Nuts & Volts - March 2006

RESISTORS

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 L_S. 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 C_Pthat 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.

Adjustable Resistors

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 C_P, 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.