| 摘要 |
PROBLEM TO BE SOLVED: To obtain an overvoltage-input protective circuit which prevents an internal circuit from being damaged when an overvoltage is applied such as when a power supply or the like is connected mistakenly to an input terminal by a method wherein a bipolar transistor is inserted across the gate and the source of a MOS transistor and the base is connected, via a resistor, to a ground or to a stable potential part inside the internal circuit. SOLUTION: In an ordinary operation, the measured current of a resistance thermometer element (RTD) 1 is turned on/off by a MOS transistor Q1 through a resistor R1 so as to flow to the RTD 1. A current value can be known by measuring a voltage across both ends of the resistor R1, and the resistance value of the RTD 1 can be found on the basis of it. A part across the emitter and the base of a bipolar transistor Q2 is reverse-biased, the transistor Q2 is turned off, and the operation of the protective circuit is not affected. When a positive overvoltage is applied to an input, the parasitic diode of the transistor Q1 is set to continuity, and an input voltage is shouldered mostly by the resistor R1. A positive overvoltage is applied to the emitter of the transistor Q2, and a part across the emitter and the base of the transistor Q2 is reverse-biased.
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