A milliohmmeter is just the tool for checking trace resistance on a printed circuit board, tracking down shorted traces, and measuring the contact resistance of a switch or connector. Its the kind of tool that would come in real handy occasionally, but not often enough to justify shelling out hundreds of dollars. Wanting one anyway, I set out to make my own. It turned out to be not only an exciting project, but a true adventure of discovery as it provided a window into the workings of lock-in amplifiers. With a lock-in amplifier topology, a microcontroller and a little firmware makes the venerable (if somewhat noisy) LM324 bipolar op-amp provide high gain and at the same time reduces noise. Improve analog performance with a microcontroller? Now THIS is a fun project! My main concern in the milliohmmeter design was how to get stable resistance readings with a suitably low test current. After all, I wanted to look at a few or few tens of milliohms, but I wanted to keep the test current low so as not to exceed the current ratings of some of the parts I want to test, and so I can run it all from a battery. For starters, 25 milliamps x 1 milliohm = 25 microvolts. This meant that I would have to be able to measure and display voltages in the range of tens of microvolts per count in a stable and repeatable manor.
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