A resistor resists the flow of electric current and drops the voltage. Resistance is measured in ohms and can be written in different ways. Resistors are made of different materials and are coated for protection. They are identified by a color coding system. Resistors can be fixed or variable, with potentiometers commonly used as volume controls. Other types include rheostats, metal oxide varistors, thermistors, and photosensitive resistors.
A resistor is a component in a circuit that resists the flow of electric current. It has two terminals through which electricity must pass and is designed to drop the voltage of the current as it flows from one terminal to the other. Resistors are primarily used to create and maintain known safe currents within electrical components.
Resistance is measured in ohms, according to Ohm’s law. This law states that electrical resistance is equal to the voltage drop across the resistor divided by the applied current. A high ohm value indicates a high current resistance. This rating can be written in several different ways: for example, 81R represents 81 ohms, while 81K represents 81,000 ohms.
The amount of resistance offered by a resistor is determined by its physical construction. A carbon composition resistor has resistive carbon packaged in a ceramic cylinder, while a carbon film resistor consists of a similar ceramic tube, but has a conductive carbon film wrapped around the outside. Metal film or metal oxide resistors are made in much the same way, but with metal rather than carbon. A wirewound resistor, made with metal wire wrapped around clay, plastic, or fiberglass tubes, offers resistance to higher power levels. Those used for applications that must withstand high temperatures are usually made from materials such as cermet, a metal-ceramic composite, or tantalum, a rare metal, so they can withstand the heat.
Resistors are coated with paint or enamel or covered with molded plastic to protect them. Because they are often too small to write on, a standardized color coding system is used to identify them. The first three colors represent the ohm value and a fourth indicates the tolerance, or how close in percent the resistor is to its ohm value. This is important for two reasons: the nature of its construction is imprecise, and if used above its maximum current, the value may change or the unit itself may burn out.
Every resistor falls into one of two categories: fixed or variable. A fixed resistor has a predetermined amount of resistance to current, while a variable one can be adjusted to provide different levels of resistance. Variable resistors are also called potentiometers and are commonly used as volume controls on audio devices. A rheostat is a variable resistor made specifically for use with high currents. There are also metal oxide varistors, which change their resistance in response to an increase in voltage; thermistors, which increase or decrease resistance as the temperature rises or falls; and photosensitive resistors.
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