FIGURE 3. A potentiometer (or pot)
acts as a variable voltage divider
by moving a wiper across the surface of a fixed-resistance element.
end. These are made to dissipate a lot
of power in sizes from one-watt to hundreds of watts! Wirewound resistors
are usually intended to be air cooled,
but some styles have a metal case that
can be attached to a heatsink or metal
chassis to get rid of undesired heat.
Because the resistive material in
these resistors is wound on a form,
they have very high LS. For this reason, wirewound resistors are not used
in audio and RF circuits. Be careful
when using a resistor from your junk
box or a grab bag in such a circuit!
Small wirewound resistors look
an awful lot like film or carbon comp
resistors. There is usually a wide
color band on wirewound resistors,
but not always. If you’re in doubt, test
the resistor at the frequencies you
expect to encounter. There are special
versions with windings that cancel
most of the inductance, but have a
much higher CP that also affects the
resistor’s performance above 50 kHz.
Ceramic and Metal Oxide
If you need a high-power non-inductive resistor, you can use cermet
(ceramic-metal mix) or metal oxide
resistors. These are constructed much
like a carbon comp resistor, substituting the cermet or metal oxide for the
carbon composition material.
There are many different types of
adjustable resistors. The simplest are
wirewound resistors with some of the
wire exposed so that a movable electrode can be attached. The most common are adjusted with a rotary shaft as
shown in Figure 3. The element
provides a fixed resistance between termi-
TOLERANCE AND TEMPERATURE COEFFICIENT
Resistors have a nominal value and a tolerance (the amount of acceptable
variation above or below the nominal value). Most resistors have a 1%, 5%, or
10% tolerance and you can find smaller or tighter tolerances. Which values of
resistors are available is determined by the tolerance series. For example, in
the 5% series, values are selected so that each is approximately twice the
tolerance or 10% from the next highest or lowest value.
Resistors also change value with temperature. The relative change of
resistance with temperature is called the temperature coefficient or tempco
and it is specified as parts per million or ppm or as percentage change per
degree Celcius of temperature change. A positive tempco means that resistor
value increases with temperature. When designing and constructing sensitive
circuits that use precision (1% or tighter tolerance) resistors, it is important to
keep them at an even temperature.
nals 1 and 3. The wiper moves to contact the element at different positions,
changing the resistance between either
end of the element and terminal 2.
If an adjustable resistor has only
two terminals (1 and 2 in the figure),
then it is called a rheostat and acts as
an adjustable resistance. Most
rheostats are intended for use in high-power circuits with power ratings from
several watts to several tens of watts.
If the adjustable resistor has
three terminals, it is called a
potentiometer or “pot” for short. Most pots
are intended to act as voltage dividers
and can be made into a rheostat by
leaving terminal 1 or 3 unconnected.
Miniature versions called trimmers
mount on a circuit board and are used
to make small adjustments or
calibrate a circuit. They are available
in single-turn or multi-turn versions.
Larger pots with 1/8” or 1/4”
diameter shafts are intended for use
as a user control. Pots are available
with resistance values from a few
ohms to several megohms and with
power ratings up to five watts.
Like fixed-value resistors, the
construction of the pot is important.
Higher-power pots may have a wirewound element that has enough
inductance to be unsuitable for audio
or RF signals. Smaller pots, particularly trimpots, are not designed to be
strong enough for use as a frequently-adjusted control. Most pots have
relatively high values of CP, as well.
Pots are also available with elements that have a non-linear taper or
change of resistance with wiper position. For example, a log taper pot has
a resistance that changes logarithmically with shaft rotation. This is
useful in attenuator circuits, for
example. An audio taper pot is used
to create a voltage divider that
mimics the loudness response of the
human ear so that volume appears to
change linearly with control rotation.
In order to save space on printed