Arrangement for reducing no-load current in an energized transformer
First Claim
1. A power transformer controlling circuit comprisinga triac for controlling the flow of electric current from an AC source to the primary winding of a power transformer,an electrical load connected through a controlling switch to the secondary winding of said power transformer,a means for allowing a test current, having a magnitude which varies directly with the input admittance of said power transformer, and is small relative to the magnitude of the current that would flow through the primary of said power transformer if said AC source were connected directly and exclusively to said power transformer, to flow from the AC source through the primary winding of said power transformer while said triac is not conducting,a means for firing said triac if the magnitude of said test current exceeds a first threshold, said first threshold having a value greater than the magnitude of the current that flows through the primary of said power transformer if said electrical load is not connected to the secondary of said power transformer, but less than the magnitude of the current that flows through said power transformer when said electrical load is connected to the secondary of said power transformer, anda means for sustaining conduction of said triac, once fired, for the duration of the time that said electrical load remains connected to the secondary winding of said power transformer.
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Abstract
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.
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Citations
11 Claims
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1. A power transformer controlling circuit comprising
a triac for controlling the flow of electric current from an AC source to the primary winding of a power transformer, an electrical load connected through a controlling switch to the secondary winding of said power transformer, a means for allowing a test current, having a magnitude which varies directly with the input admittance of said power transformer, and is small relative to the magnitude of the current that would flow through the primary of said power transformer if said AC source were connected directly and exclusively to said power transformer, to flow from the AC source through the primary winding of said power transformer while said triac is not conducting, a means for firing said triac if the magnitude of said test current exceeds a first threshold, said first threshold having a value greater than the magnitude of the current that flows through the primary of said power transformer if said electrical load is not connected to the secondary of said power transformer, but less than the magnitude of the current that flows through said power transformer when said electrical load is connected to the secondary of said power transformer, and a means for sustaining conduction of said triac, once fired, for the duration of the time that said electrical load remains connected to the secondary winding of said power transformer.
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3. A power transformer controlling circuit comprising
a triac for controlling the flow of electric current from an AC source to the primary winding of a power transformer, an electrical load, connected through a controlling switch, to the secondary winding of said power transformer, a means for periodically allowing a test pulse of current, having a magnitude which varies directly with the input admittance of said power transformer, and having a peak magnitude which is small relative to the peak magnitude of the current that would flow through the primary of said power transformer if said AC source were connected directly and exclusively to said power transformer, to flow from the AC source through the primary winding of said power transformer while said triac is not conducting, a means for firing said triac if said test pulse of current exceeds a first threshold, said first threshold having a value greater than the peak current that flows through the primary of said power transformer if said electrical load is not connected to the secondary of said power transformer but less than the peak current that flows through the primary of said power transformer when said electrical load is connected across the secondary of said power transformer, and a means for sustaining conduction of said triac, once fired, if each half-cycle of current through said triac has a peak magnitude sufficient to exceed a second threshold, said second threshold having a value greater than the peak magnitude of the current that would flow through the primary of said power transformer if said AC source were connected directly to the primary of said power transformer with said electrical load not connected to the secondary of said power transformer, and less than the peak magnitude of the current that flows through the primary of said power transformer when the triac is conducting and said electrical load is connected to the secondary of said power transformer.
Specification