Safety-enhanced transformer circuit
AssigneeFrance/Scott Fetzer Company
InventorsWilliam Thomas Hopkins, Alan M. Smith
Life of this patent
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Abstract
A transformer control circuit which includes a ground fault detection circuit that detects ground fault currents in the transformer secondary and delivers a ground fault detection signal to the primary circuit. This circuit derives power from the secondary winding and transmits the ground fault detection signal through an optical isolation barrier, thus avoiding the use of an auxiliary transformer. The ground fault detection circuit further includes safety features for detecting whether AC power is being supplied without earth ground being connected, and/or whether there is an undesired electrical connection between a transformer output connection and earth ground, which would defeat the ground fault detection circuit.
Claims16
A protected circuit for connection to a primary winding of a transformer, for detecting reverse wiring or failure to connect a ground terminal, said circuit comprising:
- a ground terminal,
- line and neutral terminals for connection to a source of alternating current power,
- first and second winding terminals for connection to said primary winding to supply alternating current electrical power thereto,
- power application circuitry for connecting said line terminal and said neutral terminal to said first and second winding terminals, respectively, said power application circuitry being responsive to a shutoff signal to disconnect at least one of said line or neutral terminals from said first or second winding terminal,
- a first electrical impedance connected between said line terminal and said ground terminal,
- a second electrical impedance connected between said ground terminal and said neutral terminal, and
- ground detection circuitry electrically coupled to said second electrical impedance, and generating said shutoff signal upon detection of more than a predetermined current flow through said second electrical impedance.
A tamper protected circuit for connection to a primary winding and a secondary winding of a transformer, each of said windings comprising first and second terminals for carrying a current therethrough, said circuit comprising:
- power application circuitry having terminals for connection to said first and second terminals of said primary winding to supply alternating current electrical power thereto, said power application circuitry being responsive to a shutoff signal to reduce electrical power applied to said first and second terminals of said primary winding,
- rectification circuitry having a first start terminal for connection to a first terminal of said secondary winding, and a return terminal for connection to a second terminal of a load, a first terminal of said load being connected to a second terminal of said secondary winding, such that current flow through said load and said secondary winding passes between said first start terminal and said return terminal, said rectification circuitry rectifying at least a portion of a current flowing between said first start terminal and said return terminal,
- a ground terminal,
- an electrical impedance connected between said ground terminal and said return terminal,
- said rectification circuitry being connected to said ground and return terminals and applying an electrical signal across said electrical impedance,
- tamper detection circuitry connected to said ground and return terminals, and generating said shutoff signal upon detection of a predetermined electrical signal across said electrical impedance.
An electrical energy collector for collecting electrical energy from a current flow between first and second secondary windings of a transformer, comprising:
- a first start terminal,
- a second start terminal,
- a capacitor having first and second terminals,
- a first rectifier connected between said first start terminal and said first terminal of said capacitor to permit only forward current flow from said first start terminal to said first terminal of said capacitor,
- a second rectifier connected between said second start terminal and said first terminal of said capacitor to permit only forward current flow from said second start terminal to said first terminal of said capacitor,
- a first zener rectifier connected between said first start terminal and said second terminal of said capacitor to permit forward current flow from said second terminal of said capacitor to said first start terminal, and to permit reverse current flow from said first start terminal to said second terminal of said capacitor only when a predetermined voltage appears between said first start terminal and said second terminal of said capacitor, and
- a second zener rectifier connected between said second start terminal and said second terminal of said capacitor to permit forward current flow from said second terminal of said capacitor to said second start terminal, and to permit reverse current flow from said second start terminal to said second terminal of said capacitor only when a predetermined voltage appears between said second start terminal and said second terminal of said capacitor,
- whereby said rectifiers divert current flow between said start terminals into said capacitor to accumulate a charge upon said capacitor.
Dependencies are inferred from the claim text.