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How to Read a Resistor? | PCB Knowledge

by: Jul 14,2023 3547 Views 1 Comments Posted in PCB Basic Information

resistor color coded resistors

As parts of the passive components, resistors are components that are designed to impede or restrict the flow of electric current. Resistors are non-polar components, meaning they do not have polarity. However, there is a convention for reading resistor values and determining their orientation.


Reading Through-Hole Resistors

The resistance value of a through-hole resistor can be read through the color-coded rings directly. Color ring resistors are commonly available in three, four, five, and six-ring configurations, with the four-ring version being the most frequently used. In a four-ring resistor, the first two rings represent the resistance value, the third ring represents the multiplier, and the fourth ring represents the tolerance.

For example, in a four-ring resistor with red, red, silver, and gold. According to the code chart, red represents the value 2, silver signifies the multiplier of 0.01, the gold indicates a tolerance of ±5%. Therefore, the resistance value of this resistor is 0.22 Ω with a tolerance of ±5%. It's important to note that due to the tolerance, the actual resistance can vary between 0.17 Ω and 0.27Ω.

Let's move on to a five-ring resistor (brown, yellow, violet, black, and green). In the five-ring resistor, there is an extra ring to indicate the significant digits, namely the first three. So, the resistance could be 147 Ω with a ±0.5% tolerance. It could be as low as 146.265 Ω and as high as 147.735Ω. 


Reading Direction and Tips

In some cases, there may be a slight variation in the spacing between adjacent color rings on a resistor. To determine the reading direction of the color code, observe the side that has the highest number of color rings. The reading direction is from the side with more rings to the side with fewer rings. Furthermore, it is worth noting that the first ring of the resistor is always close to one of the leads, while the last ring, typically gold or silver in color, represents the tolerance.


Reading SMD Resistors

Unlike through-hole resistors with a color ring to provide resistance information, SMD resistors have three code systems: the three-digit system, the four-digit system, and the EIA-96 system. In the three and four-digit systems, the first two or three digits represent the significant figures of the resistance value, and the third or fourth digit represents the multiplier or number of zeros. For example, 103 can be interpreted as 10 Ω x 103 = 10,000 Ω (10KΩ) and 7500 can be understood as 750 Ω x 10=750Ω.

When the resistance value is below 10 Ohms, it is expressed with a combination of “R” and numbers. In this case, the "R" represents the decimal point. For example, if you see a resistor labeled as "8R2," it means the resistance value is 8.2 Ohms.

Furthermore, the EIA-96 series is a coding system used for resistors with a 1% tolerance, providing a wider range of resistance values and greater precision. In this three-digit code system, the first two digits represent a value selected from the E96 Code Table, while the third digit indicates the multiplier associated with the chosen value. Let's take code 54B as an example. The “54” corresponds to a value of 357 according to the E96 Code Table, and the “B” indicates a multiplier of 10. Therefore, the resistance value for this code is 3570 Ω with a tight tolerance of ±1%.


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