Introduction
Various electrical components play specific roles in a printed circuit board (PCB). A skilled PCB designer should possess a comprehensive understanding of these components and their functions in solving design challenges. This article serves as a detailed guide to resistors, aiming to help our readers understand their purpose in electronic circuits, their electrical symbols, how to install them, interpret their markings, and calculate their power ratings. Let’s explore these topics in greater detail.
What are Resistors?
Resistors are electronic components designed to provide a specified resistance to electrical current. Their primary function is to limit the flow of electrons within a circuit. Classified as passive components, resistors do not generate energy but rather consume it. They are integrated into circuits to complement other active components, such as microcontrollers and operational amplifiers. In other words, resistors are used to limit current flow, divide voltage, and pull up or pull down input/output lines. The resistance of a resistor is measured in Ohms, and its symbol is the Greek letter omega, Ω.
Resistor Symbol
Figure 1: Resistor Symbols: R1 represents the Universal Symbol, while R2 is the American Symbol
From the symbols above, it’s evident that resistors are two-terminal components, with terminals located at each end. These resistors can be installed using two main termination methods: through-hole technology (THT) and surface-mount technology (SMT), as described below.
Through-hole
Through-hole technology (THT), often referred to as PTH (Plated Through-Hole), involves resistors with long leads that can be inserted into a breadboard or soldered by hand onto prototyping boards or PCBs.
- Through-hole resistors have long leads that are trimmed after the soldering process.
- This technology is particularly useful during the prototyping phase.
- While they take up more space compared to SMD resistors, through-hole resistors offer stronger, more durable connections.
- The main advantage of THT resistors is their robust joint compared to their SMD counterparts.
Figure 2: Through-hole Resistor
Surface-mount
Surface-mount resistors are directly mounted onto the surface of the PCB. These are tiny, rectangular components with conductive ends, usually placed on opposite longer sides.
- They are positioned on top of the PCB and fixed using automated soldering systems that place and solder the components via a reflow oven.
- Surface-mount resistors are ideal for compact, high-volume production processes.
Figure 3: Surface Mount Resistor
Resistor Decoding
Decoding resistors involves determining their value. For PTH resistors, this is done using the color-code system, while SMD resistors employ a value-marking system. Both methods enable experts to identify the resistance value of each resistor.
Let’s begin by decoding the color bands on PTH resistors:
Figure 4: PTH Resistor Color Bands
Four-Color Band Resistor
In a four-band resistor:
- The first two bands indicate the two most significant digits of the resistance value.
- The third band represents a multiplier, showing how the first two digits should be multiplied by powers of ten.
- The fourth band shows the tolerance of the resistor, indicating the possible deviation from the nominal resistance value.
The first and last bands are easy to identify. The last band, which represents tolerance, is usually either gold or silver and is distinctly separated from the value bands.
Figure 5: The Four-Color Band Resistor
In some cases, you may encounter five- or six-band resistors.
- Five-band resistors have an extra band between the first two and the multiplier band, and they often offer a wider tolerance range.
- Six-band resistors are essentially five-band resistors with an additional band at the end, indicating the temperature coefficient, which shows how resistance changes with temperature.
How to Decode Resistor Color Bands?
Decoding THT resistors is straightforward with the help of a color code table, shown in Figure 6 below:
Figure 6: Resistor Color Code Table
To decode the first two bands on the resistor, match their colors with their corresponding values in the table.
Let’s apply this to the resistor shown in Figure 5:
- The first band is yellow, which corresponds to the value 4.
- The second band is violet, which corresponds to the value 7.
- The third band is red, which indicates a multiplier of 10² (or 100).
- The fourth band is gold, indicating a tolerance of ±5%.
- Thus, the resistor has a value of 4700Ω with a tolerance of 5%.
SMD resistors are marked differently. For example, components such as the 0603, 0805, and 1206 packages typically feature markings printed on their surface.
SMD resistors are usually labeled using two systems:
- E24: All markings are numerical.
- E96: The first two characters are numbers, while the last one is a letter.
E24 Markings
The E24 marking system is similar to the color-band system for PTH resistors. The first two digits represent the most significant digits, while the third digit denotes the multiplier.
Let’s explore this system further with Figure 7 below:
Figure 7: E24 Marked Resistors
- Resistor 104 represents 100kΩ.
- Resistor 105 represents 1MΩ.
- Resistor 205 represents 20MΩ.
- Resistor 751 represents 750Ω.
- Resistor 750 represents 750kΩ.
Decoding E24 markings is straightforward.
E96 Markings
The E96 system uses three characters: the first two digits represent the resistor’s value, while the final letter represents the multiplier. The value of the resistor can be determined using the tables below.
