Development of a Multi-Range Microcontroller Based Lock-In Digital Milliohm Meter

Abstract

Milliohm Meter is an instrument for measuring very small amount of resistance in a conductor, electric motor and transformer windings and in many other current sensing applications that cannot be measured with standard digital ohmmeters. The cost of commercially available milliohm meter is relatively high, making it unaffordable for hobbyists.  In this work, a low-cost microcontroller based digital milliohm meter having three measurement sub-ranges was designed, constructed, tested and calibrated. The instrument, based on the PIC16f877A 8-bit microcontroller, used the lock-in detection technique to improve the measurement signal-to-noise ratio without requiring high current for measurement. The milliohm meter’s firmware was written with MicroC software by Mikroelektronika® and was simulated using Proteus® software by Labcenter Electronics. After construction and calibration, it was found to have resolutions of 82.9320 counts/mΩ, 9.1394 counts/mΩ, and 0.9221 counts/mΩ for the measurement ranges of 0 – 10mΩ, 0- 100mΩ and 0 – 1000mΩ respectively. Thus, the instrument is capable of measuring resistance value between 0Ω to 1Ω with good resolution values without requiring the computing power and display capabilities of a digital computer.