Hot Wire Anemometer

摘要

A hot wire anemometer circuit has a calibrate subcircuit and an operate subcircuit which are selectively invoked via a user-activate mode switch. The calibrate subcircuit includes a detector configured to compare an amplified sensor voltage with an amplified drive voltage and output an indicator signal when the two voltages match. The drive voltage is derived from a user-adjusted drive resistance of a bridge subcircuit. The operate circuit includes a feedback loop which provides a signal to a bride node of a bridge circuit. After a sensor is connected to the anemometer circuit, the drive resistance is adjusted until the indicator signal is produced, signifying that the circuit is tuned and impedances are matched to prevent oscillations. A slewing network protects the sensor by shorting the drive resistance upon switching the anemometer circuit to the calibrate mode from the operate mode, and gradually restoring the drive resistance when switching back.

主权项

1. An anemometer circuit connectable to a sensor, the anemometer circuit being selectively switchable between a calibrate mode and an operate mode, the anemometer circuit comprising:
(a) a mode selection switch (S1) configured to switch the circuit between the calibrate mode and the operate mode; (b) a bridge subcircuit comprising:
a first node (N1), a second node (N2) a third node (N3), and a fourth node (N4) which may be a ground node (N4);a first bridge resistance (R3) connected between the first and second nodes (N1, N2);a second bridge resistance (R4) connected between the first and third nodes (N1, N3);a termination resistance (R9) connected between the second node (N2) and the third node (N3); andan adjustable drive resistance (40, 140) connected between the third node (N3) and the fourth node (N4), the adjustable drive resistance configured to present a drive resistance signal (POT) at the third node (N3); wherein:the second node (N2) is configured to receive a sensor output signal (SNSR) when connected to a sensor; (c) an operate subcircuit receiving input from the bridge subcircuit and configured to:
(i) amplify a voltage difference between a sensor output signal (SNSR) and the drive resistance signal (POT), when the sensor is connected to the anemometer circuit; and(ii) output an anemometer output signal in response thereto, and (d) a calibrate subcircuit for calibrating the anemometer circuit, comprising:
a first constant current source (82) connected to the sensor (30) to thereby produce a first sensor voltage;a second constant current source (84) connected to the drive resistance to thereby produce a first drive voltage;a first gain amplifier (92) configured to amplify the first sensor voltage by a first gain to produce an amplified sensor voltage;a second gain amplifier (90) configured to amplify the first drive voltage by a second gain to produce an amplified drive voltage;a detector (94) configured to compare the amplified sensor voltage and the amplified drive voltage and output an indicator signal, if the amplified sensor voltage matches the amplified drive voltage; anda ratio K of the first gain to the second gain is >1.0.