How to Read Resistor Color Codes
Resistors are a fundamental component in electrical circuits. Early scientists came to understand the concept of resistance shortly after they were conducting tests to determine the results of passing electricity through all kind of materials and subsequently discovered electrical current. While copper, gold, and aluminum were found to be great conductors with low resistance, air, mica, and ceramics were considered resistors because of their ability to greatly limit the flow of electrical current. Although people in the industry have been aware of their fundamental capabilities for decades, a reliable resistor as we know them today did not come about until 1961 when Otis Boykin created a low-cost, reliable resistor that allowed a precise amount of electricity to pass to a component. With his breakthrough, resistors were less affected by extreme temperatures and shock, and it was finally possible to economically manufacture them. As the US military, IBM, and many consumer electronics manufactures placed orders for Boykin’s new resistor, they began to be incorporated into everything from household appliances and computers to guided missiles.
Resistors are ubiquitous in modern day electronics. As passive devices, they dissipate power but never provide power. They have many uses in circuits, such as for example regulating current flow to LEDs or controlling the amount of voltage reaching an active device such as a transistor. Resistors can be used to terminate a transmission line and prevent reflections or as a pull-up or pull down resistor on the GPIO of a microcontroller to add stability to a system. Using a resistor and capacitor together can create a timing source necessary for light flashers or electronic siren circuits. A “daisy chain” of resistors connected in series can create a voltage divider useful for components that need to operate at a lesser voltage than that supplied by the input.
Resistors are available in many different values, shapes, and physical sizes. Practically all leaded resistors with a power rating up to one watt have a pattern of colored bands that are used to indicate resistance value, tolerance, and sometimes even the temperature coefficient. There can be anywhere from three to six colored bands on the body of a resistor, with four bands being the most common variation.
Three or Four Band Resistors
The first two bands always denote the first two digits of the resistance value in ohms. On a three or four-band resistor, the third band represents the multiplier. This multiplier will basically shift your decimal place around to change your value from mega ohms to milliohms and anywhere in between. The fourth color band signifies tolerance. Keep in mind that if this band is absent and you are looking at a three-band resistor, the default tolerance is ±20%.
Five or Six Band Resistors
Resistors with high precision have an extra color band to indicate a third significant digit. If your resistor has five or six color bands, the third band becomes this additional digit along with bands one and two. Everything else shifts to the right, making the fourth color band the multiplier and the fifth band the tolerance. A six-band resistor is basically a five-band type with an additional ring indicating the reliability, or the temperature coefficient (ppm/K) specification. Using brown, the most common sixth band color, as an example, every temperature change of 10°C changes the resistance value by 0.1%.
How do I know which end of the resistor to start reading from?
1. Many resistors have some of the color bands grouped closer together or grouped toward one end. Hold the resistor with these grouped bands to your left. Always read resistors from left to right.
2. Resistors never start with a metallic band on the left. If you have a resistor with a gold or silver band on one end, you have a 5% or 10% tolerance resistor. Position the resistor with this band on the right side and read your resistor again from left to right.
3. Basic resistor values range from 0.1 Ohm to 10 Megaohms. With that knowledge, realize that on a four-band resistor the third color will always be blue (106) or less and on a five-band resistor, the fourth color will always be green (105) or less.
Why does my high voltage resistor not use metallic colors?
Gold and silver are replaced with yellow and grey in high voltage resistors to ensure the lack of metal particles in the exterior coating.
What is a zero-ohm resistor?
Easily recognized by their single black band, zero-ohm resistors are basically wire links used to connect traces on a printed circuit board. They are packaged like a resistor, so the same automated equipment used to place resistors can also be used to place these on the circuit board. This design negates the need for a separate machine to install a jumper wire.
Is there a fancy way to memorize the order of colors on the chart?
While there are several mnemonics on the internet to help you memorize the color order for the resistor color code chart, some are more pleasant than others. Another way to set the color chart to memory is to think of black as the absence of color, so it is “0”, while white is the combination of all colors, so it is the highest value, “9”. In the middle of the color chart you will find the standard rainbow colors in their regular order for numbers 2 through 7, so your childhood ROY-G-BIV acronym comes into play, minus the color indigo. Just remember that brown fits between black and red as number “1” and grey fits between violet and white as “8” and you’ve got it!
What is a “reliability” band?
Military specified resistors often include an extra band on four-band resistors to indicate reliability, or the failure rate (%) per 1000 hours of service. This is seldom utilized in commercial electronics.