Input & Output Voltages Essentials

Input AC Voltage Standards

  • Two common mains levels:
    • Low-line 120 V AC120\ \text{V AC} (range ≈ 110110–125 V125\ \text{V}) — typical in USA.
    • High-line 230 V AC230\ \text{V AC} — typical in Europe, Asia.
  • Historical development of electrical grids created regional differences; devices must match local standard.

Power-Supply Compatibility

  • Older units: manual switch ("115115/230230") to select input.
  • Modern units: auto-sensing / dual-voltage — accept either 120120 or 230 V AC230\ \text{V AC} without user action.

Consequences of Mismatch

  • 120 V120\ \text{V}-only PSU into 230 V230\ \text{V} outlet → over-voltage: PSU failure, smoke, possible fire.
  • 230 V230\ \text{V}-only device into 120 V120\ \text{V} outlet → under-voltage: device simply will not power on.
  • Always verify outlet rating (e.g.
    • USA homes can have 240 V240\ \text{V} dryer outlets).

Internal DC Output Voltages (Rails)

  • PSU converts AC → lower DC via transformer, filters, rectifiers.
  • Three key DC levels delivered to system connectors:
    • +3.3 V DC+3.3\ \text{V DC}
    • +5 V DC+5\ \text{V DC}
    • ±12 V DC\pm12\ \text{V DC}
  • Term "rail" = wire carrying current at a specific voltage (e.g., "1212-volt rail").

Importance of 1212-Volt Rail

  • +12 V+12\ \text{V} carries highest load for modern motherboards, CPUs, GPUs.
  • PSU selection often based primarily on its +12 V+12\ \text{V} current/wattage rating.

Quick Recall Keys

  • Know regional mains: 120120 vs 230 V AC230\ \text{V AC}.
  • Verify PSU switch/auto-sense before plugging in.
  • Remember DC outputs: 3.33.3, 55, 1212 V DC\text{V DC}.
  • "Rail" = individual voltage line; focus on +12 V+12\ \text{V} rail capacity.