| 摘要 |
PURPOSE:To make it possible not to damage the circuit of the gas sensor for a four- line type centralized monitor system even if the power source line and the signal line are connected in the opposite direction by mistake by connecting a thermistor with positive characteristic to the signal line. CONSTITUTION:A DC-AC inverter 6 for raising voltage is connected in the power source line 1, and the AC output of this inverter 6 is supplied to the heater 17 for a gas sensor element 16. When a sintered body that consists of SNO2, etc. as main ingredients and installed between the heater 17 in the element 16 and the other electrode 18 contacts gas, its resistance drops. By making use of this voltage drop, the secondary voltage of a boosting transformer 11 is transmitted to the electrode 18. The thyristor 20 is turned on and the relay 4 of a signal line 2 is actuated by the logical sum of the output of a comparator (COM)14 that becomes ''H'' level by the output of this element 4 and the output of COM16 for preventing mistaken alarming at the initial stage after throwing in power. A thermistor 21 with positive characteristics is connected in series between this signal line 2 and the thyristor 20. With this circuit, when the power line 1 and the signal 2 are connected in the opposite direction by mistake, the thyristor 20 and the DC power source are not directly connected and the damage to the circuit can be avoided. |
