Posted on Apr 24, 2012 6702
The circuit consists of a 5-V regulator, constant-current source Dl, D2, and Ql, and op amp gain stage Ul. Power is provid
ed by a 9-V battery whose output is regulated to +5 Vdc by the 3-terminal regulator. The emitter of Ql is always 0.6 V below the +5-V line. Resistor R1 sets the current through both diodes Dl and D2 to 5 mA. The resulting 0.6 Vdc across one of the multiturn trimmer potentiometers (R2 and R3), as selected by switch section S2A, sets the current through Ql and the resistor-under-test. When R2 is selected, the test current is 1 mA; when R3 is selected, the test current is 10 mA. On the lower two ranges, 1 and 10, the voltage across resistance-under-test is applied directly to the DMM terminals. On the upper two ranges, op amp gain stage Ul is switched into the circuit and the DMM measures the voltage between op amp output pin 6 and the test resistor. When switch S2 is in position 3 (x 100) the current set by the constant-current source is 1 mA; the multiplying factor is 100. When S2 is in position 4, 1000, the current is 10 mA and the multiplying factor is 100 10 = 1000. Multiturn trimmer-potentiometer R6 adjusts the offset of the op amp so that, with no voltage across the resistor-under-test (i.e., with the measurement terminals short-circuited), the output is zero.