An electrical device’s power rating provides a general idea of the amount of power that it can safely dissipate. This issue can become quite complex, because in practice the maximum allowable power dissipation is affected by ambient temperature, air flow, and the presence of heat sinks or printed-circuit-board features that help to move heat away from the component. In this introductory discussion we’ll consider the simplest case: an ordinary resistor carrying a constant current.
In the previous page, we looked at three ways to calculate the power dissipation of a resistor:
The second and third formulas can be rearranged to solve for voltage and current:
These expressions are not commonly used to determine voltage and current based on a known power dissipation; usually we know voltage or current and must calculate power. However, they can be helpful in a different way if we interpret P as a power rating. In this case, the formulas solve for the maximum current or voltage that a resistor can safely handle.
For example, let’s say we purchase a 100 Ω resistor that has a 0.5 W power rating.
These calculations tell us that we will exceed the resistor’s power rating if the voltage across the resistor is greater than 7.07 V or if the current flowing through the resistor is 0.07 A.
And by the way, 0.07 A would more commonly be expressed as 70 mA, where mA stands for “milliamp,” i.e., a thousandth of an ampere. We’ll cover the topic of engineering units later in this chapter.
If a resistor is dissipating 2 W of power, it is converting 2 joules of electrical energy into heat every second. That may not seem like a lot compared to, say, an electric space heater that converts 1000 J of electrical energy into heat every second. But 2 W can be a large amount of power relative to the size of the resistor. The resistor has a limited ability to transfer heat to the surrounding environment, and if too much heat accumulates in the resistor, the resulting increase in temperature can cause damage.
In general, a power rating is the amount of power that a device, such as a resistor, can dissipate without failing to perform as described. In some cases, exceeding the power rating will result in damage or even total loss of functionality.
However, it’s important to recognize that power dissipation causes damage by means of increases in temperature, and consequently the actual maximum power dissipation of a component is strongly influenced by thermal conditions. The power rating of a resistor is accurate only for a specific temperature, perhaps 25°C (77°F), and the specification assumes very little ventilation.
In reality, most resistors make very close contact with a printed circuit board, and much of the heat is transferred to the PCB. This means that a resistor may be able to safely handle power dissipation that is higher than its rating. The actual maximum power will be even higher if the design includes a fan that helps to move heat away from electronic components. In fact, that warm flow of air that you notice at the back of a personal computer is generated by the unit’s internal fan, which ensures that the power dissipated by the computer’s circuitry doesn’t cause dangerous increases in temperature.
In some systems, the power dissipated by a resistor is an intentional portion of the design. Resistors can serve as simple heaters that increase the temperature near components, such as integrated circuits, that cannot function in extremely cold conditions.
In general, a higher power rating means that the resistor will be larger and more expensive. Of course, circuit designers try to use the smallest, cheapest devices that will provide adequate functionality. In the engineering world, it’s often very important to reduce size and minimize cost, but of course, we don’t want devices that will fail in the field, or, worse yet, catch fire.
As such, most OEMs (original equipment manufacturers) have engineers on staff that deal specifically with thermal issues. Usually mechanical engineers rather than electrical engineers, these professionals will provide guidance regarding how to maximize heat flow away from sensitive components, using the smallest fans or heat sinks possible while packing the required electronic functionality into as small an area as possible.
A resistor comes with a detailed specification sheet, usually called the datasheet. If you are buying in sufficiently large quantities, the manufacturer may provide customized design support from their in-house experts, because when you succeed, they succeed, too.
We covered some basic information about power ratings and how environmental conditions affect a component’s maximum allowable power dissipation. We also saw that power dissipation, in turn, affects environmental conditions, because electrical energy is being converted into thermal energy.
In the next page we’ll take a closer look at the relationship between energy and power.