Abstract:
A protection circuit is coupled between a secondary winding of a transformer and a load. When voltage at the secondary winding of the transformer becomes abnormal, the protection circuit disconnects the power connection to the load so as to protect the load from damage. When the voltage at the secondary winding of the transformer returns normal, the protection circuit restores power to the load.

Description:
BACKGROUND 
   The disclosure relates in general to a protection circuit, and in particular to a protection circuit for voltage overload. 
   Voltage sources deliver current to electronic device, normally through a transformer. For safety, an protection circuit between the voltage source and the electronic device safeguards abnormal power supply, preventing damage. 
     FIG. 1  is a block diagram of a electronic system with an protection circuit. The system comprises a voltage source  10 , an protection circuit  12 , a transformer  15 , and a load  16 . Protection circuit  12  incorporates a breaking circuit  121  and a detection feedback circuit  122 . Transformer  15  comprises a primary winding  151  coupled to voltage source  10  via a breaking circuit  121 , and a secondary winding  152  coupled to load  16  and a detection feedback circuit  122 . Load  16  may be any type of electronic device, such as a light tube. Load  16  is coupled to ground. Detection feedback circuit  122  is further coupled to breaking circuit  121 , and detects abnormal voltage at secondary winding  152 . If the voltage at secondary winding  152  renders an abnormal value, i.e. output voltage exceeds a predetermined value, the detection circuit  122  generates a signal, such that breaking circuit  121  interrupts the circuit. 
   Protection circuit  12  does not restore breaking circuit  121 , nor does it resume operation of load  16 , despite the voltage returning to normal. This imposes a design limitation to the applications. As a compensation a restore circuit is incorporated into the circuitry, results in more complex circuitry, increased component count, and manufacturing cost increases. 
   Thus an protection circuit is in need, providing over voltage protection upon abnormal voltage detection, and power supply restoration under normal condition. 
   SUMMARY 
   A protection circuit according to the present invention comprises a first impedance coupled to a secondary winding and a load, a second impedance coupled to the secondary winding, a third impedance coupled to the first impedance, a temperature-controlled variable resistor coupled to the second impedance; and a switch comprising a first note, a second node, a third node and a fourth node, the first node coupled to the temperature-controlled variable resistor, the second node coupled to the third impedance, the third node coupled to the load, and the fourth node coupled to ground. 
   Resistance of the temperature-controlled variable resistor increases along with temperature. 
   When voltage at the first node is not less than voltage at the second node, the third node and the fourth node of the switch short; when the voltage at the first node is less than at the second node, the third node and fourth node of the switch open. 
   The second impedance and the third impedance are resistors or capacitors. 
   The circuit further comprises a fourth resistor, coupled to the second impedance, and to the ground, wherein the fourth resistor is a resistor or a capacitor. 
   The circuit further comprises a fifth resistor, coupled to the third impedance, and the ground. 
   The first impedance is a resistor or a capacitor. 
   A primary winding of the transformer is coupled to a voltage source. 
   The switch is a photodiode or a optoelectronic switch. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the detailed description, given hereinbelow, and the accompanying drawings. The drawings and description are provided for purposes of illustration only and, thus, are not intended to be limiting of the present invention. 
       FIG. 1  is a block diagram of a conventional protection circuit. 
       FIG. 2  is a block diagram of an protection circuit, according to an embodiment of the present invention. 
       FIG. 3  is a circuit diagram of a protection circuit, according to an embodiment of the present invention. 
       FIG. 4  is a circuit diagram of an protection circuit, according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 2  is a functional block diagram of an electronic system with a protection circuit, as an embodiment in the present invention. The system comprises a voltage source  20 , a transformer  25 , an protection circuit  22 , and a load  26 . Voltage source  20  supplies voltage. The transformer  25  comprises a primary winding  251  coupled to voltage source  20 , and a secondary winding  252  coupled to protection circuit  22 . Protection circuit  22  is further coupled to ground  28  and load  26 , which can be any electronic device, such as a light tube. 
   Protection circuit  22  monitors voltage at secondary winding  252 . When the voltage rises beyond a predetermined value, protection circuit  22  intercepts the connection to load  26 , protecting load  26  against over voltage damages. When the voltage falls below the predetermined value, protection circuit  22  restores the connection, and load  26  resumes operation. Thus protection circuit  22  delivers automatic power restoration for load  26 . 
     FIG. 3  is a circuit schematic diagram for the electronic device system with an protection circuit in  FIG. 2 , according to an embodiment of the present invention. As shown in  FIG. 3 , protection circuit  22  is coupled between secondary winding  252  of transformer  25  and a load  26 , comprising a first impedance R 1 , a second impedance R 2 , a third impedance R 3 , a temperature-controlled variable resistor RT, and a switch SW. The second resister R 2  has a first end coupled to secondary winding  252 . The first impedance R 1  has a first end coupled to secondary winding  252  and a second end coupled to a first end of the load  26 . The third impedance R 3  has a first end coupled to the second end of the first impedance R 1 . Temperature-controlled variable resistor RT has a first end coupled to a second end of the second impedance R 1 . Switch SW comprises a first node P 1 , a second node P 2 , a third node P 3 , and a fourth node P 4 . The first node P 1  is coupled to a second end of the temperature-controlled variable resistor RT. The second node P 2  is coupled to a second end of the third impedance R 3 . The third node P 3  is coupled to a second end of load  26 . Finally the fourth node P 4  is coupled to ground  28 . Switch SW may be a photodiode switch or an optoelectronic switch. 
   When the voltage at voltage source  20  is less than a predetermined value, protection circuit  22  operates normally. The second impedance R 2  and temperature-controlled variable resistor RT are connected in series, in turn connected in parallel to the third impedance R 3 , such that the voltage at the first node P 1  always exceeds that of the second node P 2  in switch SW. Consequently, the third node P 3  and the fourth node P 4  of switch SW remains short circuited, since switch SW is a photodiode switch or an optoelectronic switch. The current from secondary winding  252  reaches load  26 , and circulates around the third node P 3  and the fourth node P 4  of switch SW, operating load  26 . 
   When the voltage at voltage source  20  overtakes the predetermined value, the excessive voltage builds up heat energy in the circuitry, resulting in an increase in the resistance of temperature-controlled variable resistor RT. Thus the voltage across the second impedance R 2  and temperature-controlled variable resistor RT increases. As a consequence, the voltage at the first node P 1  is less than that at the second node P 2  in switch SW. At this point, the third node P 3  and the fourth node P 4  of switch SW are disconnected, isolating load  26  from voltage damage. Protection circuit  22  thus shields the electronic device against excessive voltage. 
   The employment of second impedance R 2  and third impedance R 3  present another embodiment of the present invention. Those in the art may make appropriate modifications to the embodiment, so long as the second impedance R 2  and the third impedance R 3  can extract voltage across the two ends of the first impedance R 1  for circuit operation. Hence second impedance R 2  and third impedance R 3  can be replaced with a first capacitor (not shown) and a second capacitor (not shown), and the modified circuit may accomplish equivalent functionality. 
   When switch SW is open, and voltage once again falls below the predetermined voltage, the voltage at the first node P 1  exceeds that at the second node P 2 . Consequently the connection between the third node P 3  and the fourth node P 4  of switch SW is renewed, rendering normal operation of load  26 . Protection circuit  22  thus provides of automatic restoration. 
     FIG. 4  is a detailed circuit diagram of the electronic device system with an protection circuit in  FIG. 2 , in another embodiment of the present invention. The circuit interconnection, other than protection circuit  22 , is identical to those in the embodiment of  FIG. 3 . Protection circuit  22  coupled between secondary winding  252  and load  26  comprises, a first impedance R 1 , a second impedance R 2 , a third impedance R 3 , a fourth resistor R 4 , a fifth resistor R 5 , a temperature-controlled variable resistor RT, and a switch SW. The first impedance R 1  has a first end coupled to secondary winding  252  and a second end coupled to a first end of the load  26 . The second impedance R 2  has a first end coupled to secondary winding  252 . The fourth resistor R 4  has a first end coupled to the second impedance R 2  and a second end coupled to the ground  28 . The third impedance R 3  has a first end coupled to the first impedance R 1 . The fifth resistor R 5  has a first end coupled to a second end of the third impedance R 3  and a second end coupled to ground  28 . Temperature-controlled variable resistor RT has a first end coupled to the second impedance R 2 . Switch SW comprises a first node P 1 , a second node P 2 , a third node P 3 , and a fourth node P 4 . The first node P 1  is coupled to a second end of the temperature-controlled variable resistor RT. The second node P 2  is coupled to a second end of the third impedance R 3 . The third node P 3  is coupled to a second end of load  26 , and the fourth node P 4  is coupled to the ground  28 . Once again, switch SW thereof may be a photodiode switch or an optoelectronic switch. 
   Based on a principle similar to the embodiment of  FIG. 3 , the embodiment takes the second impedance R 2  and the fourth resistor R 4  in series, in conjunction with series connecting the third impedance R 3  and the fifth resistor R 5  in parallel, rendering application in various fields such as high voltage circuits, and alternative current circuits, and others. As the voltage at voltage source  20  falls below a predetermined value, the voltage at the first node P 1  of switch SW is not less than that at the second node P 2 , thus the connection between the third node P 3  and the fourth node P 4  is short. The current from secondary winding  252  passes through load  26 , and circulates along the third node P 3  and the fourth node P 4  to the ground, resulting in normal operation of load  26 . When the voltage at voltage source  20  exceeds the predetermined value, the resistance of temperature-controlled variable resistor RT increases, leading to a voltage decrease at node  1 . Consequently the voltage at the first node P 1  is less than the voltage at the second node P 2 , the third node P 3  and the fourth node P 4  disconnect, and the current thereof cannot be conducted through load  26 , protecting load  26  from damage. 
   The second impedance R 2 , the third impedance R 3 , the fourth resistor R 4 , and the fifth resistor R 5  are provided here merely as an illustration. With an appropriate selection of the second impedance R 2 , the third impedance R 3 , the fourth resistor R 4 , and the fifth resistor R 5 , a circuit may be configured corresponding to the circuit operation of embodiment in  FIG. 4 , through the principle of voltage dividing. Hence the second impedance R 2 , the third impedance R 3 , the fourth resistor R 4 , and the fifth resistor R 5  may be replaced with a second capacitor (not shown), a third capacitor (not shown), a fourth capacitor (not shown), and a fifth capacitor (not shown), yet achieving equivalent functionality. 
   Moreover, it is valid to construct a design with the first node P 1  coupled to secondary winding  252  via temperature-controlled variable resistor RT, and the corresponding secondary winding P 2  coupled to load  26 , as long as the voltage at first node P 1  is not less than that at the second node P 2  when voltage is less than the predetermined value; and the voltage at first node P 1  is less than that at the second node P 2  when voltage exceeds the predetermined value. 
   The circuit configuration of the embodiment in  FIG. 4  also provides automatic restoration functionality, as the circuit configuration in  FIG. 3 . 
   While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.