1. Field of the Invention
This invention relates to power supplies and system grounding, and more particularly to power supplies and systems having electrically conductive chassis electrically coupled to an earth ground electrical potential.
2. Description of the Related Art
Most electronic devices require direct current (DC) electrical power. Portable electronic devices typically dissipate relatively small amounts of electrical power during use, and are typically powered by a battery producing DC electrical power. Electronic devices designed for stationary indoor operation typically include a power supply which converts alternating current (AC) power, e.g. from an electrical outlet in a wall, to DC power.
FIG. 1 is a diagram of a typical power supply 10 including a power supply circuit 12 producing a DC voltage between a DC voltage (DCV) terminal 14 and a return terminal 16. Power supply circuit 12 may be a portion of a linear power supply circuit or a switching power supply circuit. Power supply circuit 12 includes a transformer 18, a rectifier 20, and a filter 22. Transformer 18 produces an AC voltage between opposed first and second ends of a secondary winding 24. A first end of secondary winding 24 is connected to rectifier 20, and a second end of secondary winding 24 is connected to return terminal 16. The second end of secondary winding 24 forms a common ground electrical potential for components of circuit 12 as indicated by symbol 26. Rectifier 20 rectifies the AC voltage produced by transformer 18, producing a time-varying DC voltage. Filter 22 receives the time-varying DC voltage produced by rectifier 20 and smoothes the time-varying DC voltage, producing a substantially constant DC voltage at an output terminal. The output terminal of filter 22 is connected to DCV terminal 14. Where power supply circuit 12 is a switching circuit, an output voltage feedback signal may be conducted from the output terminal of filter 22 to a switching circuit driving a primary winding of transformer 18 as indicated in FIG. 1.
Power supply circuit 12 is housed within an electrically conductive metal enclosure or chassis 18. Metal chassis 18 is typically connected to an earth ground electrical potential for safety reasons as indicated by symbol 28. Metal chassis 18 completely surrounds power supply circuit 12, thus preventing a user from contacting high voltages present within power supply circuit 12 during operation. Where power supply circuit 12 is a switching circuit, grounded metal chassis 18 also provides a measure of shielding which reduces an amount of electromagnetic radiation produced by the switching circuit during operation and received by other electronic circuits located near power supply 10.
As is typical, the common ground of power supply circuit 12 is electrically connected to chassis 18 within chassis 18. In FIG. 1, this electrical connection is represented by conductor 30. Conductor 30 presents a problem in that conductor 30 provides a path for noise voltages to be conducted between the common ground potential of power supply circuit 12 and chassis 18. For example, a system including power supply 10 also includes other electronic devices having different connections to earth ground. Where the connections to earth ground of two or more of the devices are physically separated from one another, the earth ground potentials may be different, causing xe2x80x9cground loopxe2x80x9d currents to flow between the differing potentials. Noise voltages are created when ground loop currents flow through conductor resistances. In FIG. 1, any noise voltage present upon chassis 18 is conducted into the common ground potential of power supply circuit 12 by conductor 30, and into any electronic device receiving DC electrical power from power supply 10. It is also true that any noise voltage coupled into the common ground potential of power supply circuit 12 (e.g., by an electronic device receiving DC electrical power from power supply 10) is coupled into chassis 18 by conductor 30, and into any other device coupled to chassis 18.
It would thus be desirable to have a power supply which provides electrical isolation between a common ground electrical potential and a metallic chassis. Such isolation would eliminate the noise voltage conduction path present between the common ground potential and the metallic chassis of typical power supplies.
A system (e.g., a power supply) is presented which allows a user to electrically couple a common ground electrical potential of a circuit (e.g., a common power supply return) to an earth grounded chassis, or to electrically isolate the common ground electrical potential from the earth grounded chassis, without having to open the chassis. The circuit is formed upon a printed circuit board (PCB). The system includes an electrically conductive fastener having a threaded shaft (e.g., a metal bolt), and an electrically conductive bracket (e.g., a metal xe2x80x9cLxe2x80x9d bracket). The bracket is mounted upon the PCB and electrically coupled to the common ground electrical potential. The bracket is adapted for receiving the threaded shaft of the fastener.
The system includes an electrically insulating bushing having a washer-like first portion and a sleeve-like second portion. A hole in the isolating bushing passes through both the washer-like first portion and the sleeve-like second portion, and is dimensioned to pass the threaded shaft of the fastener. An outer diameter of the washer-like first portion is greater than an outer diameter of the sleeve-like second portion.
The electrically conductive chassis houses the PCB, and is adapted for coupling to an earth ground potential (e.g., for safety reasons). A portion of the chassis is adjacent to the bracket, and has a hole dimensioned to receive the sleeve-like second portion of the insulating bushing. The hole in the portion of the chassis adjacent to the bracket may, for example, have a diameter greater than or equal to the outer diameter of the sleeve-like second portion of the insulating bushing, and less than the outer diameter of the washer-like first portion of the insulating bushing.
In one embodiment, the fastener is a metal bolt. The bracket has a hole dimensioned to pass the threaded shaft of the bolt, and an electrically conductive nut is mechanically and electrically coupled to the bracket. The nut has a hole therethrough and a threaded inner surface for receiving the threaded shaft of the bolt.
The system may include an electrically insulating member disposed between, and in physical contact with, the bracket and the portion of the chassis adjacent to the bracket. The insulating member may have a hole dimensioned to pass the threaded shaft of the bolt.
In order to electrically isolate the common ground electrical potential of the circuit from the earth grounded chassis, the user may insert the sleeve-like second portion of the insulating bushing into the hole in the portion of the chassis adjacent to the bracket. The user may push the threaded shaft of the bolt through the hole in the insulating bushing, the hole in the insulating member, and the hole in the bracket. The user may turn the bolt to engage the threaded shaft of the bolt and the threaded inner surface of the nut, thereby tightening the bolt. When adequately tightened, the bolt holds the insulating bushing in place. The bracket is electrically isolated from the chassis, and the common ground electrical potential is electrically isolated from the earth ground potential.
The system may also include an electrically conductive washer having a hole dimensioned to pass the threaded shaft of the bolt. In order to electrically couple the common ground electrical potential of the circuit to the earth grounded chassis, the user may align the hole in the conductive washer with the hole in the portion of the chassis adjacent to the bracket, and push the threaded shaft of the bolt through the hole in the conductive washer, the hole in the portion of the chassis adjacent to the bracket, the hole the insulating bushing, the hole in the insulating member, and the hole in the bracket. The user may turn the bolt to engage the threaded shaft of the bolt and the threaded inner surface of the nut, thereby tightening the bolt. When adequately tightened, the bolt holds the conductive washer in place between a shoulder portion of the bolt and an outer surface of the chassis. The bolt and the conductive washer form a conductive electrical path between the bracket and the earth grounded chassis, and the common ground electrical potential is electrically coupled to the earth ground potential.
The insulating bushing and the insulating member may be formed from an electrically insulating plastic material. The insulating member, and the insulating bushing when installed as described above, preferably has a dielectric strength sufficient to withstand an electrical potential of about 3,750 volts applied between the bracket and the chassis for a time period of approximately 60 seconds. The insulating member, and the insulating bushing when installed as described above, is preferably made from a material which can withstand a temperature of 100xc2x0 C. over a significant portion of an expected lifetime of the material.
The above described system components may be advantageously employed in systems having multiple cabinets (or complex cabinet systems) to allow a user to achieve a single point of grounding, if necessary. Further, the above described system features may also be employed to selectively isolate or couple a power supply return to a chassis ground in devices other than power supplies.