# Complete CompTIA A+ Guide to PCs: Disassembly and Power

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This chapter is from the book

## Electronics Terms

Voltage, current, power, and resistance are terms commonly used in the computer industry. Voltage, which is a measure of the pressure pushing electrons through a circuit, is measured in volts. A power supply’s output is measured in volts. Power supplies typically put out +3.3 volts, +5 volts, +12 volts, and –12 volts. You will commonly see these voltages shown in power supply documentation as +5V or +12V. Another designation is +5VSB. This is for the computer’s standby power. This power is always provided, even when the computer is powered off. This supplied voltage is why you have to unplug a computer when working inside it.

The term volts is also used to describe voltage from a wall outlet. Wall outlet voltage is normally 120VAC (120 volts AC). Exercises at the end of the chapter explain how to take both AC and DC voltage readings. Figure 4.16 shows a photograph of a multimeter being used to take a DC voltage reading on the power connectors coming from a power supply. When the meter leads are inserted correctly, the voltage level shown is of the correct polarity.

The reading on the meter could be the opposite of what it should be if the meter’s leads are reversed. Since electrons flow from one area where there are many of them (negative polarity) to an area where there are few electrons (positive polarity), polarity shows which way an electric current will flow. Polarity is the condition of being positive or negative with respect to some reference point. Polarity is not important when measuring AC. Figure 4.17 shows rules to observe when working with meters.

Monitors and power supplies can have dangerous voltage levels. Monitors can have up to 35,000 volts going to the back of the CRT. Note that flat-panel displays and mobile device displays use low DC voltage and AC voltage, but not at the voltage levels of CRTs. 120 volts AC is present inside the power supply. Power supplies and monitors have capacitors inside them. A capacitor is a component that holds a charge even after the computer is turned off. Capacitors inside a monitor can hold a charge for several hours after the monitor has been powered off.

Current is measured in amps (amperes), which is the number of electrons going through a circuit every second. In the water pipe analogy, voltage is the amount of pressure applied to force the water through the pipe, and current is the amount of water flowing. Every device needs a certain amount of current to operate. A power supply is rated for the amount of total current (in amps) it can supply at each voltage level. For example, a power supply could be rated at 20 amps for the 5-volt level and 8 amps for the 12-volt level.

Power is measured in watts, which is a measurement of how much work is being done. It is determined by multiplying volts by amps. Power supplies are described as providing a maximum number of watts. This is the sum of all outputs: For example, 5 volts × 20 amps (100 watts) plus 12V 8 amps (96 watts) equals 196 watts. An exercise at the end of the chapter explains how current and power relate to a technician’s job.

Resistance is measured in ohms, which is the amount of opposition to current in an electronic circuit. The resistance range on a meter can be used to check continuity or check whether a fuse is good. A continuity check is used to determine whether a wire has a break in it. A conductor (wire) in a cable or a good fuse will have very low resistance to electricity (close to zero ohms). A broken wire or a bad fuse will have a very high resistance (millions of ohms, sometimes shown as infinite ohms, or OL). For example, a cable is normally made up of several wires that go from one connector to another. If you measure the continuity from one end of a wire to the other, it should show no resistance. If the wire has a break in it, the meter shows infinite resistance. Figure 4.18 shows examples of a good wire reading and a broken wire reading.

Digital meters have different ways of displaying infinity. Always refer to the meter manual for this reading. When checking continuity, the meter is placed on the ohms setting, as shown in Figure 4.18. The ohms setting is usually illustrated by an omega symbol (Ω).

Polarity is not important when performing a continuity check. Either meter lead (red or black) can be placed at either end of the wire. However, you do need a pin-out diagram (wiring list) for the cable before you can check continuity because pin 1 at one end could connect to a different pin number at the other end. An exercise at the end of the chapter steps through this process.

The same concept of continuity applies to fuses. A fuse has a tiny wire inside it that extends from end to end. The fuse is designed so that the wire melts (breaks) if too much current flows through it. The fuse keeps excessive current from damaging electronic circuits or starting a fire. A fuse is rated for a particular amount of current. For example, a 5-amp fuse protects a circuit if the amount of current exceeds 5 amps.

Take a fuse out of the circuit before testing it. A good fuse has a meter reading of 0 ohms (or close to that reading). A blown fuse shows a meter reading of infinite ohms. Refer to the section on resistance and Figure 4.18. An exercise at the end of this chapter demonstrates how to check a fuse.

A technician needs to be familiar with basic electronics terms and checks. Table 4.1 consolidates this information.

#### Table 4.1. Basic electronics terms

 Term Value Usage `Voltage` `Volts` `Checking AC voltage on a wall outlet (typically 120VAC). Checking the DC output voltage from a power supply (typically +/– 12, +3.3, and +/– 5 VDC).` Current Amps (amperes) Each device needs a certain amount of current to operate. A power supply is rated for total current in amps for each voltage level (such as 24 amps for 5-volt power and 50 amps for 12-volt power). `Resistance` `Ohms` `Resistance is the amount of opposition to electric current. Resistance is used to check continuity on cables and fuses. A cable that shows little or no resistance has no breaks in it. A good fuse shows no resistance. If a cable has a break in it or if a fuse is bad, the resistance is infinite.` Wattage (power) Watts Watts is a measure of power and is derived by multiplying amps by volts. Power supply output is measured in watts. Also, A UPS (uninterruptible power supply) is rated in volt-amps. The size of UPS to purchase depends on how many devices will plug in to it.