Power filter circuit responsive to supply system fault conditions

Abstract

A power filter circuit for protecting electronic equipment from electromagnetic interference introduced by a supply system circuit includes a fault/condition sensing circuit capable of detecting whether the supply system circuit is properly wired. The sensing circuit controls switching circuits in accordance with the determination of whether the supply system is properly wired. The sensing circuit activates a first switching circuit to form a short circuit between the neutral and ground conductors of the power filter circuit when it is determined that the supply system circuit is properly wired. If any wiring fault conditions, including a reverse polarity wiring, is detected by the sensing circuit, the first switch is not activated and the short circuit is not formed between the neutral and ground conductors. The sensing circuit controls a second switching circuit which corrects the polarity when a reverse polarity condition is detected and which does not change the polarity when a correct polarity condition is detected. The sensing circuit can be used with the first and second switching circuits in combination. In this case, the first switching circuit can form a short circuit between the line conductor of the power filter circuit and the ground conductor when a reverse polarity condition is detected to provide superior common mode electromagnetic interference suppression.

1. A circuit for controlling electromagnetic interference, comprising:
 a line conductor, a neutral conductor and a ground conductor, each connectable to a load and to a supply system having line, neutral and ground connectors, to provide an electrical connection between the supply system and the load;  
 a wiring fault condition sensing circuit connected to said line conductor, said neutral conductor and said ground conductor, said sensing circuit being configured to determine whether the line, neutral and ground connectors of the supply system are correctly wired and connected; and  
 a switch activated by said sensing circuit to form a short circuit between said neutral conductor and said ground conductor, irrespective of a voltage level appearing across said neutral conductor and said ground conductor, when all of the line, neutral and ground connectors of the supply system are correctly wired and connected.  
 
     
     
       2. The circuit according to  claim 1 , wherein said switch is not activated by said sensing circuit and a short circuit is not formed between said neutral conductor and said ground conductor when said sensing circuit detects that any one or any combination of the line, neutral and ground connectors of the supply system is not correctly wired or connected. 
     
     
       3. The circuit according to  claim 1 , wherein said switch has a first contact connected directly to said neutral conductor and a second contact connected directly to said ground conductor, said first and second contacts being connected by closure of said switch. 
     
     
       4. The circuit according to  claim 1 , wherein said short circuit suppresses all voltage transients present across said neutral conductor and said ground conductor. 
     
     
       5. The circuit according to  claim 1 , wherein said switch comprises a switching relay having a switching relay coil and a switch element connectable between first and second contacts. 
     
     
       6. The circuit according to  claim 5 , wherein said sensing circuit comprises:
 a voltage rectifier circuit connected between said line conductor and said ground conductor;  
 a visual indicator adapted to produce an indication of a state of the supply system; and  
 a first transistor and a second transistor arranged as a Darlington pair and responsive to a rectified voltage produced by said voltage rectifier circuit to supply drive current to said visual indicator and to said switching relay coil in order to activate said visual indicator and said switching relay coil when all of the line, neutral and ground connectors of the supply system are correctly wired and connected.  
 
     
     
       7. The circuit according to  claim 6 , wherein:
 said voltage rectifier circuit comprises a first resistor, a diode and a second resistor connected in series between said line conductor and said ground conductor;  
 a base of said first transistor is connected to a terminal of said first resistor that is connected to a terminal of said first diode;  
 a base of said second transistor is connected to an emitter of said first transistor, an emitter of said second transistor is connected to said line conductor, and a collector of said second transistor is connected to a collector of said first transistor;  
 said sensing circuit further comprising:  
 a second diode and a third resistor connected in series with an emitter-collector junction of said second diode and said visual indicator between said line conductor and said neutral conductor;  
 a capacitor and a fourth resistor connected in series with the emitter-collector junction of said second transistor and said second diode between said line conductor and said neutral conductor, wherein said switching relay coil is connected across said capacitor; and  
 a suppression circuit connected in parallel with said capacitor and adapted to suppress switching transients produced by said switching relay coil.  
 
     
     
       8. The circuit according to  claim 7 , wherein said suppression circuit comprises a diode and a Zener diode connected in series. 
     
     
       9. The circuit according to  claim 5 , wherein said switching relay comprises a single pole, single throw relay. 
     
     
       10. The circuit according to  claim 1 , further comprising:
 an overload protection stage adapted to protect the load from overload conditions;  
 a diverter stage adapted to suppress transient overvoltages;  
 a normal mode filtering stage adapted to suppress normal mode electromagnetic interference; and  
 a clamping stage adapted to suppress residual voltage transients.  
 
     
     
       11. A circuit for detecting and automatically correcting for an incorrectly wired AC power supply system, comprising:
 an input line conductor and an input neutral conductor, each connectable to a supply system having line, neutral and ground connectors;  
 an output line conductor and an output neutral conductor, each connectable to a load;  
 a ground conductor connectable to said supply system and said load;  
 a switch coupled to the input and output line and neutral conductors and having a first state in which the input line conductor is connected to the output line conductor and the input neutral conductor is connected to the output neutral conductor and a second state in which the input line conductor is connected to the output neutral conductor and the input neutral conductor is connected to the output line conductor; and  
 a reverse polarity wiring fault condition sensing circuit connected to said input line conductor, said input neutral conductor and said ground conductor, said reverse polarity sensing circuit controlling said switch to be in the second state only when said reverse polarity sensing circuit detects that all three of the line, neutral and ground connectors of the supply system are connected, with the ground connector correctly wired but with the line and neutral connectors reversed, such that when an AC voltage is present across the neutral and ground connectors of the supply system and across the neutral and line connectors of the supply system, the AC voltage is present across the output line conductor and the ground conductor at the load.  
 
     
     
       12. The circuit according to  claim 11 , wherein when said reverse polarity sensing circuit does not detect that all of the line, neutral and ground connectors of the supply system are connected with the ground connector correctly wired but with the line and neutral connectors reversed, said switch remains in the first state. 
     
     
       13. The circuit according to  claim 11 , wherein said switch comprises a double pole, double throw relay having a first contact connected to said input line conductor, a second contact connected to said output line conductor, a third contact connected to said output neutral conductor, a fourth contact connected to said input neutral conductor, a fifth contact connected to said output neutral conductor, and a sixth contact connected to said output line conductor, wherein:
 in the second state, said first and third contacts are connected and said fourth and sixth contacts are connected; and  
 in the first state, said first and second contacts are connected and said fourth and fifth contacts are connected.  
 
     
     
       14. The circuit according to  claim 11 , wherein said switch comprises a switching relay having a switching relay coil, a first switching element controlled by said switching relay coil to be in one of two states, and a second switching element controlled by said switching relay coil to be in one of two states, current to said visual indicator and to said switching relay coil in order to activate said visual indicator and said switching relay coil when the integrity of all of the line, neutral and ground connectors of the supply system is sound and the supply system presents a reverse polarity voltage across the line and neutral connectors. 
     
     
       15. The circuit according to  claim 14 , wherein said sensing circuit comprises:
 a voltage rectifier circuit connected between said input neutral conductor and said ground conductor;  
 a visual indicator adapted to produce an indication of a state of the supply system; and  
 a first transistor and a second transistor arranged as a Darlington pair and responsive to a rectified voltage produced by said voltage rectifier circuit to supply drive current to said visual indicator and to said switching relay coil in order to activate said visual indicator and said switching relay coil when all of the line, neutral and ground connectors of the supply system are connected, with the ground connector correctly wired but with the line and neutral connectors reversed.  
 
     
     
       16. The circuit according to  claim 15 , wherein:
 said voltage rectifier circuit comprises a first resistor, a diode and a second resistor connected in series between said input neutral conductor and said ground conductor;  
 a base of said first transistor is connected to a terminal of said first resistor that is connected to a terminal of said first diode;  
 a base of said second transistor is connected to an emitter of said first transistor, an emitter of said second transistor is connected to said input neutral conductor, and a collector of said second transistor is connected to a collector of said first transistor;  
 said sensing circuit further comprising: 
 a second diode and a third resistor connected in series with an emitter-collector junction of said second diode and said visual indicator between said input neutral conductor and said input line conductor;  
 a capacitor and a fourth resistor connected in series with the emitter-collector junction of said second transistor and said second diode between said input neutral conductor and said input line conductor, wherein said switching relay coil is connected across said capacitor; and  
 a suppression circuit connected in parallel with said capacitor and adapted to suppress switching transients produced by said switching relay coil.  
 
 
     
     
       17. The circuit according to  claim 16 , wherein said suppression circuit comprises a diode and a Zener diode connected in series. 
     
     
       18. The circuit according to  claim 11 , further comprising:
 a normal polarity, wiring fault condition sensing circuit connected to said output line conductor, said output neutral conductor and said ground conductor, said normal polarity sensing circuit being configured to determine whether an integrity of each of the line, neutral and ground connectors of the supply system is sound and whether a correct polarity voltage is present across said output line conductor and said output neutral conductor; and  
 a short-circuit switch activated by said normal polarity sensing circuit to form a short circuit between said output neutral conductor and said output ground conductor, irrespective of a voltage level appearing across said neutral conductor and said ground conductor, when the integrity of all of the line, neutral and ground connectors of the supply system is sound and a correct polarity is present across said output line conductor and said output neutral conductor.  
 
     
     
       19. The circuit according to  claim 18 , wherein said short-circuit switch is not activated by said normal polarity sensing circuit and a short circuit is not formed between said output ground conductor and said output neutral conductor when said normal polarity sensing circuit detects that any of the line, neutral and ground connectors of the supply system is disconnected. 
     
     
       20. The circuit according to  claim 19 , wherein said short-circuit switch has a first contact connected directly to said output neutral conductor and a second contact connected directly to said ground conductor, said first and second contacts being connected by closure of said short-circuit switch. 
     
     
       21. The circuit according to  claim 18 , wherein said short-circuit switch is a single pole, single throw relay. 
     
     
       22. The circuit according to  claim 18 , further comprising:
 an overload protection stage adapted to protect the load from overload conditions;  
 a diverter stage adapted to suppress transient overvoltages;  
 a normal mode filtering stage adapted to suppress normal mode electromagnetic interference; and  
 a clamping stage adapted to suppress residual voltage transients.  
 
     
     
       23. A circuit for controlling electromagnetic interference, comprising:
 an input line conductor and an input neutral conductor, each connectable to a supply system having line, neutral and ground connectors;  
 an output line conductor and an output neutral conductor, each connectable to a load;  
 a ground conductor connectable to said supply system and said load;  
 a normal polarity wiring fault condition sensing circuit connected to said input line conductor, said input neutral conductor and said ground conductor, said normal polarity sensing circuit being configured to determine whether the line, neutral and ground connectors of the supply system are correctly wired and connected;  
 a normal polarity switch circuit activated by said normal polarity sensing circuit to connect said input line conductor to said output line conductor and to connect said input neutral conductor to said output neutral conductor when all of the line, neutral and ground connectors of the supply system are correctly wired and connected;  
 a reverse polarity wiring fault condition sensing circuit connected to said input line conductor, said input neutral conductor and said ground conductor said reverse polarity sensing circuit being configured to determine whether each of the line, neutral and ground connectors of the supply system is connected and whether an AC voltage is present across the neutral and ground connectors; and  
 a reverse polarity switch circuit activated by said reverse polarity sensing circuit to connect said input line conductor to said output neutral conductor and to connect said input neutral conductor to said output line conductor when all of the line, neutral and ground connectors of the supply system are connected and an AC voltage is present across the neutral and ground connectors.  
 
     
     
       24. The circuit according to  claim 23 , wherein said normal polarity switch circuit is not activated by said normal polarity sensing circuit, said input line conductor is not connected to said output line conductor, and said input neutral conductor is not connected to said output neutral conductor when said normal polarity sensing circuit detects that any one or any combination of the line, neutral and ground connectors of the supply system is not correctly wired or connected. 
     
     
       25. The circuit according to  claim 23 , further comprising:
 an overvoltage protection circuit causing said normal polarity sensing circuit not to activate said normal polarity switch circuit during an overvoltage condition, whereby said input line conductor is not connected to said output line conductor, and said input neutral conductor is not connected to said output neutral conductor.  
 
     
     
       26. The circuit according to  claim 23 , wherein said normal polarity switch circuit comprises a double pole, single throw switching relay having a switching relay coil, a first switch element connectable between said input line conductor and said output line conductor and a second switch element connectable between said input neutral conductor and said output neutral conductor. 
     
     
       27. The circuit according to  claim 26 , wherein said normal polarity sensing circuit comprises:
 a voltage rectifier circuit connected between said line conductor and said ground conductor;  
 a visual indicator adapted to produce an indication of a state of the supply system; and  
 a first transistor and a second transistor arranged as a Darlington pair and responsive to a rectified voltage produced by said voltage rectifier circuit to supply drive current to said visual indicator and to said switching relay coil in order to activate said visual indicator and said switching relay coil when all of the line, neutral and ground connectors of the supply system are correctly wired and connected.  
 
     
     
       28. The circuit according to  claim 27 , wherein:
 said voltage rectifier circuit comprises a first resistor, a diode and a second resistor connected in series between said line conductor and said ground conductor;  
 a base of said first transistor is connected to a terminal of said first resistor that is connected to a terminal of said first diode;  
 a base of said second transistor is connected to an emitter of said first transistor, an emitter of said second transistor is connected to said line conductor, and a collector of said second transistor is connected to a collector of said first transistor;  
 said sensing circuit further comprising: 
 a second diode and a third resistor connected in series with an emitter-collector junction of said second diode and said visual indicator between said line conductor and said neutral conductor;  
 
 a capacitor and a fourth resistor connected in series with the emitter-collector junction of said second transistor and said second diode between said line conductor and said neutral conductor, wherein said switching relay coil is connected across said capacitor; and  
 a suppression circuit connected in parallel with said capacitor and adapted to suppress switching transients produced by said switching relay coil.  
 
     
     
       29. The circuit according to  claim 27 , further comprising an overvoltage protection circuit, including:
 a non-linear device connected between said input line conductor and said input neutral conductor, said non-linear device conducting only in response to an overvoltage condition; and  
 a silicon controlled rectifier responsive to said non-linear device to remove the drive current from the Darlington pair when said non-linear device is conducting, thereby causing said normal polarity sensing circuit not to activate said normal polarity switch circuit during an overvoltage condition, such that said input line conductor is not connected to said output line conductor, and said input neutral conductor is not connected to said output neutral conductor.  
 
     
     
       30. The circuit according to  claim 29 , wherein:
 said non-linear device is a PNPN diode;  
 said overvoltage protection circuit further includes a first resistor and a second resistor connected in series with said PNPN diode between said input line conductor and said input neutral conductor; and  
 an anode of said silicon controlled rectifier is connected to a base of the first transistor, a cathode of said silicon controlled rectifier is connected to said input line conductor, and a control gate of said silicon controlled rectifier is connected between said first and second resistors.  
 
     
     
       31. The circuit according to  claim 23 , further comprising a short-circuit conductor connected between said output neutral conductor and said ground conductor and forming a permanent short-circuit between said output neutral conductor and said ground conductor. 
     
     
       32. The circuit according to  claim 31 , further comprising:
 an overload protection stage adapted to protect the load from overload conditions;  
 a diverter stage adapted to suppress transient overvoltages;  
 a normal mode filtering stage adapted to suppress normal mode electromagnetic interference; and  
 a clamping stage adapted to suppress residual voltage transients.  
 
     
     
       33. The circuit according to  claim 23 , further comprising:
 an overload protection stage adapted to protect the load from overload conditions;  
 a diverter stage adapted to suppress transient overvoltages;  
 a normal mode filtering stage adapted to suppress normal mode electromagnetic interference; and  
 a clamping stage adapted to suppress residual voltage transients.  
 
     
     
       34. The circuit according to  claim 23 , wherein said reverse polarity switch circuit is not activated by said reverse polarity sensing circuit, said input line conductor is not connected to said output neutral conductor, and said input neutral conductor is not connected to said output line conductor when said reverse polarity sensing circuit detects that any of the line, neutral and ground connectors of the supply system is disconnected or an AC voltage is present across the line and ground connectors of the supply system. 
     
     
       35. The circuit according to  claim 23 , wherein said reverse polarity switch circuit comprises a double pole, single throw switching relay having a switching relay coil, a first switch element connectable between said input line conductor and said output neutral conductor and a second switch element connectable between said input neutral conductor and said output line conductor. 
     
     
       36. The circuit according to  claim 23 , further comprising a short-circuit conductor connected between said output neutral conductor and said ground conductor and forming a permanent short-circuit between said output neutral conductor and said ground conductor. 
     
     
       37. A circuit for sensing an overvoltage condition between a line conductor and a neutral conductor connectable to a supply system, comprising:
   an overvoltage protection circuit coupled to the line and neutral conductors that generates an overvoltage signal having a first state indicating an overvoltage condition in which an rms voltage between the line and neutral conductors exceeds a predetermined level, and a second state indicating an non - overvoltage condition between the line and neutral conductors; and        a wiring fault condition sensing circuit coupled to said overvoltage protection circuit, to the line and neutral conductors and to a ground conductor, the wiring fault condition sensing circuit including a switching device for selectively supplying a drive current to produce a switch control signal in response to a state of the overvoltage signal and to the supply system being correctly wired and connected to the line, neutral and ground conductors, wherein the switching device receives the overvoltage signal from said overvoltage protection circuit such that the switching device is inhibited from supplying the drive current in response to the overvoltage signal indicating an overvoltage condition, and operation of the switching device is unaffected by the overvoltage signal indicating a non - overvoltage condition.     
     
     
       38. The circuit of  claim 37 , wherein said switching device is a Darlington pair. 
     
     
       39. The circuit of  claim 37 , further comprising a switch configured to couple the line conductor to a load in response to the switch control signal. 
     
     
       40. A circuit for sensing an overvoltage condition between a line conductor and a neutral conductor connectable to a supply system, comprising:
   an overvoltage protection circuit coupled to the line and neutral conductors and configured to generate an overvoltage signal having a first state indicating a non - overvoltage condition between the line and neutral conductors and a second state indicating an overvoltage condition between the line and neutral conductors, wherein the overvoltage protection circuit comprises: a switch control circuit coupled to the line and neutral conductors and configured to generate an overvoltage switch control signal in response to a voltage between the line and neutral conductors; and a switching element coupled to the switch control circuit and configured to generate the overvoltage signal in response to the overvoltage switch control signal; and        a wiring fault condition sensing circuit coupled to the overvoltage protection circuit, to the line and neutral conductors and to a ground conductor, said wiring fault condition sensing circuit generating a switch control signal in response to the supply system being correctly wired and connected to the line, neutral and ground conductors and a state of the overvoltage signal, wherein said switch control circuit comprises: a non - linear device, a first resistor and a second resistor connected in series between the line and neutral conductors, wherein the non - linear device conducts only in response to an overvoltage condition between the line and neutral conductors, and wherein the overvoltage switch control signal is generated at a junction between the first and second resistors.     
     
     
       41. The circuit of  claim 40 , wherein said non- linear device is PNPN diode.   
     
     
       42. The circuit of  claim 40 , wherein said switching element is a silicon controlled rectifier, an anode of said silicon controlled rectifier is connected to said wiring fault condition sensing circuit, a cathode of said silicon controlled rectifier is connected to the line conductor, and a control gate of said silicon controlled rectifier is connected to the junction between said first and second resistors.