Heat-wire type air flow measurement apparatus

摘要

A heat generating element having a thermal characteristic such that its resistance varies in response to changes in temperature is provided in the intake pipe of an engine. A heating power, the voltage of which is set by a reference voltage source, is supplied to the heat generating element via a transistor. The supply of this heating power is controlled by a start pulse signal periodically generated by a set flip-flop circuit when the ignition switch is on. When the heat generating element reaches a predetermined temperature, the supply is cut off by the flip-flop circuit. A measurement signal having a pulse width corresponding to the time period of heating power supply is generated from the flip-flop circuit. The opening of the ignition switch is detected and a one-shot multivibrator is driven to produce a burn-off signal having a pulse width to which the burn-off period corresponds. This burn-off control signal turns the transistor on and heating power is supplied to the heat generating element and the voltage of the reference power source is reduced so that the voltage of the heating power is switched to a low level.