WATER PLANT OPERATOR TRAINING PROGRAM

There are many instances where only a portion of an output voltage from a signal source is needed. If we allowed the full output voltage from a home CD player to be driven into the input of an amplifier, the amplifier would play at or near full power at all times. This would become quite annoying in a very short period of time. To reduce the overall volume, we need to allow only a fraction of the full signal through to the amplifier. To control the level of the signal, we use a potentiometer. A potentiometer (also know as a 'pot') is a modified resistor. Potentiometers can be used to allow a change in the resistance in a circuit or as a variable voltage divider (in the case of a volume control). If you have a rotary volume control on your TV or radio, it is (more than likely) a potentiometer being used as a variable voltage divider.* A potentiometer generally has 3 terminals. 2 of the terminals are connected to the opposite ends of a resistive element. The 3rd terminal (usually, is physically in-between the other 2 terminals) is called the wiper. The wiper is a contact (actually, generally many very small contacts) that slides along the resistive element. The diagram below shows the schematic symbol for a pot. *If your volume control clicks and steps the volume up or down with each click, it's probably a rotary encoder (a switch), not a potentiometer. The following diagram shows how the schematic symbol relates to the drawing of the potentiometer. Voltage Dividers A Potentiometer as a Variable Voltage Divider: We covered voltage dividers on the resistors page. A pot connected as it is in the following diagram will act to divide the voltage like the 2 individual resistors. In the diagram below, you can see that the linear taper‡ potentiometer is in the middle of its range of travel. You can also see that 12 volts is applied to terminals 'A' and 'B' are connected to the 12 volt battery. This means that the output† from the slider will be 6 volts. This means that the resistance of the resistive element increases in direct proportion to the distance traveled along the resistive element. In the middle of travel, the resistance from the sliding terminal to either of the other terminals is half of the total resistance. †The output is simply the voltage at the point where the wiper contacts the resistive element.