Continued from Electrical Resistance - Part 2...
A standard color code for resistors is used to determine and show the manufactured ohmic value, the tolerance, and the reliability level of that resistor.
In the standard color code system that is most widely used today, four color coded bands are painted on the side of the resistor. The color of each band indicates the value of that bands represented significant digit. The color of the first band indicates the value of the first significant digit. The color of the second band indicates the value of the second significant digit. The third color band represents a decimal multiplier by which the first two digits must be multiplied by in order to obtain the resistance value of the resistor.
|Color||Significant Figure||Decimal Multiplier||Resistance Tolerance||Reliability Level Per 1000 Hrs|
For example, lets say that we have a resistor (see example resistor below) that has the following color bands in this order: Red, Violet, Orange, and then Silver. Since red is the color of the first band, the first significant digit is 2. The second band is violet, therefore the second significant digit is 7. The third band is orange, which indicates that the number formed as a result of reading the first two bands is multiplied by 1000. In this case 27 x 1000 = 27,000 ohms. The last band on the resistor indicates the tolerance, which is the manufacturer's allowable deviation from the numerical value given on the reistor. In this instance its color is silver, which indicates that this resistor has a tolerance of 10 percent plus or minus of the value of the resistor. For this particular example resistor, the allowed limit of variation in ohmic value is between 24,300 to 29,700 ohms.
When measuring resistors, there may be situations in which the quantities to be measured may be extremely large, and the resulting number using the basic unit of ohm may prove too cumbersome. Therefore, a metric system prefix is usually attached to the basic unit of measurement to provide a more manageable unit. Two of the most commonly used prefixes are kilo (used to represent thousand and is abbreviated k) and mega (used to represent million and is abbreviated M).
Reusing our example above, the 27,000 ohm resistor could have been written as 27 kilohms, or 27 kΩ. Other examples are: 1,000 ohms = 1 kΩ. Likewise, 1,000,000 ohms is written as 1 megohm or 1 MΩ and 10,000,000 ohms = 10 MΩ.
Resistors are the most common components used in electronics. Because of this, electricians and technicians must be able to quickly identify, select, check, remove, and replace resistors, based on their color codes.
To help a technician remember the resistors color codes, there is a funny memory aid that they can learn to sing or say to themselves, in efforts to remember the color code in its proper order. Each word of the memory aid phrase starts with the first letter of the color: "Bad Boys Run Over Yellow Gardenias Behind Victory Garden Walls" equates to "Black Brown Red Orange Yellow Green Blue Violet Gray White" respectively. There are many other memory aid sentences that could be used instead, this is to simply demonstrate one technique that might be used to help memorize the color codes in their order.
If you make a mistake identifying the first or second significant colors, it usually is not too serious. However, if you make a mistake identifying the third band then you are in trouble because the value is going to be at least 10 times too high or too low.
Some important points to remember about the third band is:
Red, orange, and yellow are the most commonly used colors for the third band.
The fourth band, which is the tolerance band, usually does not present too much of a problem, unless working with microcomputers and cellphones. If there is no fourth band, it means that the resistor has a 20 percent tolerance. A silver fourth band has a 10 percent tolerance, and a gold fourth band has a 5 percent tolerance.
In some cases, the third band will be silver or gold. It then becomes a multiplier, and you multiply the first two bands by 0.01 if it is silver, and 0.1 if it is gold.
If the resistor has a fifth band, then the fifth band indicates the reliability level per 1000 hours of operation of that resistor. The color Brown indicates 1.0%, Red indicates 0.1%, Orange indicates 0.01%, and Yellow indicates 0.001%. For example, if a resistor has a fifth color band that is brown, the chance of failure will not exceed 1 percent for every 1000 hours of operation of that resistor.
Some resistors, both wirewound and composition, will not use the resistor color code. These resistors will have the ohmic value and tolerance imprinted on the resistor itself.
Thank you for reading, I hope you found this blog post educational and helpful in some way.