One of the most important considerations circuit designers have to make when allocating resistors in their designs is the appropriate power rating for components. The resistor power rating is based on the amount of thermal loading a component will be able carry on a continuous basis and are expressed in watts. There are a good range of standard watt resistors with many more supplemental ratings made to order for specific applications. Typical, off-the-shelf power ratings range from 1/8 watt to 50 watts although far larger examples are regularly custom built. Generally a resistor power rating up to two watts will be of a linear carbon design; larger examples include ceramic base, wire wound components.
As their name implies, resistors function by offering a resistance to the flow of electrical current through them. This process generates heat in the resistor, the extent of which is a product of the current magnitude involved. In other words, the higher the current flow through the resistor, the hotter it gets. If a resistor is subjected to excessive current and resultant heat loads, it will eventually be destroyed. The current conducted by a resistor varies according to several other features of the circuit, thereby making the correct resistor power rating a critical part of circuit design.
Resistor power ratings are expressed in watts, with common ratings running from small 1/8 watt variants to specialist high watt resistors of several hundred watts. The average resistors encountered on most circuit boards are linear carbon types with ratings of 1/8 to 2 watts. Resistors over 2 watts no longer follow the standard carbon body design and employ a ceramic support with exposed wire windings. These types of high watt resistors are typically rated at between 5 and 50 watts and sometimes feature several tap connections which allow for a selection of different resistance values in one package. Larger watt resistors are generally purpose built and may include integral heat sinks to dissipate the high thermal loads they develop.
Power ratings on most smaller, carbon resistors are generally established visually according to the physical size of each one. Larger wire wound or metal casing resistors generally have the resistor power rating stenciled onto them. To calculate which watt resistor will be required in any given application, Ohm's law must first be applied to establish the amperage the resistor can carry. Once this has been established, the standard power calculation formula of watts equals amps times volts can be used to determine which watt resistors are required. In other words, a resistor which will pass 12 volts at 6 milliamps — 12 x 0.006 amps – requires a 0.072 watt resistor; this means that any of the standard 1/8 watt resistors will be sufficient.