milliohmmeter with AT90S2313

Posted on Aug 14, 2012

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. In this circuit, shown in the block diagram in figure 1, a 1 kHz burst of 5 volt peak pulses is applied to a series dropping resistor to establish a pulsing current through the resistance under test.

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The IR drop across the resistor under test is a voltage proportional to the value of the resistance. After passing through a high pass filter to eliminate the DC component of the signal, it is rectified by a synchronous demodulator. For a period of time corresponding to the burst, the rectified signal is applied to an integrator, which charges up, and then at the end of the burst, the time it takes the integrator to discharge at a constant rate is measured to determine the size of the charge, and that is how the average amplitude of the signal applied to the integrator is determined. Since random noise is not synchronized with the switching in the synchronous demodulator, it is not rectified and it averages out to nearly zero. The longer the integration time, the less proportionate effect a given small pulse of noise has on the integrator's output, and the more gain the integrator has for the 1 kHz burst. Integration is the wonder of lock-in .amplification Getting The Drop \ figure 2 The 1 Khz switching signal to the base of the 2N2907 causes 5 volts peak-to-peak to be applied to the 220 ohm resistor in series with the resistance under test, to cause 23 milliamps peak-to-peak to flow through it. One critical part of measuring a very low resistance is developing an IR drop across the resistance, and measuring only that. Getting the IR drop itself was easy, a PNP switching transistor is driven into saturation...

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