Resistors are ubiquitous in electrical devices. These simple, passive components play a significant role in circuit design, coming in various types and serving multiple functions. Primarily, a resistor’s role is to offer resistance against electrical current. Additional functions include:
- Dividing voltage
- Generating heat
- Powering LEDs
- Matching and loading circuits
- Controlling gain
- Fixing time constraints
Choosing the right resistor type for a project depends on several pre-planned factors before sourcing them for mass production. Engineers must consider the following factors when selecting a resistor:
- Resistance
- Tolerance
- Rated power dissipation
- Package and mounting
- Voltage rating
- Material construction
- Inductance and capacitance
- Thermal range
- Operating noise
Resistors come in various types with distinct ratings and sizes. Understanding each type’s advantages and functions can significantly aid in circuit design.
Common Types of Linear Resistors
Linear resistors react according to Ohm’s Law, changing value in direct proportion to the applied voltage and temperature. These resistors are typically classified into two categories: fixed resistors and variable resistors.
Fixed Resistors
These resistors offer consistent resistance across a circuit and are commonly used in printed circuit boards and electronic devices. Fixed resistors come in a variety of sizes and material compositions. The most common types include:
Carbon Composition Resistors: One of the oldest types, these resistors are made from a mix of powdered carbon and ceramic. Although still available, they are less commonly used due to their higher cost and less efficient specifications compared to other types of fixed resistors.
Wire Wound Resistors: Made with an insulated metal wire wound around a non-conductive core, these resistors are highly durable and stable under high power and temperature conditions. However, they are costlier and unsuitable for high-frequency applications.
Thin Film Resistors: Available as carbon film and metal film resistors, these are ideal for applications requiring high stability, precision, and low noise, such as medical devices and audio equipment.
Thick Film Resistors: Mostly used in consumer electronics, these resistors are constructed similarly to thin film resistors but with thicker films of metal oxides or cermet oxides. They are cost-effective and widely available.
Fusible Resistors: Providing resistance and acting as a fuse, these resistors can break a current in case of an overload. They are commonly used in high-end electronic devices and safety equipment.
Variable Resistors
Unlike fixed resistors, variable resistors allow resistance values to be adjusted through a dial, knob, or screw, making them essential in applications like radio and audio equipment. The common types of variable resistors are:
Potentiometers: Controlled via a dial or knob, these resistors feature a moving contact regulating resistance. They are commonly found in audio/visual equipment.
Rheostats: Similar to potentiometers, these resistors use a sliding contact to regulate voltage. They are used in audio/visual equipment and transducers.
Types of Non-Linear Resistors
Non-linear resistors differ in that their resistance value changes with temperature, light, or voltage rather than following Ohm’s Law. Common types include:
Thermistors: These resistors vary voltage according to temperature changes and are used in appliances, automobiles, and batteries.
Varistor Resistors: Made of semiconductor materials, these resistors vary resistance with applied voltage. They are used in telecommunication lines and power strips.
Photo Resistors (LDR): These resistors change resistance with light exposure and are used in light sensors and photography equipment.
Surface Mount Resistors (SMDs): Mounted directly onto PCBs, these resistors save space and speed up production, mainly in computing equipment.
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