Potentiometers
Common Potentiometer Failures:
#1) Dirty. Spray with good cleaner (Deoxit D5 or Deoxit F5 for sliders) and exercise!
#2) Broken. Usually must be replaced, but sometimes you can repair it by taking it apart.
#3) Shorted, case to outside terminal. The first time I saw this, I though I was looking at something rare, but it is not! Old style 7/8", 24mm type pots (think guitar controls, Fender amps, etc) seem to be most prone to this fault. Because you don't suspect it, it can be hard to find. Usually you can take the pot apart and repair this.
Potentiometers or pots for short, are 3 terminal, passive devices, that are very useful for voltage division and making variable resistors.
If we return to the basic voltage division experiment, two 1k resistors across a 9V battery, we find that the "output" voltage between the two resistors can be varied by adjusting one of the resistor values.
For instance, if we change one of the resistors to a 2k resistor, the "output" voltage moves from 4.5V to 6V. A change to a 500Ω resistor results in a 3V output.
If we are using two resistors, and changing the values, we do get a change in voltage division, but we also get a change in current. Sometimes we want this, but often we don't. The potentiometer overcomes this unwanted effect by keeping a fixed voltage across the power supply terminals, and using a movable contact, called a "wiper," to "pick off" the voltage we want.
The image at left is a drawing of the insides of a potentiometer. The brown circular strip is made of carbon, just like a fixed resistor, and comes in many resistive values. When the outside connections are used, the resistive strip behaves just like a regular resistor.
In the center is a movable contact called the wiper because it slides along, or wipes, the resistive strip as it is rotated. The wiper is mechanically connected the shaft you can rotate with your fingers (not shown in the internal view).
The wiper connection sets the potentiometer apart from regular resistors. If we apply a voltage across the resistive strip (the outside terminal connections), we can "pick off" any voltage between the maximum and minimum voltage at the wiper terminal.
The illustrations show the potentiometer at various rotations, and the resulting voltages at the wiper terminal due to the voltage divider effect. We are using a 9V battery as a power supply, shown as a schematic symbol (two parallel lines, one longer than the other). A voltmeter reads the wiper voltage, taking the battery negative (-) terminal as "0V."
At the extreme maximum and minimum, we simply get 9V and 0V. In a central position, we get 1/2 the supply voltage, or 4.5V in this case. Any arbitrary voltage between 0V and 9V can be reached, as illustrated by the 3.14V reading.
Notice that I did not specify the resistance of the pot when using it as a voltage divider. The voltage division effect is indifferent to the total resistance. What matters for voltage division is the ratio of the two legs of resistance.
We will now look at a potentiometer's resistance value, and see how we can make a variable resistor using just two of the three terminals.
The voltmeter has been exchanged for an ohmmeter. This tells us how resistive something is. The pot is specified as a 1k linear pot. Linear means the resistance changes evenly from one end of the strip to the other, so that 1/2 of the total resistance is available at 1/2 rotation. Other types of tapers will have non-linear changes as the wiper is rotated. Logarithmic changes are the next most common.
Reading our 1k pot with a ohmmeter from the outside terminals would just give us 1k. However, if we hook up our meter with just one outside terminal, and the wiper terminal, we can "pick off" any resistance we like between 0Ω and 1000Ω.
For analog audio electronics, potentiometers are commonly used as volume controls, tone controls, gain controls, and panning or balance controls. They can also control audio effects and control things like rate of an effect, the depth of an effect, and more. Understanding how the insides of a potentiometer works, how voltage division is achieved, and how variable resistances are produced, will allow you to understand circuits using pots as control devices.
Here is a schematic of a 9V battery, and a potentiometer hooked up as a voltage divider. The symbol for a pot is the same as a resistor, with an arrow pointing to the middle of the resistor. The arrow terminal represents the wiper connection. When you see a pot on a schematic, use your "mind's eye" to move the arrow from maximum to minimum, and quickly note the effect at the wiper terminal. In this example, you would imagine that the wiper can "pick off" any voltage between 9V and 0V.
Here is nearly the same schematic, but with an ac voltage source, instead of a dc voltage. 1kHz (that's "one kilohertz") is an audio frequency that we can hear if we end up connecting this circuit to a speaker. It might be very loud (I did not specify the amplitude of the ac voltage here), so the potentiometer can be used as a volume control to quiet the 1kHz tone. Let's say that the amplitude of the 1kHz tone is 9V, and the wiper is connected to the input of an audio amplifier. Now our voltage divider is dividing ac voltage and the output, as "picked off" by the wiper, can be a loud 1kHz tone, or a quiet 1kHz tone.