Posted on Feb 6, 2014

Simple opamp circuit with one transistor gives reliable hot-wire anemometer for measuring flow of gases or liquids. R2 is heated above ambient temperature in Wheatstone bridge including overheat resistor R1which is calibrated to be 30% larger than cold resistance of R2. Bridge is fed from power tran. sistor which is within feedback loop of opamp th

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at senses bridge unbalance. Output of bridge is fed back to power transistor in correct phase for maintaining constant- temperature condition in which R2 is approximately equal to R1. Article covers construction of hot-wire probe made from Wollaston wire. -W. Bank, Build Your Own Constant-Temperature Hot-Wire Anemometer, EDN Magazine, Aug. 1, 1972, p 43.

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