Tab Electronics Guide to Understanding Electricity ... - Sciences Club

32 Chapter Two
A potentiometer has a round body, about 1 2
to 1 inch in diameter, a
rotating shaft extending from the body, and three terminals (or leads) for
circuit connection. It consists of a fixed resistor, connected to the two
outside terminals, and a wiper connected to the center terminal. The
wiper is mechanically connected to the rotating shaft, and can be
moved to any point along the fixed resistance by rotating the shaft. A
rheostat can be made from a potentiometer by connecting either outside
terminal to the wiper terminal.
Potentiometers are specified according to their power rating, fixed
resistance value, taper, and mechanical design. The fixed resistance value
is usually indicated somewhere on the potentiometer body (it can be
determined by measuring the resistance value between the two outside
terminals with a DVM). If the power rating is not marked on the body,
you will probably have to estimate it (if you cannot cross the manufacturer’s
part number to a parts catalog). Standard-size potentiometers
(about 1 inch in diameter) will typically be rated at 1 or 2 watts. Taper
refers to the way the resistance between the three terminals will change
in respect to a rotational change of the shaft. A linear taper potentiometer
will produce proportional resistance changes. For example, rotating
the shaft by 50% of its total travel will result in 50% resistance changes
between the terminals. Logarithmic taper potentiometers are nonlinear, or
nonproportional, in their operation. They are often called audio taper
potentiometers because they are commonly used for audio applications.
The sensitivity-to-volume level of the human ear is nonlinear. Logarithmic
potentiometers closely approximate this nonlinear sensitivity and
are used for most audio volume-level controls, as well as many other
nonlinear applications. The mechanical construction of potentiometers
will vary according to the intended application. Most are single-turn,
meaning that the shaft will rotate only about 260 degrees. For some precision
applications, multiple-turn potentiometers are used.
The term trim-pot is used to describe small, single-turn potentiometers
intended to be mounted on printed circuit boards and adjusted
once, or very infrequently. Figure 2-2 illustrates some common types of
potentiometers and trim-pots.
If you work with various types of commercial or industrial electrical
or electronic equipment, you might run across some dedicated rheostats.
These are rheostats that were not made from potentiometers. They are
usually intended for high-power applications, and are specially designed
to dissipate large amounts of heat.
Adjustable resistors should not be confused with potentiometers or
rheostats. Potentiometers and rheostats are commonly used in applica-