Building Potentiometers

To help you better understand potentiometers, you are now going to read through a description of how one is designed. To start, imagine that you are designing a circuit and find that you need a certain voltage to be delivered to one part of the circuit to power a small lamp.

You could use a fixed resistor, but then you might have to redesign the circuit later if a lamp of a higher wattage is needed. You decide to design in an adjustable resistor, which can be adjusted during assembly.

Therefore, you need a resistor which has a tap whose position can be changed by mechanical motion.

You could connect two terminals with a length of uninsulated resistance wire. Next, you would need to fashion a clamp to make contact with the wire at any point between the terminals. The result would probably look similar to the one back in Figure 2.23.

This device would work, but it is larger than your entire circuit board! Not only that, but to change the resistive value, you would have to do one or more of the following:

• change the wire to one made of a material with more, or less, conductivity
• change the wire to one having a larger, or smaller diameter
• increase, or decrease, the length of the wire

What is wrong with these options? Plenty!

The choice of wire materials is limited by cost and practicality. Using a smaller wire increases both the fragility of your device and the difficulty of making proper terminations and contact with the sliding tap. Finally, increasing the length of the wire will increase the size of the device. What do you think can be done?

A clever way to increase the length of the wire in a practical manner is to wind it around a bar of insulator material. This bar is known as a mandrel.

Figure 2.27 shows a fiberboard tube mandrel.

Figure 2.27

Figure 2.28 shows a fiberboard or ceramic bar mandrel.

Figure 2.28

Unlike our overly large, straight wire potentiometer (Figure 2.23), this new design will permit a smooth, continuous change in the tap, or wiper position. Now, the wiper will electrically jump across the wires, as shown in Figure 2.29.

Figure 2.29

Additional problems to be considered are the possible shorting due to the closeness of the coils, and the need to wind the wire with uniform tension and spacing.

The next development might be to curve the flat mandrel, as in Figure 2.30.

Figure 2.30

Two benefits result:

1. You have a longer resistance wire with less bulk.
2. The center post allows more convenient positioning of the wiper.

If you were to keep the diameter of the round mandrel (Figure 2.27) small, you could gain some valuable benefits.

A round mandrel is more easily wound than a flat one, which means less cost. Also, a small diameter of resistance wire causes smaller jumps as the contact moves from one wire to the next. This feature means smoother performance.

Finally, a round mandrel could be shaped into a spiral, or helix. This design provides a longer resistance range because of the extreme length of the mandrel. Yet, it fits in a small package. An example is shown in Figure 2.31.

Figure 2.31

Now that we have developed several varieties of the wirewound resistive element, let us consider improving the ease and precision of setting the device. How can we quickly set the slider at the exact spot that we want?

If our potentiometer uses a linear mandrel, we could design a lead screw arrangement, as shown in Figure 2.32.

Figure 2.32

It will require several turns of the screw to move the contact from one end to the other. Now it would be easy to set the contact to any point on the resistive element.

We could add a similar mechanical improvement to our rotary design. A worm gear would be perfect, as you can see in Figure 2.33.

Figure 2.33

Figure 2.34

The adjusting screw would rotate the gear attached to the shaft controlling the contact. We could even add a dial for visual display, as illustrated by Figure 2.34.

So far we have considered only wirewound resistive elements. In recent years other materials have been developed.

In addition to wire wrapped around a mandrel, there is cermet, a mixture of fine particles of glass and precious metal, which is applied in paste form to a flat ceramic substrate as displayed in Figure 2.35.

Figure 2.35

Figure 2.36

The other material is conductive plastic, a mixture of carbon powder and plastic resin, and applied as a film. Figure 2.36 is an example of a conductive plastic resistive element.