A buck–boost transformer is a type of auto transformer used to make small adjustments to the voltage applied to alternating current equipment. It was developed to supply voltages for two main categories; the first of which is when the local power is consistently not high or low enough to correctly and safely run the equipment at its rated power load.

For large adjustments in voltage (more than 15% to 20%), usually a two-winding transformer is used with the required voltage ratio, for example 240VAC to 120VAC. These transformers are more costly than buck–boost transformers since both windings must carry the full power delivered to the load, whereas the buck–boost winding must only carry a fraction of the load power.

For loads demanding slightly higher voltages than available a  Buck Boost Transformer can be designed such that the majority of the line load is passed directly through to the power source while only that amount of kVA needed to adjust to corrected specifications is actually added. This allows much smaller Buck Boost Transformers to service a significantly greater supply to a load than the kVA that is indicated on the transformer nameplate.

Common examples of this application of Buck Boost Transformers would be where motors and HVAC equipment is operating on a line supplying 110V or 230V when the equipment demands 120V or 240V service. A Buck Boost Transformer is designed to lower (buck) or raise (boost) line voltage in a range of 5% to 20%.

Buck Boost Transformers are also used when significantly lower voltages are needed, as in low voltage lighting and landscape lighting.

Primary voltages running from 110V to 440V are lowered to secondary voltages as low as 12 volts, but can also be 16, 24, 32 or 48 volts and can be configured by custom design to fit other voltages.