A 1A, 5V supply will require much less than 1A at 120VAC input. It is usually safe to assume at least 70% efficiency at full load (higher for more modern supplies), so the power supply will only need perhaps 1.5A at 120VAC input. Power supplies are not 100% efficient, thus it will require 5% to 30% more than 120W of input power to produce 120W of output power. As (for example) a motor load such as a refrigerator or saw briefly draws a much higher starting current, in order to avoid 'nuisance trips' a 20A rating does not instantly remove power when that load is exceeded.Ī V BB power supply rated (for example) 12V at 10A can provide up to 12V x 10A = 120W (Watts) of DC power. ![]() A typical US AC wall outlet provides 110V to 120V and is protected by a fuse or circuit breaker with a 15A or 20A rating.
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