Patent ID: 7481574

Claim:
A flow rate sensor measuring flow rate of a fluid thermally, comprising: a heat-generating resistor generating heat in response to a supplied current that is supplied to said heat-generating resistor; a bridge circuit including four nodes and four legs, two of the legs being connected in series at each of the nodes, wherein said heat-generating resistor is proximate a first leg of said bridge circuit but is not electrically connected to any leg or any node of said bridge circuit, a first temperature detector in the first leg of said bridge circuit and having a resistance value varying in response to temperature of said heat-generating resistor, and a second temperature detector in a second leg of said bridge circuit and having a resistance value varying in response to temperature of the fluid; a comparison portion supplying a digital output indicating difference between voltages at two of the nodes of said bridge circuit; a digital-to-analog converter converting an output from said comparison portion to an analog signal and supplying the analog signal to said heat-generating resistor as the supplied current; and an output operation portion sequentially adding outputs from said comparison portion for a prescribed time period and outputting the outputs that are sequentially added as a flow rate, for the prescribed time period, of the fluid.