Matching Resistors

I had a need for matched resistors in preparation for a coming project. I could have purchased some 0.1% tolerance parts, but since the value wasn't critical I thought I would have a go at matching by hand using a Wheatstone bridge. As it turned out I exceeded the 0.1% tolerance I could have purchased with ease.

Since I had no constraints on the resistance value I grabbed 25 or so 36k resistors. I don't know the tolerance specification of these but I expect they are 5% tolerance. Since I didn't think I would ever use these it didn't matter if I wasted them. I soldered three of them onto a piece of veroboard with connections for the power supply and meter. The fourth resistor would be manually placed across the tracks of the veroboard and held there with an eraser or other non-conductive stick.

The voltage difference represents how equal in ratio the resistor being tried is to those already in the circuit. It doesn't matter that none of the fixed resistors are matched. We are simply measuring the voltage difference for each resistor we trial. When the voltage difference is the same then the resistors we tried are the same.

In no time I had 2 pairs of resistors matched to within 1mV. I did the maths and that represents a 12 ohm difference, or 0.03% of 36k.

Who would have thought it could be that easy. Viva la Wheatsone Bridge.

Regards
Richard