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When mechanical shafts are rotating, a simple gear train can deliver power to a shaft at one speed from a shaft at a different speed. However, fluid power can be converted from a source with one pressure-flow ratio to another pressure-flow level without rotation by using the switching action of a hydraulic ram. Similarly, when AC power is being delivered from an AC source, a simple transformer can be used to convert power at one voltage level to power at another voltage level with low losses. Likewise, the switched action of an SMPS can convert DC power with low losses. http://en.wikipedia.org/wiki/Switched-mode_power_supply