LOW_VOLTAGE_OHMMETER

Published:2009/7/20 20:31:00 Author:Jessie | From:SeekIC

Combines stable constant-current source U1-CR1-RY with DC amplifier U2 having gain of 10 to keep applied voltage down to 0.1V. Output is linearly proportional to unknown resistance. Resistances well below 1 ohm can be measured accurately, U2 scales 0-100 mV unknown voltage to 0-1 V at output, so 1K resistor under test can be read as 1.000K on DVM scale. U2 should be offset-nulled to eliminate zero error, for best low-scale accuracy, by shorting input and adjusting R4 for 0.000 V out of U2. Full-scale calibration involves trimming individual range values ofRY for correct output, while using reference value for RX. Fuse and clamp diodes protect range resistors, and R5 protects opamp.-W. Jung, An IC Op Amp Update, Ham Radio, March 1978, p 62-69.



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