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OVERCURRENT_PROTECTION_FOR_400_V_SUPPLY

2016-09-23 14:10  
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OVERCURRENT_PROTECTION_FOR_400_V_SUPPLY
R1 and Q1 form current detector for bias supply. At normal current levels, voltage drop is very small and Q2 is reverse-biased. When current reaches 400 μA, voltage drop across R1 forces gate of Q1 to near pinchoff. Combined voltage drop across R1 and Q1 then becomes large so Q2 is forced almost to full conduction.Q3 and Q4 then tum on Q5, to provide overcurrent-alarm signal foractivating logic circuit that shuts off power supply.-J. P. Thompson, Overcurrent Alarm Protects HV Supply, EDN Magazine, Nov. 20, 1978, p 321-322.