Upgrade Your Decade Resistance Box!

by: Kevin C. Carpenter, CBET

Often obscure resistances are needed to calibrate electronic equipment. For example, a service manual may read "Insert a 6.85 Ohm resistor and the display should indicate 98.6 Degrees Fahrenheit." The problem is, who has a 6.85 ohm resistor? By upgrading your decade resistance box you can handle most of these situations with relative ease.

The upgrade is simple. Mount a ten-turn potentiometer and two banana jacks in the case of the decade resistance box. Wire the wiper arm terminal on the potentiometer to one of the banana jacks. Then wire either of the two remaining terminals to the remaining banana jack. The upgrade is complete. Using an ohm meter you can adjust the potentiometer to the appropriate resistance and then apply it to the test circuit.

Some care should be taken when selecting a potentiometer for this upgrade. The lower the resistance the better. A low resistance is important for two reasons. One, a lower total resistance yields a higher resolution per turn. Secondly, most decade resistance boxes already have a minimum resistance of 1 ohm. While a 1 ohm potentiometer would be ideal they are usually very expensive or physically too large for this project. We used a 100 Ohm ten-turn potentiometer and got good results.


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This page was published on 9/18/98