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Buying a simple resistor can be a complicated task. Here are some of the parameters you will need to decide on:

  • resistance - measured in ohms, typically from 1 to several million (a.k.a., "megaohm"), the resistance measures the amount of current the device will allow to pass it in normal operation. The resistance of most resistors are marked with color bands on the body for easier identification.
  • power rating or wattage - this describes the heat tolerance of the device and typically range from a quarter of a watt up to several watts. Since higher resistance components allow less current to pass through, they typically are lower wattage devices. For example, in a five volt DC circuit, a 10 ohm resistor would need to handle 0.5 amps and 2.5 watts, whereas a 100 ohm resistor in the same circuit would only need to handle 0.05 amps and 0.25 watts.
  • tolerance - common carbon resistors are designed to be within 5-20% of their rating; more expensive devices are designed to tolerances of 1% or less. If the actual resistance is important in your circuit, you will need to order one of the more expensive, more precise resistors. "Banded" resistors commonly show their tolerance with a color band.
  • composition - in addition to carbon resistors, other choices include carbon and metal film, cermet film, wirewound, and metal glaze.
  • packaging - the most common shape for a resistor is axial, which means that the two leads protrude from the center of the body of the resistor as if they were a single wire running straight through it. Resistors are also available in surface mount and inline and dual inline arrays.

Resistors are typically sold in packages of 5 to 10, but some companies will sell them individually.

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