An ignition device for an internal combustion engine includes a power transistor connected to a primary coil of an ignition coil, a current sensing resistor connected in series with the power transistor, a constant current circuit and an abnormal oscillation control circuit that connects the gate re
An ignition device for an internal combustion engine includes a power transistor connected to a primary coil of an ignition coil, a current sensing resistor connected in series with the power transistor, a constant current circuit and an abnormal oscillation control circuit that connects the gate resistor with the feedback circuit when the constant current circuit controls the primary current to be constant. The constant current circuit has a gate resistor connected in series to the control gate of the power transistor and a feedback circuit connected between the gate resistor and the current sensing resistor and controls current supplied to a control gate of the power transistor so as to control the primary current to be constant.
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What is claimed is: 1. An ignition device for an internal combustion engine comprising: a power transistor, having a main gate to be connected to a primary coil of an ignition coil and a control gate, for supplying primary current to the primary coil by supplying gate current to the control gate; a
What is claimed is: 1. An ignition device for an internal combustion engine comprising: a power transistor, having a main gate to be connected to a primary coil of an ignition coil and a control gate, for supplying primary current to the primary coil by supplying gate current to the control gate; a current sensing resistor connected in series with said power transistor; a constant current circuit, having a gate resistor connected in series to the control gate of said power transistor and a feedback circuit connected between the gate resistor and said current sensing resistor, for controlling the primary current to have a constant amount; and switching means for switching connection between the gate resistor and the feedback circuit, wherein said switching means turns on when said constant current circuit controls the primary current to have the constant amount. 2. The ignition device as claimed in claim 1, wherein said switching means turns off to disconnect the gate resistor from the feedback circuit when said constant current circuit does not operate to control the primary current. 3. The ignition device as claimed in claim 1, wherein said switching means turns on to connect the gate resistor and the feedback circuit when an amount of the primary current reaches a first preset value that is smaller than the constant amount. 4. The ignition device as claimed in claim 3, wherein said switching means turns off to disconnect the gate resistor from the feedback circuit when the amount of the primary current does not reach the first preset value. 5. The ignition device as claimed in claim 3, further comprising fail signaling means, connected to the current sensing resistor, for providing a fail signal if an amount of the primary current is larger than a second preset value. 6. The ignition device as claimed in claim 5, wherein the second preset value is smaller than the first preset value. 7. The ignition device as claimed in claim 5, wherein said switching means stops said fail signaling means from providing the fail signal when the amount of the primary current reaches the first preset value. 8. The ignition device as claimed in claim 7, wherein said fail signaling means starts providing the fail signal when the amount of the primary current reaches a third preset value that is smaller than the first preset value. 9. The ignition device as claimed in claim 1, further comprising coil's abnormality detecting means and second switching means for stopping the gate current supplied to the gate of said power transistor when said coil's abnormality detecting means detects an abnormality. 10. The ignition device as claimed in claim 1, wherein said feedback circuit comprises: a constant current controlling comparator having an input terminal connected to the current sensing resistor; and a transistor having a control gate connected to the constant current controlling comparator and a main gate connected between the gate resistor and a ground. 11. The ignition device as claimed in claim 5, wherein said fail signaling means comprises: a fail detecting comparator having an input terminal connected to the current sensing resistor and an output terminal providing an output signal when the potential of the input terminal thereof reaches a first threshold level of the comparator; and a logic circuit, connected to the output terminal of said fail detecting comparator, for providing the fail signal when the logic circuit receives the output signal from the fail detecting comparator. 12. The ignition device as claimed in claim 11, wherein: said fail signaling means comprises another comparator having an input terminal connected to the current sensing resistor and an output terminal connected to the logic circuit, and said another comparator provides an output signal for stopping the fail signal when the potential of the input terminal thereof becomes a second threshold level that is higher than the first threshold level. 13. The ignition device as claimed in claim 12, wherein said switching means comprises a transistor having a control gate connected to said another comparator to turn on said transistor to connect the gate resistor and the feedback circuit when said another comparator provides the output signal. 14. An ignition device for an internal combustion engine comprising: a power transistor, having a main gate to be connected to a primary coil of an ignition coil and a control gate, which supplies primary current to the primary coil by supplying gate current to the control gate; a current sensing device connected in series with said power transistor; a constant current circuit, having a gate resistor connected in series to the control gate of said power transistor and a feedback circuit connected between the gate resistor and said current sensing device, which controls the primary current to have a constant amount; and a transistor which switches connection between the gate resistor and the feedback circuit so that the transistor turns on when said constant current circuit controls the primary current to have the constant amount. 15. The ignition device as in claim 14, wherein said transistor turns off to disconnect the gate resistor from the feedback circuit when said constant current circuit does not operate to control the primary current. 16. The ignition device as in claim 14, wherein said transistor turns on to connect the gate resistor and the feedback circuit when an amount of the primary current reaches a first preset value that is smaller than the constant amount. 17. The ignition device as in claim 16, wherein said transistor turns off to disconnect the gate resistor from the feedback circuit when the amount of the primary current does not reach the first preset value. 18. The ignition device as in claim 14, further comprising a fail signaling circuit, connected to the current sensing device, which provides a fail signal if an amount of the primary current is larger than a preset value. 19. The ignition device as in claim 16, further comprising a fail signaling circuit connected to the current sensing device, which provides a fail signal if an amount of the primary current is larger than a second preset value. 20. The ignition device as in claim 19, wherein the second preset value is smaller than the first preset value. 21. The ignition device as claimed in claim 20, wherein said transistor stops said fail signaling circuit from providing the fail signal when the amount of the primary current reaches the first preset value. 22. The ignition device as in claim 18, wherein said fail signaling circuit comprises: a fail detecting comparator having an input terminal connected to the current sensing device and an output terminal providing an output signal when the potential of the input terminal thereof reaches a threshold level of the comparator; and a logic circuit, connected to the output terminal of said fail detecting comparator, which provides the fail signal when the logic circuit receives the output signal from the fail detecting comparator.
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