One of the problems with common multimeters, especially analog ones, is that their low resistance scales are limited and not linear. With the circuit shown in the figure, we can use a common voltmeter or multimeter on the 0-15 V DC voltage scale to function as a low resistance linear ohmmeter. An important application for this simple circuit is to allow the use of a common voltmeter or even a multimeter on the voltage scale with low-value resistive sensors. The circuit functions as a constant current source connected to the resistor being measured. The source is adjusted in such a way (by the emitter resistor scale) that the current flowing through the resistor under test produces a voltage drop proportional to its value, which can be directly converted from the instrument's indication. Yes, it is possible to measure low resistances such as 0 to 1500 ohms in a linear manner on a 0 to 15 V scale. The circuit must be powered by a 24 V source, and the diode should preferably be of the germanium type such as 1N34 or OA90. The accuracy of the circuit will depend on the accuracy of the resistors selected by the switch in the various scales.
