Patent ID: 7568387

Claim:
A thermal flow sensor in which a first bridge circuit is composed of two temperature detection elements that are arranged in the vicinity of a heating element at upstream and downstream sides thereof, respectively, with respect to a forward flow of fluid for detecting the temperature of said heating element, and a plurality of first resistors; a second bridge circuit is composed of said heating element, a fluid temperature detection element arranged at a location free from the influence of heat from said heating element, and a plurality of second resistors; and a flow rate of said fluid is detected by using the fact that an amount of heat transmitted from said heating element to said fluid in a state where said heating element is always held at a control temperature higher by a predetermined value than the temperature of said fluid detected by said fluid temperature detection element depends on the flow rate of said fluid; said thermal flow sensor comprising: an amplifier section that amplifies a voltage across opposite ends of at least one of said plurality of second resistors; a current control section that is controlled based on an output voltage of said amplifier section; and an output terminal that is connected to one end of each of said two temperature detection elements for outputting a detection result corresponding to the flow rate of said fluid; wherein said amplifier section comprises an amplifier part that amplifies an input signal to said amplifier section, and an amplification factor control part that controls an amplification factor of said amplifier part; said amplification factor control part changes the amplification factor of said amplifier part by means of an electric signal so that the temperature of said heating element is adjusted to said control temperature.