Choosing a Type of Resistor: Fixed or Variable
Resistors come in two basic flavors: fixed and variable. Both types are commonly used in electronic circuits. Here’s the lowdown on each type and why you would choose one or the other:
A fixed resistor supplies a constant, factory-determined resistance. You use it when you want to restrict current to within a certain range or divide voltage in a particular way. Circuits with LEDs use fixed resistors to limit the current, thus protecting the LED from damage.
A variable resistor, commonly called a potentiometer (pot for short), allows you to adjust the resistance from virtually zero ohms to a factory-determined maximum value. You use a potentiometer when you want to vary the amount of current or voltage you’re supplying to part of your circuit. A few examples of where you might find potentiometers are light-dimmer switches, volume controls for audio systems, and position sensors, although digital controls have largely replaced potentiometers in consumer electronics.
Take a closer look at fixed and variable resistors. The following figure shows the circuit symbols that are commonly used to represent fixed resistors, potentiometers, and another type of variable resistor called a rheostat. The zigzag pattern should remind you that resistors make it more difficult for current to pass through, just as a kink in a hose makes it more difficult for water to pass through.
Fixed resistors are designed to have a specific resistance, but the actual resistance of any given resistor may vary (up or down) from its nominal value by some percentage, known as the resistor’s tolerance.
Say you choose a
that has a 5% tolerance. The actual resistance it provides could be anywhere from
(because 5% of 1,000 is 50). You might say that the resistance is
give or take 5%.
There are two categories of fixed resistors:
Standard-precision resistors can vary anywhere from 2% to (gulp) 20% of their nominal values. Markings on the resistor package will tell you just how far off the actual resistance may be (for instance, +/–2%, +/–5%, +/–10%, or +/–20%). You use standard-precision resistors in most hobby projects because (more often than not) you’re using resistors to limit current or divide voltages to within an acceptable range. Resistors with 5% or 10% tolerance are commonly used in electronic circuits.
High-precision resistors come within just 1% of their nominal value. You use these in circuits where you need extreme accuracy, as in a precision timing or voltage reference circuit.
Fixed resistors often come in a cylindrical package with two leads sticking out so you can connect them to other circuit elements. Feel free to insert fixed resistors either way in your circuits — there’s no left or right, up or down, or to or from when it comes to these little two-terminal devices.
Most fixed resistors are color-coded with their nominal value and tolerance, but some resistors have their values stamped right onto the tiny package, along with a bunch of other letters and numbers guaranteed to cause confusion. If you aren’t sure of the value of a specific resistor, pull out your multimeter, set it to measure resistance in ohms, and place its probes across the resistor (either way), as shown. Make sure your resistor is not wired into a circuit when you measure its resistance; otherwise, you won’t get an accurate reading.
Circuit designs usually tell you the safe resistor tolerance to use, whether for each individual resistor or for all the resistors in the circuit. Look for a notation in the parts list or as a footnote in the circuit diagram. If the schematic doesn’t state a tolerance, you can assume it’s okay to use standard-tolerance resistors (+/–5% or +/–10%).