Patent ID: 7562569

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