When wires become components

-September 26, 2013

You probably have a resistor decade box in your lab used to find the best fixed-value resistor for an analog circuit. You may also use a decade box to verify the functionality of a circuit or system under load.

A calibration-grade decade box has resistors with accuracies of 0.01% and better. IET Labs manufactures, supports, and calibrates such decade boxes, as well as capacitance and inductance boxes. The company also manufactures reference standards of all three types used in calibration labs around the world. I visited the company's Boston, Mass., facility on May 13. Engineer Joel Goldberg explained how some of these products are designed and made.

Many of the products produced and supported at the Boston facility were originally made by GenRad. IET Labs acquired the products in 2000. Recently, the company acquired LCR meters, formerly manufactured by Quadtech in Marlboro, Mass.

IET Labs makes resistors out of wire - by hand - and calibrates each one for the proper resistance. Figure 1 shows wire used to make low resistances. The long loop wire on the right is 10-gauge wire. There are four terminals: two attached to the long loop for the excitation current and two for the voltage measurement.

Figure 1: Wires are used to form low-value resistors in resistance boxes.

Note the placement of the measurement wires on the loop wire. The placement of the wires is critical and custom for every unit, for their location sets the measured value of the loop wire. The higher up the loop, the less resistance will be produced. Sometimes, technicians will actually change the resistance of a wire. They may file it to increase resistance or add solder to decrease resistance.

Now let's look at a resistance decade box. Figure 2 shows the outside of a decade box connected to a Fluke precision multimeter used for resistance measurements only. In this photo, the meter is used to calibrate the decade box.

Figure 2: A resistance decade box under test, connected to a reference multimeter.

Note that the two probes connect to four inputs on the meter. That's because one side of the probe delivers excitation current while the other is used for the voltage measurement. Thus, two probes facilitate a four-wire Kelvin connection, eliminating the loss in the probe wires.

Because resistance decade boxes can produce resistance in steps as small as 0.001Ω, wires cut to precise lengths are used for the lower resistances. Figure 3 shows the lowest decade switch with wires instead of resistors. Each wire is trimmed to get its resistance within tolerance.

Figure 3: Wires in a resistance decade box for low-value resistances down to 100mΩ or 10mΩ.

For even smaller resistances, say, 0.001Ω, the connections get down to no wire at all. See Figure 4.

Figure 4: At 0.001Ω, even wires are too long.

Wires not only become resistors, but inductors as well. Figure 5 shows spools of wire used to make inductors for decade boxes and for reference inductors used in calibration labs. These inductors are hand wound, measured, then adjusted to get the proper values.

Figure 5: Spools of wire become inductors for decade boxes and reference inductors.

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