3.5 - Computer Power

Question 1

WARNING

Answer 1

• Always disconnect from the power source when
  working on a device
  – Always. Seriously.
  – Some devices store a charge in capacitors
  – Know how to discharge before touching
• Never connect your body to any part
  of an electrical system
  – Do not connect yourself to the
  ground wire of an electrical system
• Respect electricity
  – It does not respect you

Question 2

Computer power supply

Answer 2

• Computer uses DC voltage
  – Most power sources provide AC voltage
• Convert 120 V AC or 240 V AC
  – To 3.3 V DC, 5 V DC, and 12 V DC
• You’ll know when this isn’t working
  – An important component

Question 3

Amp and volt

Answer 3

• Ampere (amp, A) – The rate of electron flow
  past a point in one second
  – The diameter of the hose
• Voltage (volt, V)Electrical “pressure”
  pushing the electrons
  – How open the faucet is

Question 4

Power

Answer 4

• Watt (W) – Measurement of real power use
  – volts * amps = watts
  – 120V * 0.5A = 60W

Question 5

Current

Answer 5

• Alternating current (AC)
  – Direction of current constantly reverses
  – Distributes electricity efficiently over long distances
  – Frequency of this cycle is important
  – US/Canada – 110 to 120 volts of AC (VAC), 60 hertz (Hz)
  – Europe – 220-240 VAC, 50 Hz
• Direct current (DC)
  – Current moves in one direction with a constant voltage

Question 6

Dual-voltage input options

Answer 6

• Voltage varies by country
  – US/Canada – 120 volts of AC (VAC), 60 hertz (Hz)
  – Europe – 230 VAC, 50 Hz
• Manually switch between 120 V and 230 V
  – Get your meter!
  – Or use an auto-switching power supply
• Don’t plug a 120 V power supply into
  a 230 V power source!

Question 7

Power supply output

Answer 7

• Different voltages
  – For different components
• Positive and negative voltage
  – Voltage is a difference in potential
  – The electrical ground is a common reference point
  – Depends on where you measure from
• At the front door of your house
  – The second floor is +10 feet
  – The basement is -10 feet
• +12 V
  – PCIe adapters, hard drive motors, cooling fans,
  most modern components
• +5 V
  – Some motherboard components
  – Many components are now using +3.3 V
• +3.3 V
  – M.2 slots, RAM slots, motherboard logic circuits
• +5 VSB
  – Standby voltage
• -12 V
  – Integrated LAN
  – Older serial ports
  – Some PCI cards
• -5 V
  – Available for ISA adapter cards
  – Most cards didn’t use it
  – Today’s motherboards
  don’t have ISA slots

Question 8

24-pin motherboard power

Answer 8

• Main motherboard power
  – Provides +3.3 V, +/-5 V, and +/- 12 V
• 20 pin connector was the original ATX standard
  – 24 pin was added for PCI Express power
• You can connect a 24-pin connector to
  a 20-pin motherboard
  – Some cables are 20-pin + 4-pin

Question 9

Redundant power supplies

Answer 9

• Two (or more) power supplies
  – Internal to the server
• Each power supply can handle 100% of the load
  – Would normally run at 50% of the load
• Hot-swappable
  – Replace a faulty power supply without
  powering down

Question 10

Power supply connectors

Answer 10

• Fixed connectors
  – Connected to the power supply
  – May have too many connectors
  – May not have enough
• Modular
  – Add cables as needed
  – Fewer leftover wires,better airflow
  – A bit more expensive

Question 11

Sizing a power supply

Answer 11

• Power supplies are rated by watts
  – Overall and by individual voltages
• Bigger isn’t necessarily better
  – More expensive
  – Doesn’t speed up your computer
• Physical size is relatively standard
  – Older cases and systems may have proprietary sizes
• Calculate the watts required for all components
  – CPU, storage devices, video adapter
  – Many online calculators
• Video adapters are usually the largest power draw
  – Many video card specifications list a recommended
  power supply wattage
• 50% capacity is a good rule of thumb
  – Power supply runs efficiently and there’s room to grow