# Chapter 10 Power Supplies Essay

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Chapter 10 Power Supplies

Measuring Electricity
Voltage—pressure of electrons in a wire Unit of measurement: volts (V)
Current— flow (or amount) of electrons in a wire -Unit: ampere (A) -When voltage is applied to a wire, electrons flow, producing current
Wattage—measure of power consumed or needed watt (W) W = V * A
Resistance—impedance or opposition to the flow ohm (Ω)
Wire has amperage rating Defines how much amperage it can handle i.e., 20 amp, 30 amp If you exceed amperage Wires heat up—may break
Circuit breakers are heat sensitive -Sense when amperage exceeds threshold -Breaks the circuit to stop the flow of electricity
Two Types of Current
• Alternating current (AC)—electrical current flows in both directions
– Electricity provided at wall socket
– Frequency of alternations measured in cycles per second, or hertz (Hz)
• Direct current (DC)—electrical current flows in one direction
– Electricity provided by batteries

Powering the PC
Types of Power
• PCs use DC voltage.
• Power companies supply AC voltage.
• The power supply in a computer converts high-voltage AC to low-voltage DC.
• PSU power supply unit. A power supply also falls into the category of field replaceable unit (FRU), which refers to the typical parts a tech should carry, such as RAM & a floppy disk drive.
Supplying AC
• In the U.S., 115 volts AC (VAC) is standard.
• Other countries use 230 VAC.
– Most power supplies are dual-voltage & compatible with both. They may have a switch on the back to accommodate multiple countries.
Outlet Voltages
• Hot and neutral provide path for AC
– Hot has 115 V
– Neutral carries no voltage
– Ground used for safety
– Returns excess electricity to ground
Multimeter
• Multimeter (or Volt-Ohm meter, VOM) measures
– Voltage
– Resistance
– Continuity
Testing AC Voltage
• Verifies wiring of outlet
– Hot should be 115 VAC
– Neutral completes the circuit
– Ground should go to ground
Imperfect Electricity
• Power companies supply imperfect power
– Voltage varies a bit
– Sags because of high demand
– Surges or spikes
• Requires 2 devices to compensate
– Surge suppressor
– Uninterruptible power supply
Surge Suppressors
• Surge suppressors provide protection against power surges
• Joule is a unit of electrical energy
– Surge suppressor rated by joules
– Higher joules = better protection
• Some protect more than AC surges
– Phone lines for traditional modems & DSL
– Cable connections for cable modems
Important: No surge suppressor in the world can handle the ESD of a lightning strike. Always unplug electronics during electrical storms!
Recommendations
• UL 1449 for 330 V rating
• Minimum of 800 joules
• UL 497A for modem protection

Uninterruptible Power Supply
• An uninterruptible power supply (UPS) provides protection against a power dip or power outage
– Contains a battery that provides continuous  AC power
– Provides surge protection & power conditioning
• Constantly charges battery—provides protection against power sags (brownouts) & blackouts
– All UPSs measured in watts & volts-amps
• Watts are what your system uses
• Volt-amps are what UPS can deliver in a perfect world
• Try manufacturers’ Web sites for matching wattage with a specific system
– Look for smart UPS w/ USB connector
– Note that devices take the power from the UPS with less than 100% efficiency. A UPS tries to deliver perfect power, but some percentage is lost to inefficient devices.

Supplying DC
• The power supply acts as a step-down transformer
– Converting AC into 5-, 12-, & 3.3-V DC
– PCs use a 12-V current to power motors on devices such as hard drives
– PCs use a 5-V/3.3-V current to support onboard electronics

Power Connectors
• Motherboard
– 20- or 24-pin P1 (ATX)
– 4-pin (P4), 6-pin, 8-pin
• Peripherals
– Molex
– Mini
– SATA
– PCIe
• Connectors are keyed
– Can plug in only 1 way