Potentiometers

A potentiometer, or pot, is a variable resistor. A small one used for internal circuit adjustments is called a trimpot. Pots and trimpots may have two, three or (rarely) four leads, but most have three. The outer two are connected to the substrate on which the resistive element is formed, with one lead at each end of the resistor. The center lead goes to the wiper, a movable metal contact whose point touches the resistive element, selecting a resistance value that rises relative to one outer lead while falling relative to the other as you turn the knob. A two-wire pot has no connection to one end of the resistive element but is otherwise the same. Some two-wire pots connect the free end of the element to the wiper, which slightly affects the resistance curve as you turn it, but it doesn’t matter  a whole lot. Two-wire pots are sometimes called rheostats.Four-wire pots, used mostly on stereo receivers to provide the “loudness” function which increases bass at low volume levels, are like three-wire pots, but with an extra tap partway up the resistive element. The amount of resistance change you get per degree of rotation is even from end to end on linear taper pots. On log taper pots, also called audio taper, a logarithmic resistance curve is used, so that audio loudness, which is perceived on a log curve, will seem to increase or decrease at a constant rate as the control is varied. Trimpots, whose primary use is for set-and-forget internal adjustments on circuit boards, are always linear taper. How much power the pot can dissipate depends on its size. It’s not marked on the part. In most applications, only small signals are applied to it, so it’s not much of an issue. Some pots have metal shafts, while others use plastic. Plastic provides insulation in applications like power supplies, where you might come in contact with a dangerous voltage. Also, some pots have switches built into them, and those may be rotary, operating at one end of the wiper’s travel, or push-pull. Most trimpots rotate through less than 360 degrees, just like pots. Those used in applications requiring high precision may be multi-turn, with a threaded gear inside providing the reduction ratio. Multi-turn trimpots use a small screw for adjustment, usually off to one side of the body of the component. Be wary of turning the screw, because there’s no visual indicator to help you set it back where it was. In stereo receivers, pots can be ganged together onto one shaft, so that turning it will affect both channels together. Each pot is internally isolated from the other, though you may find one end of all of them tied to ground at the terminals.

Markings

Pots are numerically marked with their resistance values using the same scheme as resistors. A B in the code indicates a linear pot, while a C means a log pot. An A, though, can mean either, as the codes have changed over the years. Some pots have LIN or LOG printed on them. Most don’t, though.

Uses

Pots are used to adjust operating parameters for analog signals and power supply voltages. Once, they were the primary method of setting just about everything. In this digital age, volume, treble, bass, brightness, contrast and such are more often selected from a menu or adjusted with up/down buttons. Trimpots on circuit boards are less common too, but you’ll still find some, especially in power supplies, including switchers.

What Kills Them 

Out-of-Circuit Testing

Test the outer two leads on a three-lead pot with your DMM’s ohms scale. It should read something close to the printed value. If the element is cracked, it’ll read open. To check for integrity of the wiper’s contact, connect the meter between one end of the pot and the wiper. Slowly turn the pot through its range, observing the change in resistance. This is one test better done with a good old analog VOM; the meter needle will swing back and forth with the position of the wiper, and it’s easy to interpret. A two-lead pot should read something close to its stated resistance at one end of its control range and 0 ohms at the other. To verify if a pot is linear taper or log taper, measure the resistance at one-third  of the rotation and again at two-thirds. See if those values are about one-third and  two-thirds of the total resistance from end to end. If it’s a log taper part, they won’t even be close.