Low Ohms Adaptor For DMMs Based On An LM317 Regulator

This adaptor circuit is essentially a 100mA constant current source. It is applied across a low-value resistor of unknown value (ie, the resistance to be measured) and the resulting voltage drop can then be measured by a digital multimeter (DMM). Setting your DMM to the 200mV range will enable it to measure up to 2O with high resolution while the 2V range will give a maximum resistance measurement of 20O. Construction could consist of mounting the LM317 adjustable 3-terminal regulator inside a small plastic box together with the battery and two resistors connected to the output and Adj pins.

 

Low Ohms Adaptor Circuit Diagram For DMMs Based On An LM317 Regulator

No on/off switch is required since no current will be drawn when no external resistance is connected across the Test terminals. Accuracy using 1% resistors should be within 5% and this could be improved by measuring the current, adjusting the resistance between the output and Adj pins of the LM317 to provide a precise 100mA. Before using the adaptor, check that your meter is not likely to be damaged by having the full output (6V+) applied when it is set to a low voltage range. Similarly, be aware that the voltage and current output of the adaptor may damage components if you use it for “in-circuit” tests.

Author: Peter Chamberlain – Copyright: Silicon Chip