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# Section6.2Power Supplies and Batteries

In almost all countries, the electrical power is supplied to our homes, schools, and businesses in the form of Alternating Current (AC). A plot of the magnitude of the voltage versus time shows a sinusoidal wave shape. Computer circuits use direct current (DC) power, which does not vary over time. A power supply is used to convert AC power to DC as shown in Figure 6.2.1.

As you probably know, batteries also provide DC power.

Computer circuits distinguish between two different voltage levels to provide logical $0$ and $1\text{.}$ For example, logical $0$ may be represented by $0.0$ volts DC and logical $1$ by $+2.5$ volts DC. Or the reverse may be used, $+2.5$ volts as logical $0$ and $0.0$ volts as logical $1\text{.}$ The only requirement is that the hardware design be consistent. Fortunately, programmers do not need to be concerned about the actual voltages used.

Electrical engineers typically think of the AC characteristics of a circuit in terms of an ongoing sinusoidal voltage. Although DC power is used, computer circuits are constantly switching between the two voltage levels. Computer hardware engineers need to consider circuit element time characteristics when the voltage is suddenly switched from one level to another. It is this transient behavior that will be described in the following sections.