ISOLATED_THRESHOLD_SWITCH

Published:2009/7/6 7:43:00 Author:May | From:SeekIC

Standard photocoupler programmed with 150-ohm resistor R1 provides threshold switching function for separating high-level noise from switching-signal pulses as short as 10μs. Current-transfer ratio of phototransistor coupler is made practically zero at some arbitrary input current, and changed rapidly back to 10% or more at slightly higher level. Programming range for threshold value extends from 60 mA for 10 ohms at R1 to 3 mA for 400 ohms. Use of 2.7-megohm resistor R2 across base-emitter terminals of coupler reduces low-current gain of phototransistor. Noise currents up to 5 mA on sensing line are rejected while operating currents as low as 10 mA are accepted.-J. Cook, Photocoupler Makes an Isolated Threshold Switch, EDN Magazine, Oct. 5, 1974, p 72, 74, and 76.



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