator  75 , defective sensing transformer circuit  250  or floating ground detection circuit  260  is a digital high, the latching circuit  230  and power switch circuit will be activated, thereby shutting down the power supply until it is reset. 
     With reference to both FIGS. 4 and 5, if the primary coil TW 21  of sensing transformer T 2  opens, the center tap M of high voltage transformer T 1  will be floating. If there is a floating center tap, the sensing transformer T 2  and sensing circuit  10  may not detect a secondary ground fault. In order to avoid this condition, the center tap M is also connected to neon lamp LP 3  through current limiting resistor R 12 . The neon lamp LP 3  is optically coupled and sealed with phototransistor Q 7 , having the emitter connected to ground and the collector connected to the inverting terminal of comparator  60 . 
     In non-fault, non-floating center tap operation, no current flows to the neon lamp LP 3 , and so the phototransistor Q 7  does not conduct. When the sensing transformer T 2  opens, however, a current and voltage are generated at current limiting resistor R 12  which is sufficient to power lamp LP 3 . Phototransistor Q 7  is then placed in the conducting state. 
     As seen in FIG. 4, the fixed reference voltage Vcc is also connected to the inverting terminal of comparator  60  across current limiting resistor R 26 . Filter capacitor C 40  is used to filter AC components from the voltage input to the non-inverting terminal. The non-inverting terminal of comparator  60  has a voltage equal to Vcc/2 applied to it, so that the inverting terminal voltage is about equal to Vcc, and the output is a digital low, until phototransistor Q 7  begins conducting. 
     Once phototransistor Q 7  begins conducting, it creates a short circuit of the voltage Vcc applied to the inverting terminal, dropping it to about zero, and causing the output of comparator  80  to go to a digital high. The high output from comparator  80  causes logic diode D 15  to begin conducting and latch circuit  230  and power switch circuit  240  to activate. The output from comparator  80  is also connected to FAIL SENSOR indicator  102 , which activates when the output is high. Thus, an open sensing transformer T 2  will be detected by the defective sensing transformer circuit  250 . 
     The floating ground protection circuit  260  works in the reverse manner to the defective sensing transformer circuit  250 . 
     A line input LINE powers neon lamp LP 1  connected in series with a current limiting resistor R 30  and earth ground  5 . The light from neon lamp LP 1  causes phototransistor Q 5  to conduct, thereby shorting reference voltage Vcc connected across resistor R 32  to the collector through the emitter to ground  15 . A neon lamp and phototransistor are preferred for use instead of an opto coupler due to an Underwriter&#39;s Laboratory (UL) limitation, UL-2161, which indicates that maximum ground leak currents should not exceed 0.5 mA. That level of current is not sufficient to power known opto coupler diodes, but can be used to power a neon lamp for use as the switch. Clearly, however, current switches which operate within this limitation can be substituted for the optically coupled neon lamp LP 3  and phototransistor Q 7 . A filter capacitor C 41  is connected to the non-inverting terminal. The collector of phototransistor Q 5 , and reference voltage Vcc, are also connected to the non-inverting terminal of comparator  85 . While phototransistor Q 5  is conducting, however, the applied voltage at the non-inverting terminal is zero. The divided reference voltage Vcc/2 is applied to the inverting terminal of comparator  85 . Thus, when the earth ground  5  is solidly connected, the output of comparator  85  is a digital low, and no current flows through logic diode D 14  to the latching circuit  230 . 
     When the earth ground  5  is disconnected, the neon lamp LP 1  stops emitting light, causing phototransitor Q 5  to stop conducting, thereby applying reference voltage Vcc to the non-inverting terminal of comparator  85 . Since the applied voltage at the non-inverting terminal is higher than the Vcc/2 voltage at the inverting terminal, the comparator begins outputting a digital high signal. Logic diode D 14  conducts the high signal to the latching circuit  230  and power switch circuit  240 , causing the power supply to shut down. The high output from comparator  85  will also activate the GNDOPEN indicator  108  connected to the output. 
     In one application of the ground fault protection of the invention, in the sensor circuit  10  shown in FIG. 4, the three inputs D 0 , D 1 , D 2  preferably correspond to one of eight color levels that can be produced using a power supply driving a gas discharge tube, depending on the output voltage of the power supply. The power supply is connected to the sensor circuit  10  via the center tap M on the secondary coil TW 12  of the high voltage transformer T 2 . The inputs D 0 , D 1 , D 2  are provided by a CPU or other controller circuit  300  based on the color level being output. 
     It is envisioned that additional sensing circuits can be connected to the latching and power supply circuits  230 ,  240  using the logical OR gate  280  by connecting the cathode of another logic diode to the common output. 
     The indicators  100 ,  102 ,  104 ,  106 ,  108  can each be a different lamp, such as an LED, or they can represent a circuit designed to make a single indicator lamp flash in different patterns or colors to convey the particular fault which has occurred to an operator of the power supply. 
     In a preferred embodiment, values for the components identified in the circuit of FIG. 4 are as follows. Resistors R 12 , R 23 , R 24 , R 25 , R 26 , R 29  and R 32  can each be 10 kohms. The resistances of R 27  and R 28  should be equal, and preferably large, such as about 50 kohms each. Values for R 33 - 40  and R 40  must be selected based on the loads which will be applied to the power supply. R 30  is preferably about 100 kohms, and adjusted to comply to UL-2161 standards. Capacitors C 38 -C 40  and C 45  can be any value which sufficiently filters AC components from the circuits. Preferably, the capacitance of C 41  is about 10 μF, and C 37 , C 44  and C 46  are each about 0.1 μF. 
     Latching diode D 10 , peak hold diode D 13  and logic diodes D 14 , D 15 , and D 16  are all preferably type 1N914. Diode type 1N4001 are preferred for rectifying diodes D 11 , D 12 . A suitable integrated circuit containing comparators  70 ,  75 ,  80  and  85  is a LM324 made by National Semiconductor. An LM358 chip from National Semiconductor is preferred for comparator  60 . Vcc is preferably set at about 5V for digital operation, and digital ground  15  is preferably 0V. A BJT transistor type 2N3906 from Motorola can be used for transistor Q 6 . 
     While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.