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POLARITY_INDICATOR

2015-03-17 16:05  
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POLARITY_INDICATOR
Q1 and Q2 at input should be Darlington transistors to minimize loading on input signal. With no input, current flows through R1D1 and R2D2 networks on alternate half-cycles, Positive signal at input 1 turns on Q1, shunting sum of current through D3 and lighting positive-indicating LED in diode bridge. Similarly, negative voltage on 1 turns on other LED. Supply requirement of 6.3 VAC can usually be obtained from digital multimeter with which indicator is used.-R. A. Snyder, Simple Polarity Indicator Suits DMM's or DPM's, EDN Magazine, Nov. 5, 1977, p 110.