This term is usually applied to a transformer that will lower (buck) or raise (boost) the input voltage applied to it.
There are two types of buck/boost transformers, self-adjusting (active) and passive.
Self-adjusting (active) transformers will automatically raise or lower the input voltage to keep it in a specified range. These are also known as ferro-resonant or voltage-regulating transformers.
Passive transformers are used for larger loads where the amount of buck or boost is fixed. An example would be connecting equipment rated for 240 Volts to a 208 Volt source (which is what you get on the secondary side of a 3-phase transformer set up in a wye configuration with the neutral connected to the center of the wye).
It is important to note that buck/boost transformers are auto-transformers that do not provide electrical isolation or harmonic mitigation between the primary and the secondary windings (since the primary and secondary are one and the same in an auto-transformer). They are generally not useful for feeding phase-control dimming systems.
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