| 摘要 |
<p>PROBLEM TO BE SOLVED: To make an upper limit of a measuring range for a thermal flowmeter set easily, and to protect heating resistor and a transistor even when an abnormal voltage is generated. SOLUTION: A midpoint of a bridge circuit having the heating resistor 11 for flow measurement and a temperature compensating resistor 12 therefor is connected to an input terminal of a differential amplifier 15, and a current supplied to the bridge circuit is regulated to bring a temperature difference between the heating resistor 11 and the temperature compensating resistor 12 into a prescribed difference via the differential amplifier 15 and the transistor 16. This flowmeter is provided with a first voltage limiting means 18 for limiting a base voltage of the transistor 16, and a second voltage limiting means 10 for limiting an emitter voltage. Setting is conducted to satisfy VB1-VBE<Vset, where VBD1 and Vset are voltage limiting values set by the voltage limiting means 18, 19, VBE is a voltage between a base and the emitter. A fuse 30 is connected to a collector side of the transistor 16.</p> |
