| 摘要 |
PURPOSE:To reduce aging, by setting the temperature of a detection element in a clean air higher than the element temperature securing the maximum sensitivity to lower the temperature of the element down to the element temperature securing the maximum sensitivity according to a rise in the gas concentration. CONSTITUTION:The output VOUT of a detection element RS in a clean air is too low to keep a light emitting element LED from emitting. As the impedance of a voltage adjusting circuit B is high, the output voltage VH of a constant voltage circuit AV is equivalent to the sum of the Zener voltage of a Zener diode ZD1 and forward voltages of diodes D1 and D2. The current element temperature of the element RS is set at T1. As the gas concentration rises, an element LED is driven with an increase in the output VOUT of the element RS and the resulting light is received with a light receiving element of the circuit B, making the impedance of the circuit B decrease. As a result, with a drop in the voltage across the diodes D1 and D2, the output voltage VH of a circuit AV lowers and the element temperature of the element RS also falls. In this manner, the control of the applied voltage VH of a heater resistance RH keeps the element temperature of the element RS at T1 while normally, no gas exists thereby reducing secular characteristic changes. |
