Designing Embedded Hardware

Resistors

Even a conductor (such as a metal wire) is not 100% efficient at conducting current flow. As current flows through the wire, energy will be lost as heat (and sometimes light). For very small currents, this energy loss is negligible, but for large currents, the loss can cause the conductor to become quite hot (an effect utilized in toasters) or glow brightly (lightbulbs). This loss of energy results in a voltage difference across the wire (or component). The component is said to resist the current flow. This resistance (also known as impedance, although impedance is somewhat more complex than simple resistance) is measured in Ohms (unit symbol Ω, equation symbol R). Schematics commonly leave off the Ω symbol, so 100kΩ is usually written as just 100k.

Tip

On a schematic, a 4.7kΩ value may be written not as 4.7k, but rather as 4k7. The reason is that it is too easy for a decimal point to be missed or lost when the document is photocopied. The solution is to place the multiplier (k) in the position of the decimal point. Resistors such as 24.9Ω are written as 24R9.

This convention is used by design engineers in most of the world. However, in North America, it is only sometimes followed.

The relationship between voltage, current, and resistance is known as Ohm's Law , and is given by:

V = I * R

For a fixed resistance, a varying voltage will produce a varying current, while a constant voltage will produce a constant current. Hence, a varying voltage source is known as an Alternating ...