Arrangement for reducing no-load current in an energized transformer

摘要

A two-terminal control and power switching circuit for substantially reducing electric current flow in the primary winding of an energized power transformer when no load is connected to the transformer secondary. A triac, in series with the power transformer primary and AC source, remains off when a load remains unconnected to the secondary of the power transformer. An oscillator circuit causes a resistive path to be periodically connected across the nonconducting triac. When a load is connected across the power transformer secondary, the resulting increase in voltage developed across a portion of this resistive path is sufficient to cause voltage-amplitude-sensing circuitry to fire the triac. After the triac is turned on by this voltage-amplitude-sensing circuit, conduction of the triac is maintained by a feedback arrangement whereby a low-impedance, current-sampling transformer, having its primary winding also in series with the triac, power transformer primary winding, and AC source, delivers lagging current from its secondary winding to the input terminals of the triac, with amplitude sufficient to fire the triac after every zero-crossing of the AC source voltage as long as the load remains connected to the power transformer secondary. When the electrical load on the power transformer secondary is removed, the magnitude of the current flowing through the current-sampling transformer falls below the level required to fire the triac, and the triac ceases to conduct at the next zero-crossing.