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Stupid resistor tricks

February 4, 2007

I needed pairs of resistors matched as precisely as possible

So I take a bunch in a taped reel, taped them to a sheet of paper, and measure them on a 3-digits digital meter (rechecking to see if the measurements repeat, and also slapping my hand on a bunch to check for thermal drift while measuring). Then I type the measurements into Excel, sort, find the values with more than one hit, and assign colors for those values. Color the tape, and we’re there — a bunch of small sets of resistors that are within .1% of each other. (I used color code ordering with gaps, so a black and a brown differ by at most 0.002).

Why am I doing this? If the “simple” synchronized rectifier doesn’t work, I think I can build one using an LM158 (low-voltage op-amp) together with a pair of precision voltage dividers (to divide inputs at or outside the supply voltages, into valid inputs for the op-amp). I’ve done this trick once before building a light-board, where I needed 24 sets of 3 matched resistors. For that, I just took 100 resistors, measured them, tossed out the outliers, binned and sorted the rest, and then just took them three at a time starting at one end of the sort. The resistors seem to stay similar over time.

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