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3.1 Power is the system budget

Lesson
Hardware safety: isolate power; never open a PSU.

Before touching hardware

  • Follow the exact model's service instructions. Shut down, unplug mains and peripherals, and disconnect any internal battery as instructed. Sleep, the PSU switch, or unplugging a laptop charger alone does not fully isolate power.
  • Let parts cool. Use a clean, dry workspace and approved ESD precautions. Hold boards by their edges, avoid contacts, account for screws, and never force connectors.
  • Never open a PSU or insert tools through its vents; stored energy can remain after unplugging. Use only modular cables confirmed compatible with the exact PSU model.
  • Isolate power before every physical change or cleaning. Powered tests are software/external observations with hands and tools out. No live probing, unspecified pin bridging, or damaged-battery servicing.
  • Stop for damage, swelling, liquid, burning smells, or unclear instructions; consult qualified service. Photos, diagrams, and supplied readings are sufficient for these exercises.

Service and ESD guidance · Example PSU cable chart. Check your exact model.

Practice lab

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Calculate

Check a repair quote against PSU capacity and wall demand

A technician proposes retaining a 550 W DC-rated PSU after a GPU upgrade. Supplied peak DC estimates are CPU 100 W, GPU 220 W, and other parts 60 W. Calculate DC demand with 25% margin and wall draw at the original 380 W load with 90% efficiency. These estimates do not certify PSU quality, transients, connectors, or support.

Lesson notes

Plan power for the whole system, transient demands, and intended upgrades.

Wattage is not the whole story

A PSU also needs the right connectors, safe cable layout, reasonable quality, and enough headroom. A big number on a weak unit is not the same as reliable power.

  • Check GPU and CPU power connectors.
  • Plan for transient spikes.
  • Prefer reliable quality over mystery wattage.

Connectors are the correct plugs

Enough wattage does not help if the part cannot be connected correctly. Avoid unsafe adapters and check the manufacturer guidance.

Worked example

Inputs: Illustrative peak: CPU 100 W + GPU 220 W + other 60 W. Margin 25%. PSU 550 W.

Task: Compute planned demand and state what arithmetic does not prove.

Check the reasoning

380 W × 1.25 = 475 W, below 550 W. Still verify manufacturer requirements, transients, connectors, quality, and exact cable compatibility.

Rating labels are not guaranteed total-system peak measurements. Never open a PSU or assume same-brand modular cables match.