A Guide to a Complete PCB Resistors Sizing

Introduction

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.

What are Resistors

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.

Resistor Symbol

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.

Through-hole

This is abbreviated as TH and in most cases will come abbreviated as PTH meaning plated through-hole.

• The through-hole resistors come with long leads that can be stacked into the breadboard or be hand soldered through the prototyping boards or PCB.
• They are very significant during the process of prototyping.
• They have long leads that should be trimmed after the soldering process is achieved.
• The disadvantage of this type of resistor termination mode is that they occupy more spacing as compared to their SMD counterparts.
• The advantage is that they form a very strong joint as compared to the SMD counterpart.

Figure 2: Through-hole Resistor

Surface-mount

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.

• They are designed to sit on top of the PCBs and fixed through soldering on top of the mating well-designed landing pads.
• Such resistors are placed and soldered onto the PCB by well-designed automated systems which place them automatically and soldered through an oven.

Figure 3: Surface Mount Resistor

Resistor Decoding

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

Four Color Band Resistor

For this type of resistor color band,

• The first two bands are used to indicate the resistor’s two most significant digits of the value of the resistance.
• The third band represents the weight value that is used for multiplying the two significant digits by power ten.
• Finally, the fourth band is used for indicating the resistance tolerance. Tolerance is used to explain the deviation of the value of the resistor from the actual value.

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.

• The five-band resistors have an extra band that occurs between the first two bands and the multiplier band. It is also accompanied by a wider tolerance range.
• The six-band resistor is just a five-band resistor that has an extra band at the end that serves as an indicator for the temperature coefficient.
• The extra band helps us to know how the value of resistance changes as the temperature of the resistor change.

How do you Decode the Resistor Color Bands?

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.

• The color of the first band is yellow and from the table yellow has a value of 4.
• The second band is violet which has a value of 7.
• The multiplier which is the third band has red as its color hence the multiplier of 10 is 2
• And the fourth and last band is gold in color and this represents tolerance. Therefore, the resistor has a tolerance of 5%as red in the table.
• Therefore, combining the values above, the resistor has a resistance of 4700 ohms and a tolerance of 5%

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:

• E24 where all the makings are numbers
• E96 where the first two characters are numbers while the last one is a letter.

E24 Markings

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

• Resistor 104 has a value of 100kΩ
• Resistor 105 has a value of 1MΩ
• Resistor 205 has a value of 20MΩ
• Resistor 751 has a value of 750Ω
• Resistor 750 has a value of 750kΩ

It is that easy when it comes to decoding E24 markings.

E96 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

• 01C is the 10kΩ
• 85A is the 750Ω
• 01B is the 1kΩ
• 01D is the 100kΩ
• 30C is the 20kΩ

Conclusion

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.