Fig. 1

# Voltage Buck-Boost Transformer Connections Tutorial

by Lewis Loflin

Caution: there is no high voltage isolation in this circuit could result in electrical shock. The current is limited by the wire smallest gauge. This is for informational purposes only and comes with no warranty.

Here I'll illustrate transformer voltage buck or boost connections. This is where we connected the secondary of a power transformer in series with the primary winding to increase voltage output (boost) or reduce voltage output (buck). Please note there's no longer electrical isolation between the primary and secondary and could cause an electrical shock.

This forms a type of autotransformer with a single winding made from 2 windings in series.

When AC transformer windings are connected in series the phase relationship will determine if the voltages add or subtract. When a voltage is induced in a transformer winding the phase is 180 degrees out of phase with the voltage that created it. In the example in Fig. 1 the dots donate those two connections that are in phase with each other.

Fig. 1 the connection from the top of the primary 120V winding to out of phase bottom of the 30V winding causes the voltage to add. 120VAC + 30VAC = 150VAC. This is the boost connection.

Fig. 2

In Fig. 2 the top of the primary winding is connected in phase with the secondary winding known as the "buck" or subtract configuration. 120VAC - 30VAC = 90VAC.

It's that simple. This completes a brief look at boost-buck transformer connections.

Fig. 3 Dual primary transformer connected as an autotransformer.

Fig. 4 Power transformer connected in voltage boost-buck or subtract configuration.

Fig. 5 Power transformer connected in voltage boost-boost or add configuration.