Thermal type air flow meter

A thermal type air flow meter adapted to be attached to a duct, the air flow meter comprising:

• a sensor portion arranged in an air passage in the duct through which air flows, the sensor portion including a heating element configured to produce heat by energization thereof;
• a temperature control unit configured to control a temperature of the heating element such that a temperature difference between the temperature of the heating element and a temperature of air flowing in the air passage becomes constant, the temperature control unit including:
• a heater temperature detection resistor configured to detect the temperature of the heating element;
• a first resistive element;
• a first arm serially connecting the heater temperature detection resistor and the first resistive element;
• an air temperature detection resistor configured to detect the temperature of air flowing in the air passage;
• a second resistive element;
• a second arm serially connecting the air temperature detection resistor and the second resistive element;
• and
• a voltage supply unit configured to supply a first voltage to the first arm, and to supply a second voltage to the second arm;
• and
• a flow rate detection unit configured to detect a flow rate of air flowing in the air passage based on a heat radiation amount from the heating element, wherein
• the voltage supply unit includes a voltage adjusting portion which is capable of adjusting at least one of the first voltage and the second voltage such that the temperature difference between the temperature detected by the heater temperature detection resistor and the temperature detected by the air temperature detection resistor becomes constant.