Resistors Basics: What They Are and How They Work?

The complexity of the electronic world is beyond imagination. When we mention certain terms, most people might not know what they represent, such as vacuum tubes, amplifiers, oscillators, and so on. However, even those unfamiliar with electronic circuits surely know about resistors, which indirectly highlights their significance. They belong to the most basic electronic components used for current limitation, voltage reduction, voltage division, resistance measurement, adjusting circuit gain, and many other applications. Now, let’s dive into the world of resistors with TechSparks!

What Is a Resistor and What Does It Do

A resistor is a fundamental passive component in electronic circuits, primarily used to provide resistance to the flow of electrical current, hence its name “resistor.” As a passive component, it does not actively supply energy to the circuit; instead, it converts electrical energy into heat and dissipates it.

Resistors are measured in ohms (Ω), and the magnitude of their resistance determines the level of impedance they offer to the flow of current. Here is a basic resistor value calculation formula:

R = V / I

Where

R: Resistance of the resistor in ohms (Ω).
V: Voltage, measured in volts (V).
I: Current, measured in amperes (A).

It’s important to note that this formula applies to fixed-value resistors. For variable resistors, the calculation may vary depending on the type and design of the resistor.

In circuit diagrams, resistors are typically represented with various symbols that help identify their location and function within the circuit. Below are schematic symbols for different types of resistors in circuits:

Resistors are typically mounted on printed circuit boards and designed to work alongside other components. Their functions include:

Utilizing internal resistance materials (usually carbon or metal) to impede the flow of current and reduce voltage.
Dividing voltage into two or more lower voltage values and creating output voltage in voltage divider circuits.
Influencing the degree of signal amplification by altering the characteristics of certain components in the circuit.

Type of Resistor

Fixed and Variable

Fixed resistors have their resistance values set during the manufacturing process and cannot be changed afterward. Their resistance values are typically indicated by color-coded bands or printed codes on the resistor itself. Common types include Carbon Film, Metal Film, Metal Oxide, Carbon Composition, and Wirewound resistors.

Variable resistors, on the other hand, are essentially potentiometers that allow users to change their resistance values during operation. This can be achieved through knobs, sliders, or digital interfaces. They are commonly found in real-life applications like adjusting light intensity, volume control in devices, automation, and remote control applications.

SMT and THT

Through-hole resistors, also known as plug-in resistors, are resistors installed in circuit boards in a plug-in form. During the assembly process, manufacturers insert the component’s pins into pre-drilled holes and then solder them on the backside of the board to secure them firmly. In modern processes, the assembly and soldering of through-hole resistors can be done manually. This is partly because plug-in components have a smaller presence in the circuit, and small workshops are unwilling to bear the high cost of automation equipment. Additionally, larger-sized components may not be compatible with the AI automatic insertion machine. Therefore, there may be some limitations in batch projects.

Surface-mount resistors, on the other hand, are resistors installed in a surface-mount form on circuit boards. They typically have a flat profile and no protruding pins. Precise surface mounting is carried out using pick-and-place robots during installation, followed by soldering as they pass through a reflow soldering oven. Due to their compact size and advantages in automation, surface-mount resistors are highly advantageous for small circuits, densely populated circuits, and batch projects.

Resistor Accuracy

The accuracy of a resistor refers to the degree of deviation or error between its actual resistance value and its nominal (or labeled) resistance value. It is categorized based on their accuracy, often denoted by tolerances such as ±1%, ±2%, ±5%, ±10%, and ±20%. Higher accuracy resistors provide more precise resistance values in electronic circuits, but they tend to be more expensive due to the need for precision manufacturing and testing processes. In electronic circuit design, engineers choose resistors with the appropriate accuracy based on the specific requirements of their application.

Here are some common resistor series and their tolerances:

E6 Series: Tolerance of ±20%. These are used in special applications, such as potentiometers in circuits.
E12 Series: Tolerance of ±10%. These low-precision resistors are mainly used in cost-sensitive projects.
E24 Series: Tolerance of ±5%. They are standard accuracy resistors suitable for general-purpose circuits.
E48 Series: Tolerance of ±2%. These are beneficial for circuits that require accuracy in analog and digital applications.
E96 Series: Tolerance of ±1%. High-precision resistors that can meet the requirements of most circuits.
E192 Series: Tolerance of 0.5%, 0.25%, 0.1%. These offer the highest precision but come at a cost.

Series, Parallel, and Mixed Resistor Circuits

Resistors can be connected in series or parallel. In series, multiple resistors are connected sequentially in a circuit path, with current passing through them in sequence. In parallel, multiple resistors have one terminal connected together and the other terminal connected together, and they share the same voltage. To enhance circuit flexibility, a combination of both can be used to create a mixed resistor circuit, as shown in the diagram below:

In the circuit diagram above, we have four resistors labeled as R1, R2, R3, and R4. The power supply voltage on the left is V, and the total current flowing through the circuit is I. The current path in the circuit can be divided into series and parallel sections.

Firstly, we can see that R2 and R3 are connected in series, meaning that they share the same current. We can represent their equivalent resistance as RA, which is equal to R2 + R3. This is shown below:

Next, R4 is connected in parallel with R2 and R3, which means they share the same voltage. We can represent the equivalent resistance of R2, R3, and R4 as RB, given by RA × R4 / (RA + R4). Now, we can simplify the original circuit as follows:

After thoroughly dissecting the hybrid circuit, we obtain the total circuit resistance REQ, which is equal to R1 + RB. The final equivalent circuit is as follows: