Most electronic devices use resistors. From telephone equipment to stereo systems, computers and electric heaters, resistors can be found in any electronic equipment. In simplest terms, the main function of a resistor is simply to limit the flow of current through an electric circuit. The amount or unit of electrical resistance (R) is measured in Ohms (Ω), where ohms are equivalent to volts (v) divided by amperes (amp).
Resistors are either variable (values can be adjusted) or fixed (values cannot be adjusted). They can also be classified according to the materials used:
- Film resistors -made from conductive metal oxide housed in an insulating ceramic rod; it can be a metal film, carbon film or metal oxide film.
- Carbon composition resistors - made of carbon dust or graphite paste.
- Wire-wound resistors - have a cylindrical ceramic core where a metal alloy wire is wrapped or wound.
As electronic components, resistors can be found extensively on circuit boards. These are attached to boards by inserting their metal leads into the thru-hole then soldered at the back. Other resistors use surface mount technology (SMT) wherein it is soldered directly on top of the board. With the advent of SMT, the circuit boards can now be built faster and smaller in size. Vishay Dale, a manufacturer of resistors, makes rectangular metal oxide film resistors that are identified by number instead of using colored bands.
When choosing a resistor, you have to consider its resistance, tolerance and power rating. Tolerance is an indicator of the accuracy of the given resistance value. The power rating, indicated in watts (W), specifies how much power can be safely tolerated by the electronic component. The power that the resistor absorbs from the circuits is converted into heat. The power ratings range from 1/8 watt resistors to more than 100W. If the current that enters the resistor is exceeded, this can lead to overheating.
To calculate the resistance value and tolerance rating of color-coded resistors, you can make use of a resistor calculator. You can find resistor calculators in the following sites:
- Online resistor calculator by Danny Goodman
- Free downloadable resistor calculator:
To use a resistor calculator for a four band resistor:
- Identify the tolerance indicator/band. This is usually, but not limited to, gold, silver, red or brown/no color. Position the resistor so that the tolerance indicator is at the right side.
- Input the colors into the calculator depending on the order of the bands, starting from left to right.
- First band - first digit of the resistor value
- Second band - second digit of the resistor value
- Third band - multiplier that indicates the number of zeroes to add after the digits are interpreted
- Fourth band - accuracy of the resistor value in percent
Once you have keyed in the colors, the resistor calculator or program will return the resistance value in ohms. For example, if you input the color sequence: brown black orange and gold, the calculator will return a resistance value of 10K ohm +/- 5%.
Calculating for resistance is now easy using a resistor calculator. You don't need to memorize the values associated with the different color bands. What's more is that resistor calculators are available for free online.