Current transformer with test wire

A current transformer includes a housing including generally cylindrical outer and inner walls defining an internal chamber, a front face enclosing one end of the internal chamber, a base, and a central opening defined by the inner wall. A generally toroidal current transformer core is disposed within the internal chamber. A secondary wiring is disposed about the transformer core and is configured to generate a current in response to magnetic flux in the transformer core. A pin housing is disposed on the front face of the housing adjacent the base. The pin housing has electrically conductive pins. A test wire passes through the central opening. The secondary wiring is electrically connected to a first pair of the pins and the test wire is electrically connected to a second pair of the pins.

BACKGROUND

The present invention relates to ground fault detection circuits and more particularly, to differential current transformers of a ground fault detection circuit.

Ground Fault Circuit Interrupt (GFCI) devices are often found used in receptacles in kitchens, bathrooms, and outdoor settings where there may be water or moisture that can pose a risk of electrocution. GFCI devices interrupt power in AC power systems when ground current faults and grounded neutral faults are detected. GFCI devices are also used in circuit breakers that protect these same areas of residential buildings. GFCI devices and other devices that detect ground faults and interrupt AC power systems may also be referred to generally as “ground fault detectors.” Ground fault detectors typically have test circuits that can detect if the ground fault detection circuit is operating correctly.

GFCIs typically detect current leak age by comparing the current flowing in the line and returning in the neutral. A difference in current levels can indicate that a ground fault exists. GFCIs typically use a differential transformer, which will also be referred to as a current transformer herein, to detect a difference in the current level.

Many GFCI devices have a “test” button for verifying the operating status of a current transformer. Testing the device may involve the creation of a small imbalance by passing a test signal current through the differential transformer's opening via a test wire. The differential transformer and detection circuitry in a properly functioning device detects the test current as an imbalance and causes the circuit to trip. This means that the circuit breaker device is working. If the circuit breaker does not trip, the circuit may have a problem that indicates further evaluation of what caused the failure, and suggest some form of remediation.

Currently, assembly of a current transformer for a GFCI device requires that the test wire is manually inserted through the opening of the current transformer and conductive material at the bare ends of the wire is soldered onto a printed circuit board (PCB). Such soldering of a test wire typically requires one or more steps in addition to the mounting and wiring of other components forming the GFCI circuitry on the PCB. Retaining the test wire in position during the placement and soldering of the test wire may require the use of fixtures, careful manual placement, and time to inspect and ensure that the wires do not become disconnected or improperly positioned or interfere with the placement or operation of other components during manufacture or installment.

There is a need for current transformers that simplify assembly to a PCB and provides reliable operation. Devices according to the present disclosure satisfy the need.

BRIEF SUMMARY

In one embodiment, the disclosure includes a current transformer that includes a housing with generally cylindrical outer and inner walls defining an internal chamber, a front face enclosing one end of the internal chamber, a base, and a central opening defined by the inner wall. A generally toroidal current transformer core is disposed within the internal chamber. A secondary wiring is disposed about the transformer core and is configured to generate a current in response to an imbalance of magnetic flux in the transformer core. A pin housing is disposed on the front face of the housing adjacent the base. The pin housing has electrically conductive pins. A test wire passes through the central opening. The secondary wiring is electrically connected to a first pair of the pins and the test wire is electrically connected to a second pair of the pins.

These and other features will be apparent from the following detailed description and accompanying drawings.

DETAILED DESCRIPTION

Now referring generally to the figures where, whenever possible, like reference numbers will refer to like elements, there is illustrated inFIGS.1-7an exemplary current transformer20(seeFIG.7). The current transformer20includes a housing22. A core shield24is disposed within the housing22. A generally circular, toroidal core26comprising a wound secondary27, which will also be referred to a secondary winding, is disposed within the core shield24. A top washer28provides a rear cover and fits to and closes the housing22with the core shield24and core26positioned within the housing.

Referring, in particular, toFIGS.2and3, the housing22may be made of plastic or hard rubber or any suitable material. The housing22has an internal chamber30that is generally circular or toroidal in shape defined by an annular outer wall32and an annular inner wall34. The inner wall34defines a central opening36. The housing22has a front face38that defines one side of the internal chamber30.

Formed on the front face38of the housing22is a secondary pin housing40and a test pin housing42. The secondary pin housing40and the test pin housing42may be positioned side by side on the front face. The secondary pin housing40and the test pin housing42may be formed of the same material as the housing22, such as plastic or hard rubber and formed as separate parts or as a one-piece construction with the housing.

The secondary pin housing40includes a block of material formed on the front face38with two bores44A,44B formed therethrough. The bores44A,44B may be parallel, and formed as blind holes or through bores. Each of the bores44A,44B may optionally include a respective conductive insert46A,46B in the form of a sleeve. Each of the conductive inserts46A,46B may be co-molded, press fit, or otherwise fixed within one of the bores44A,44B.

The test pin housing42includes a block of material formed on the front face38with two bores48A,48B formed therethrough. Each of the bores48A,48B may include a respective conductive insert50A,50B in the form of a sleeve. The bores48A,48B may be parallel, and formed as blind holes or through bores. Each of the conductive inserts50A,50B may be co-molded, press fit, or otherwise fixed within one of the bores48A,48B.

The pin housings40,42are arranged on the front face38of the housing22. Because the front face38of the housing22is annular, and the pin housings40,42require a sufficient amount of material to house the pins52, the pin housings may be staggered on the front face to provide maximum attachment contact. In other words, the secondary pin housing40may be attached to the front face38nearest a base portion58of the housing, whereas assuming that the base portion58is located at a lower part of the housing22, the secondary pin housing is located at the lowest part of the front face. The test pin housing42may therefore be located upwardly and aside the secondary pin housing40.

In embodiments, each of the bores44A,44B,48A, and48B may have an electrically conductive pin52fitted into a respective conductive insert46A,46B,50A, and50B to make an electrically conductive assembly. The inserts and pins are formed of any suitable electrically conductive material, such as aluminum or copper. The inserts and pins may be soldered together after assembly to form an integral, electrically conductive part. It may be preferred that the pins52are inserted into the bores44A,44B,48A,48B without inserts to produce a more simple construction.

While the test pin housing42is offset in the described manner, the bores46A/B and48A/B may still be aligned. However, in this embodiment, the pins52disposed in the test pin housing42will necessarily be longer in length than the pins52disposed in the secondary pin housing40so as to terminate at the same position.

The pins52are shaped and sized to be received into a suitably shaped and sized electrical receptacle or socket80located on a printed circuit board (PCB)54(seeFIG.7). In one embodiment the pins52are solid and cylindrical with a beveled or cone-shaped terminus for easy location of the pins in a PCB socket (not shown). In particular, the pins52are positioned so as to electrically attach to the PCB in one single step or attachment operation.

Circuitry on the PCB, which is placed in communication with a current transformer20according to the present disclosure when the transformer is installed, may include any suitable circuitry configured for testing Ground Fault Circuit Interrupt (GCFI) devices. Because the circuitry can be any suitable, well-known configuration, the details of such circuitry will not be set out herein. Many such GFCI devices have a “test” button for verifying the health of a device. Actuation of the test button creates a small imbalance by passing a stimulus signal current through the core opening of the transformer. For example, pressing the test button may cause a 120 volt AC power supply to be drawn across a resistor along a test wire that passes through the transformer. In one illustrative example, a current of 8 mA (milliamperes rms), which is greater than a 6 mA leakage current detection requirement for GFCI circuits, passes through the current transformer. The transformer and detection circuitry on the PCB in a properly functioning device would detect the test current as an imbalance and trip the circuit.

The pin housings40,42, have buttress material56attached to the housing22and extending to the housings to reinforce the pin housings so as to reduce flexing of the pin housings during manufacture, insertion of the inserts and pins, and during the subsequent attachment of the current transformer20to the PCB54, which reduces the possibility of breakage of the housings and electrical connections.

The housing22has a base portion58formed on the outer wall32of the housing in a position that defines the bottom60of the current transformer20. The bottom60of the base portion58is shaped to engage a correspondingly shaped part of the PCB54and provide stability to the current transformer. The base portion58includes a pair of grooves62that extend from the front face38to a rear side64of the housing. The pair of grooves62terminate at the front face38at a position that at or aligned with the two parallel bores44A,44B of the secondary pin housing40. Each of the pair of grooves62is sized and shaped to contain and protect one of the pair of wires66from the core26and permit the wires to pass between the current transformer20and the PCB54to pins52disposed in the secondary pin housing40.

Since the wires66originate from the core26, which is positioned within the housing22, there are a pair of notches70formed on the rear side64of the outer wall32to permit the wires66to pass from the internal chamber30, extend along and within the grooves62, and terminate at respective pins52, where they are electrically connected, via soldering for example, to the pins, so as to electrically connect the windings27of the secondary to circuitry on the PCB54when the assembled transformer20is installed onto the PCB to form a GFCI device.

While the base58or bottom60may be planar with a correspondingly shaped portion of the PCB54to receive the transformer20, there may also be features that provide guidance or assistance in proper location of the transformer on the PCB. For example, referring in particular toFIG.3, the base60may be configured as two adjoined planar sections comprising a flat land portion84and a raised portion86that extends from the flat land portion. Where portions84,86meet, there may be a step or stop portion88that transitions from the elevations of the land and raised portions. The step or stop portion88may be a surface normal to the land portion84or beveled at an angle that is not 90 degrees. In addition, the stop portion88may lie in a plane parallel to that of the front face38or angled with respect to the front face. The step88and shape of the raised portion86permit a positive engagement with a corresponding feature (not shown) formed on the PCB54for proper location of the transformer20on the PCB.

Returning toFIG.1, the core26is enclosed within a core shield24as is well known. After the core26and core shield24assembled and positioned within the internal chamber30of the housing22, a top washer28is attached to outer and inner wall32,34on the side of the housing opposite the front face38to fully enclose the core. The top washer28may be a flat, circular plate with a washer opening72that aligns with the central opening36such that primary leads may be passed through the transformer20as is well known.

Both the core shield24and the top washer28include openings for wires66forming the ends of the wound secondary27can extend from the internal chamber30to the outside of the transformer20. The core shield24may have a window74that is aligned during assembly with a notch76formed on the top washer28for that purpose.

A test wire78passes through the central opening36and around the outside of the toroid or the exterior of the housing22. Each conductive end of the test wire78is electrically connected, via soldering for example, to respective pins52of bores48A,48B of the test pin housing42. When assembled and the test wire78and wires from the wound secondary66are connected to pins52correctly, the transformer20is connected to the PCB54via the four pins52by insertion of the pins into the PCB54. After mounting of all of the electrical components onto the PCB54, the components and transformer20may be wired to the PCB in a single step via conventional methods.