Protection circuit

#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.

#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. 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 R1, a second impedance R2, a third impedance R3, a temperature-controlled variable resistor RT, and a switch SW. The second resister R2 has a first end coupled to secondary winding 252. The first impedance R1 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 R3 has a first end coupled to the second end of the first impedance R1. Temperature-controlled variable resistor RT has a first end coupled to a second end of the second impedance R1. Switch SW comprises a first node P1, a second node P2, a third node P3, and a fourth node P4. The first node P1 is coupled to a second end of the temperature-controlled variable resistor RT. The second node P2 is coupled to a second end of the third impedance R3. The third node P3 is coupled to a second end of load 26. Finally the fourth node P4 is coupled to ground 28. Switch SW may be a photodiode switch or an optoelectronic switch.