Summary: This article is a good guide on how to size your resistors based on the color of the bands and the value written on the top casing of your resistor.
Various electrical components are used to achieve various roles in a printed circuit board. Therefore, a good PCB designer should have a complete understanding of such components and what they can achieve to fulfill their PCB designs problems. This article is a good guide about resistors and the main objective is to help our readers understand what resistors are, what they do in an electronic circuit, their electrical symbols, how to terminate the resistors, decode their markings, and finally how to calculate the power rating of each resistor. Let us dive deeper.
Resistors can be understood as electronic components that have a specified and never changing electrical current resistance. Their core role is limiting the electron flow through any given electric circuit. They are classified as passive elements thus they do not generate but only consume power. They are incorporated into the circuit to complement other connected active components such as microcontrollers, op-amps, etc. In other words, they are used to limit current flow, act as voltage dividers, and pull up the input/output lines. The resistor resistance is measured in Ohms and its symbol is the Greek capital omega, Ω.
Figure 1: Resistor Symbols: Resistor R1 is the Universal Symbol While R2 is the American Symbol
From the symbols above, it is noticeable that resistors are two-terminal components, with the terminals connected on each end of the resistor. These resistors can be terminated using two-termination modes namely through-hole technology, and surface mount technology as discussed below.
This is abbreviated as TH and in most cases will come abbreviated as PTH meaning plated through-hole.
Figure 2: Through-hole Resistor
The surface mount resistors are mounted on the surface of the printed circuit board as the name suggests. They are very tiny black rectangular-shaped components that are terminated on either opposite longer sides by other tiny silvery conductive edges.
Figure 3: Surface Mount Resistor
Decoding is the process that is used to determine the value of each resistor. The PTH resistors make use of the color-coding system and the SMD resistors make use of the value-marking system. Both systems allow the experts to determine what amount of resistance rating every resistor has.
Let us start by decoding the resistor color bands;
Figure 4: PTH Resistors Color Bands
For this type of resistor color band,
Identifying the first and the last band in a resistor is very simple. The last band which is the tolerance band is either gold or silver and is expansively separated from the value bands.
Figure 5: The Four-Color Band Resistor
Sometimes you will come across five and six-band resistors.
In decoding the THT resistors, we consider the color bands. The decoding is made easy through the color code table shown in Figure 6 below.
Figure 6: Color Code Table for Resistors
For the first two bands on the resistor, you should read the color’s corresponding value from the table above.
Using the table in Figure 6, let us calculate the resistance of the resistor in Figure 5 and see its value.
In decoding the surface mount resistor marking, there is a different alternative that is used to display their values. When you observe resistors such as the 0603, 0805, and 1206, you will notice some common markings on these resistors. The resistors have three to four well-marked characters that have been printed on the case.
Two methods are used to mark such SMD resistors namely:
This type of making has some similarities to the color band marking used with the PTH resistors. The first two numbers represent the first two most significant digit while the last number represent the magnitude.
Let us use Figure 7 below to understand this system further.
Figure 7: E24 Marked Resistors
It is that easy when it comes to decoding E24 markings.
Here three characters are used to mark the E96 marking system that is two numbers at the start and one letter at the end. The two numbers represent the first three digits of the resistor value which are then read using the table below to determine the value of the resistor.
Figure 8: Two First Value Markings Table
For the letter on the resistor, it represents the multiplier which can be read from the table in Figure 9 below.
Figure 9: E96 Letter markings value table
Let us use Figure 10 below to understand how the tables in Figures 8 and 9 work.
Figure 10: E96 Marked Resistors
From the tables
The most significant part of this article is knowing how to calculate the value of the resistor just by observing the colors of the band or by reading the value written on top of it.