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.
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