Patent ID: 8217320

Claim:
A thermal type flow sensor including heat resistive elements having a temperature dependency for measuring a flow rate of a fluid, comprising: a first series circuit including at least a first resistive element and a second resistive element; a second series circuit including at least a third resistive element and a fourth resistive element; wherein the first, second, third, and fourth resistive elements comprise a bridge circuit including a heat resistive element; the first, second, third, and fourth resistive elements have a temperature dependency; the first resistive element comprises the heat resistive element, and the second, third, and fourth resistive elements are temperature sensitive configured to operate at substantially room temperature; and said bridge circuit is configured by connecting the first series circuit and the second series circuit in parallel; the thermal type flow sensor further comprising: a driver circuit that receives a first intermediate voltage between the first resistive element and the second resistive element and a second intermediate voltage between the third resistive element and the fourth resistive element to control a current that flows in the first, second, third, and fourth resistive elements in accordance with a difference between the first and second intermediate voltages; a fifth resistive element comprising another heat resistive element that is connected in parallel to said bridge circuit, the fifth resistive element having a power consumption larger than that of the first resistive element, wherein the driver circuit supplies a heating current to the heat resistive elements; and a semiconductor substrate having a thin-walled portion therein, wherein the first resistive element and the fifth resistive element are formed on the thin-walled portion of the substrate; and all of the first, second, third, fourth, and fifth resistive elements are formed on the semiconductor substrate.