Equivalent Noise Resistance

Equivalent Noise Resistance is any spurious variation in the electrical output not found in the input, appearing between the contact and resistive element when the shaft is rotated. You will remember that cermet and conductive plastic resistive elements present a continuous, smooth path for the wiper. With wirewound elements, the wiper must jump and bridge from one turn of the resistance wire to the next. This simple drawing in Figure 2.49 points out this bridging action.

Figure 2.49

Depending on the relative width of the contact to the wire size and spacing, the wiper actually touches several turns at once.

In the example above, the wiper is wide enough to touch two turns in position A and one turn in position B. When two turns are simultaneously contacted, that portion of the resistive element bridged by the wiper is shorted. As a result, the resistance of the shorted turn will decrease and change the output voltage of the device.

The most common specification of ENR is 100 Ωs maximum.

To look at ENR another way, consider the circuit in Figure 2.50. The potentiometer is used as a voltage divider.

Figure 2.50

The electrical signal present at terminals 1 and 3 is the input and that at terminal 2, the output. If the voltage division performed by the potentiometer were ideal, a graph of the output as the wiper moved from 1 to 3 would be a straight line from 0 to E± (see small graph). It would have a slope equal to the ratio of total input voltage to total resistance.

However, when the output is precisely monitored with an oscilloscope, it can be seen that the potentiometer not only deviates from the ideal, but also some noise is present in the output waveform. This distortion is caused by the device itself.