The Silent Architects of Electronics: Understanding Resistors

A resistor is an electrical component designed to oppose and limit the flow of current in a circuit. It works based on the principle that certain materials naturally resist the movement of electric charges, converting some electrical energy into heat. Resistors are essential for controlling voltage levels, setting bias conditions, dividing voltages, protecting components from high current, limiting inrush current, providing stable loads, and shaping signals in electronic filters.

Written By Khorshed Alam

Member of AUST Robotics Club

Student | Department of EEE, AUST

How does Resistor work?

This image compares electric current to cars on a road. Just as traffic signs or narrowed lanes slow vehicles down, a resistor slows the movement of electrons. It acts as a controlled obstacle that regulates how fast current can flow through a circuit.

Here, flowing water represents electric current, and the narrow section of the pipe represents resistance. When the pipe becomes tighter, the water flow decreases and pressure changes. In the same way, a resistor restricts current and creates a measurable drop in electrical pressure (voltage).

As current passes through a resistor, some energy is lost, creating a voltage drop. The voltmeter measures this drop just like measuring pressure changes in water flow.

Electrons collide with atoms inside the resistor, causing opposition to current flow and converting part of the electrical energy into heat.

Types of Resistors

1. Carbon Film Resistor

A carbon film resistor uses a thin carbon layer on an insulating rod to create resistance. Its colored bands show the resistance value and tolerance. It is inexpensive, stable, and widely used in everyday electronic circuits.

The image shows carbon-film resistors with their outer coating removed, exposing the resistive layer. A spiral cut is made through the film, and the resistance varies depending on how long and narrow this spiral path is. By adjusting the tightness of the spiral, manufacturers control the final resistance value.

Color Code Reference

The resistor color code assigns numbers and multipliers to colored stripes, making it easy to read resistance and tolerance without printed labels. The shown example corresponds to a 100-ohm resistor with ±5% tolerance.

Those are also carbon-film resistors, designed with a larger body so they can safely handle higher voltage and power levels. Their increased size and robust construction allow them to dissipate more heat, making them suitable for circuits where significant current flows. Because of this durability, they are commonly used in power supplies, load testing setups, and other demanding high-power applications.

2. Surface Mount Device (SMD) Resistor

SMD resistors are tiny rectangular components soldered directly onto a circuit board’s surface. They replace larger, leaded resistors and allow electronics to be smaller, lighter, and more reliable. Their printed number codes make value identification easy, and their compact size is essential for modern devices like phones, laptops, and compact control systems.

A groove cut in an SMD resistor adjusts its resistance precisely by increasing the current path. This ensures accurate values, tight tolerances, and reliable performance in compact electronic circuits

SMD resistors mainly use three types of value indicators:

1. N-digit code

2. R-notation

3. C-notation

3. Potentiometer

A potentiometer is a variable resistor used to control voltage and signal levels. It has three terminals: two ends of a resistive track and one moving wiper. Turning or sliding the wiper changes the output voltage.

Potentiometers come as rotary, slider, and trimmer types; all adjust resistance using a movable wiper on a resistive track.

Inside a potentiometer is a circular resistive track and a wiper connected to the middle terminal. Rotating the knob moves the wiper along the track, changing the resistance between the terminals. This shifting resistance produces a variable output voltage, allowing smooth control in electronic circuits.

When a fixed voltage—such as 9V—is applied across the two outer terminals, the wiper outputs a fraction of that voltage. A multimeter reading changes as the knob is rotated, showing how the potentiometer divides the voltage based on the wiper position.

Value Markings & SMD Codes:

Potentiometer bodies display value like B500K, where the letter indicates taper type (B = linear) and the number indicates total resistance.

SMD versions use three-digit codes: the first two digits are the base value, and the last digit is the multiplier. For example, 101 represents 100 ohms.

4. Rheostat

A rheostat is a variable resistor designed to control current in a circuit. Unlike a typical potentiometer that uses all three terminals as a voltage divider, a rheostat usually uses two terminals, one fixed end and one sliding contact to vary only the resistance in series.

Construction & Working

A rheostat consists of a long resistive wire wound around an insulating ceramic body. A movable slider travels along the length of this resistive track.

When the slider is positioned near the beginning of the coil, the resistance is low, allowing more current to flow. Sliding it further increases the resistance, reducing the current.

Left image: The slider is near the low-resistance end, so current flows easily and the bulb glows brightly.

Right image: The slider is moved toward the high-resistance end, causing the bulb to dim due to reduced current.

5. Fusible Resistors

A fusible resistor is a special type of resistor designed to act both as a resistor and as a fuse. Under normal operation it works like a standard resistor, limiting current in a circuit. However, if excessive current flows due to a short circuit or component failure, the fusible resistor heats up and safely “opens” the circuit by melting its internal element. This prevents damage to sensitive components and reduces fire risk. Fusible resistors are commonly used in power supplies, adapters, chargers, TVs, and other devices where over-current protection is essential.

They are usually marked with their resistance value and sometimes include a flame-retardant coating to ensure safe failure. The main purpose of a fusible resistor is to provide controlled resistance during normal use and reliable protection during fault conditions.

A fusible resistor uses the same color-band value code as a carbon-film resistor; only its function differs.

6. Varistor

A varistor is a protective component used to guard circuits against voltage spikes and transient surges. Most types are MOVs (Metal Oxide Varistors), recognizable by their flat, disk-like shape. At normal voltage the varistor has very high resistance, but during a surge its resistance drops instantly, allowing it to divert excess energy away from sensitive components.

A graph in the image shows how a varistor reacts to voltage changes. In the normal range, its resistance is extremely high and almost no current flows. When voltage exceeds a threshold (around 150V in the example), the resistance drops sharply. The varistor then clamps the voltage and absorbs the surge, protecting the circuit from events like switching transients, lightning spikes, or line disturbances.

The diagram shows a varistor connected across an AC supply after a fuse. Under normal conditions it does not conduct, and power flows smoothly to the load. During a spike, the varistor turns conductive and diverts the excess energy. If the surge is strong, the fuse blows, isolating the circuit and preventing damage.

The marking 14D121K on a varistor provides all its essential specifications. The number 14 indicates the device’s diameter in millimeters, while the letter D shows it is a disk-shaped varistor. The code 121 represents its voltage rating, meaning 12 multiplied by 10 equals 120 volts. Finally, the letter K denotes a tolerance of ±10%. These markings help identify the correct MOV for surge-protection applications.

7. Thermistor

A thermistor is a resistor whose resistance changes with temperature. NTC thermistors decrease in resistance as temperature rises, while PTC thermistors increase resistance. They are widely used for temperature sensing, controlling, and protecting circuits, such as limiting inrush current, monitoring batteries, or preventing overheating. Their small size and sensitivity make them essential in electronics and industrial applications.

A black NTC thermistor and a blue PTC thermistor are placed together to highlight their opposite behaviors. The NTC decreases in resistance with increasing temperature, while the PTC becomes more resistive, demonstrating their different roles in circuit protection and control.

A temperature–resistance graph compares both thermistor types. The NTC curve slopes downward, indicating resistance falls with heat, while the PTC curve rises, showing resistance increases. The contrast helps explain how each device responds to temperature changes.

Thermistor applications are illustrated through symbols representing inrush-current limiting and temperature control. These examples show how thermistors protect circuits, manage heat, and support stable operation in electrical and electronic systems.

8. Light Dependant Resistor (LDR)

An LDR is a light-sensitive resistor whose resistance decreases when light increases and rises sharply in darkness. The serpentine pattern on its surface is the active light-sensing material that changes conductivity based on illumination.

Different LDR models have different sensitivity levels. Their light resistance (10 Lux) is in the kilo-ohm range, while dark resistance (0 Lux) goes into mega-ohms.

9. Strain Gauge Resistor

A strain gauge is a thin resistive sensor that changes its resistance when stretched or compressed. It is bonded to a surface to measure mechanical strain.

When the gauge is compressed, its length decreases and resistance drops. When it is stretched, its length increases and resistance rises. This happens because the cross-sectional area changes with the applied force.

In real systems, a strain gauge is connected to electronics that detect tiny resistance changes. This setup is used in load cells, pressure sensors, and industrial machines to measure force or pressure accurately.

Wrapping Up

Resistors are far more than simple current-limiting components—they are the silent architects that shape signals, protect circuits, and enable precise measurement and sensing. From fixed resistors to specialized types like LDRs and strain gauges, each device brings its own ability to interpret and control the physical world through electrical behavior. By understanding how these different resistors function, you not only strengthen your grasp of circuit fundamentals but also open the door to more advanced, creative, and reliable electronic designs. With this knowledge, you are better equipped to explore the deeper layers of electronics with clarity and confidence.