Abstract:
A device for use with a printed circuit board, PCB, is provided. The device is arranged to electrically connect to a component on the PCB. The device comprises a first layer having a first hole therethrough and a second layer having a second hole therethrough. The second hole is arranged to receive a connector to connect the device to the PCB. Each of the first and second holes has a width in the plane of the respective first or second layer. The first and second holes are substantially coaxial. The width of the first hole is greater than the width of the second hole. Thus proper electrical clearance is provided between the component on the PCB an the device when the connector is not present.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit and priority of British Patent Application No. GB1119160.8 filed Nov. 4, 2011. The entire disclosure of the above application is incorporated herein by reference. 
     FIELD 
     The invention relates to an earth busbar. In particular, the invention relates to reducing leakage of current from an electrical contact on a printed circuit board (PCB) to an earth busbar when the electrical contact is not connected to the earth busbar. 
     BACKGROUND 
     A busbar is typically a strip of conducting material for use in an electrical apparatus. A busbar may be electrically connected to a PCB to provide a return path for electrical current for electrical components on the PCB. Such a busbar is referred to herein as an earth busbar. 
     A busbar typically has a number of electrical connections to respective electrical contacts on a surface of a PCB. Most typically three electrical connections are made from the busbar to three respective electrical contacts on a PCB. Each connection is realized by a threaded screw, comprising a conductive material, which is inserted through a hole in the PCB and received into the busbar. The head of the screw contacts the respective electrical contact on the surface of the PCB. The first connection is a conventional earth connection, which will be used in most applications. The second connection is for connecting an electromagnetic compatibility (EMC) capacitor to earth. The third connection is for connecting a metal oxide varistor (MOV) to earth. 
     In some applications, it may be desirable not to have the second or third connection at least some of the time. For example, electrical noise in a circuit may be reduced by removing the EMC capacitor. An MOV provides protection from transient voltages, which may result from a lightning strike, for example. In some applications, for example on a ship, the power supply is isolated and so such protection is not required. 
     In applications where the second or third connection is not used, a sufficiently large gap between the second or third electrical contact on the PCB and the conducting material of the earth busbar must be maintained to ensure proper electrical clearance and to prevent current leakage from the PCB to the earth busbar. According to known methods, the gap is typically maintained by inserting an insulating spacer between the PCB and the earth busbar during installation. 
     The insulating spacer is typically formed of a thermoplastic material. The spacer is therefore subject to creep over time, which can cause the connector between the PCB and the busbar to come loose, resulting in a poor electrical connection. Furthermore, the spacer adds complexity to the installation process, and is sometimes omitted from the installation entirely because of human error. When the spacer is omitted, leakage currents can flow between the PCB and the busbar even when no connecting screw has been installed, producing unpredictable and undesirable consequences. 
     An invention is set out in the claims. 
     SUMMARY 
     A device for use with a printed circuit board, PCB, is provided. The device is arranged to electrically connect to a component on the PCB. The device comprises a first layer having a first hole therethrough and a second layer having a second hole therethrough. The second hole is arranged to receive a connector to connect the device to the PCB. Each of the first and second holes has a width in the plane of the respective first or second layer. The first and second holes are substantially coaxial. The width of the first hole is greater than the width of the second hole. 
     An assembly is provided. The assembly comprises a PCB, a device and a first connection between the PCB and the device. The device comprises a first layer having a first hole therethrough and a second layer having a second hole therethrough. The second hole is arranged to receive a connector to form a second connection between the device and the PCB. Each of the first and second holes has a width in the plane of the respective first and second layer. The first and second holes are substantially coaxial. The width of the first hole is greater than the width of the second hole. 
     Thus, when the connector is absent, the first layer of the device provides electrical clearance between a component on a PCB and the second layer by spacing the second layer from the PCB. Furthermore, the width of the first hole, which is greater than the width of the second hole, provides electrical clearance between the component on the PCB and the first layer. 
     Thus, when the device is connected to a PCB but there is a contact on the PCB that is not to be electrically connected to the device, all portions of the device are separated from the electrical contact on the PCB to provide electrical clearance without requiring additional spacers. Eliminating the need for additional spacers between the PCB and the device allows for easier installation, and reduces the risk of improper installation through human error, resulting in a safer and more effective device. 
     The first and second layers of the device may be integrally formed. The first and second layers of the device may be formed of a continuous sheet of material. The continuous sheet of material may be folded to form the first and second layers. Because the device is formed of a single, folded sheet of material, it is simple and inexpensive to manufacture. 
     The first and second layers of the device may be formed of a conducting material. Thus the “spacer” (the first layer of the device) is not a thermoplastic and is not subject to creep over time. Therefore the device retains its structure and a connection between the device and a PCB will remain secure, even over a long period of time. The first layer may comprise an electrically conductive portion surrounding the first hole. 
     The second layer of the device may comprise a lip. The lip may surround the second hole in the second layer. The lip may be arranged to receive the connector. The lip provides a larger contact area between the second layer of the device and a connector, thereby providing both a secure electrical connection between an electrical component on the PCB and the device and a secure mechanical connection between the PCB and the device. 
     The first and second layers of the device may be substantially planar. The first and second layers may be substantially parallel to one another. 
     The device may be a busbar. The device may be an earth busbar. 
    
    
     
       DRAWINGS 
       Embodiments and aspects will now be described with respect to the figures, of which: 
         FIG. 1  shows a perspective view from the front left-hand-side of an earth busbar and a PCB installed in a housing. 
         FIG. 2  shows a perspective view from the front right-hand-side of the earth busbar and the PCB. 
         FIG. 3  shows the earth busbar and PCB in cross section along the line A-A. 
         FIG. 4  shows the earth busbar in cross section along the line B-B, without the other components shown in  FIGS. 1 to 3 . 
         FIG. 5  is a plan view of the earth busbar, when not installed in a housing. 
         FIG. 6  shows a view of the earth busbar in cross section along the line A-A, when not installed in a housing. 
         FIG. 7  shows a perspective view from the rear right-hand-side of the earth busbar, when not installed in a housing. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a busbar  100  and PCB  200  installed in a housing  300 . A first end  102  of the busbar  100  is connected to the housing  300  via a screw  104 , as shown in  FIG. 2 . A second end  106  of the busbar  100  is connected to the PCB  200  via screws  108 ,  110  and  112 . A number of electrical components  202  are mounted on the PCB  200 . The busbar  100  provides an earth connection between the electrical components  202  on the PCB  200  and the housing  300 . 
     The first end  102  of the busbar  100  as shown in  FIGS. 1 to 3  herein comprises a substantially planar portion having a plurality of gaps therein. It sits substantially horizontally at the front of the housing  300 . A wall extends substantially vertically upwards from one side of the substantially planar portion. An upper part of that wall extends laterally and rearwards, away from the first end  102  of the busbar  100  towards the second end  106  of the busbar  100 . 
     The description below focuses on the features at the second end  106  of the busbar  100 . 
       FIG. 4  shows a cross-sectional view of the second end  106  of the busbar  100  along the line A-A shown in  FIG. 1 , but without the other features depicted in  FIG. 1  being present. The busbar  100  comprises a wall section  114 , an upper layer  116  and a lower layer  118 . In the embodiment shown in  FIG. 4 , the wall section  114 , the upper layer  116  and the lower layer  118  are formed of a single, continuous sheet of conducting material. The wall section  114  and the upper layer  116  are joined via a first bend or fold  124  of the conducting material. The upper layer  116  and the lower layer  118  are joined via a second bend or fold  120  of the conducting material. Both folds have curved profiles, with the first fold  124  being approximately 90° and the second fold  120  being approximately 180°. 
     As can be seen in  FIGS. 3 ,  5  and  6 , the upper layer  116  of the busbar  100  has three holes  126 ,  128  and  130  located therein. The holes  126 ,  128  and  130  in the upper layer  116  of the busbar  100  are arranged to correspond to holes  204 ,  206  and  208  located in the PCB  200 , such that holes  126 ,  128  and  130  are substantially coaxial with holes  204 ,  206  and  208 , respectively, when the busbar  100  and PCB  200  are aligned. 
     With reference to  FIGS. 3 ,  4  and  6 , the lower layer  118  of the busbar  100  also has three holes  132 ,  134  and  136  located therein. The holes  132 ,  134  and  136  in the lower layer  118  correspond to and are substantially coaxial with the holes  126 ,  128  and  130  in the upper layer  116  of the busbar  100 . The first  126  and second  128  holes in the upper layer  116  of the busbar  100  each have a diameter larger than the diameter of the first  132  and second  134  holes in the lower layer  118  respectively. 
     Each of the holes  132 ,  134  and  136  in the lower layer  118  of the busbar  100  is surrounded by a lip portion  138 . The lip portions  138  extend downwardly from the lower layer  118  of the busbar  100  in a direction substantially perpendicular to the plane of the lower layer  118  and away from the upper layer  116  of the busbar  100 . 
     The PCB  200  is provided with first  210 , second  212  and third  214  solder pads, which surround the first  204 , second  206  and third  208  holes in the PCB  200 , respectively. The diameter of the inner surface of each of the first  210 , second  212  and third  214  solder pads is larger than the respective diameter of the first  204 , second  206  and third  208  holes in the PCB  200 . Thus, the solder pads  210 ,  212  and  214  are set back from the edges of the holes  204 ,  206  and  208 , respectively, which increases the distance between the solder pads  210 ,  212  and  214  and the busbar  100 , providing increased electrical clearance. Each of the solder pads  210 ,  212  and  214  can be electrically connected to one or more of the electrical components  202  on the PCB  200  as required. 
     In operation, screws  108 ,  110  and  112  are inserted through the solder pads  210 ,  212  and  214 , the holes  204 ,  206  and  208  in the PCB  200  and the holes  126 ,  128  and  130  in the upper layer  116  of the busbar  100 , and are received by the lip portions  138 , which define and surround the holes  132 ,  134  and  136  of the lower layer  118 . The lip portions  138  are tapped to receive the threads of the screws. The screws  108 ,  110  and  112  and the lip portions  138  provide a secure mechanical connection between the busbar  100  and the PCB  200 . Furthermore, a reliable electrical connection is provided between the electrical components  202  and the busbar  100 , via the solder pads  210 ,  212 ,  214 , the screws  108 ,  110 ,  112  and the lip portions  138 . 
     The first screw  108 , solder pad  210 , PCB hole  204 , upper layer hole  126  and lower layer hole  132  are provided for connecting an MOV to earth. The second screw  110 , solder pad  212 , PCB hole  206 , upper layer hole  128  and lower layer hole  134  are provided for connecting an EMC capacitor to earth. The third screw  112 , solder pad  214 , PCB hole  208 , upper layer hole  130  and lower layer hole  136  are provided for connecting other electrical components  202  on the PCB  200  to earth. 
     When one or both of the first  108  and second  110  screws are not present, the busbar described above provides the required electrical clearance between the solder pads  210 ,  212  and the busbar  100  by ensuring that any conducting portions of the busbar  100  are located far enough away from the solder pads  210 ,  212 . This achieved by the inclusion of the upper layer  116 , which acts as a (vertical) spacer between the PCB  200  and the lower layer  118 , maintaining the conducting portions of the lower layer  118  which are most proximal to the solder pads  210 ,  212  in a substantially vertical direction at a required distance from the solder pads  210 ,  212  to ensure sufficient electrical clearance. Furthermore, because the diameter of the holes  126 ,  128  in the upper layer  116  is larger than the diameter of the holes  132 ,  134  in the lower layer  118 , the conducting portions of the upper layer  116  which are most proximal to the solder pads  210 ,  212  in a substantially vertical direction are far enough away from the solder pads  210 ,  212  in a lateral (or substantially horizontal) direction to ensure the required electrical clearance. 
     The holes  126 ,  128  and  130  in the first layer  116 , the holes  132 ,  134  and  136  in the second layer  118  and the holes  210 ,  212  and  214  in the PCB  200  respectively are also laterally spaced apart from one another to provide electrical clearance between, for example, the second solder pad  212  and the portion of the upper layer  116  surrounding the first hole  126 . 
     Because the screw  112  is always present when the busbar  100  and PCB  200  are in use, it is not necessary for the diameter of the hole  130  in the upper layer  116  to be larger than the diameter of the hole  136  in the lower layer  118 ; no electrical clearance is required. 
     Because the busbar  100  comprises a single, folded piece of material, it is simple and inexpensive to manufacture. For example, the holes  126 ,  128 ,  130 ,  132 ,  134  and  136  may be cut into a sheet of material using a die and press, or they may be drilled. In the case of a die and press, the lip portions  138  are formed by the die in the same process. In the case that the holes  126 ,  128 ,  130 ,  132 ,  134  and  136  are drilled, the lip portions  138  are formed in a subsequent die and press process. Once the holes  126 ,  128 ,  130 ,  132 ,  134  and  136  have been formed, the sheet of material is folded to form the upper  116  and lower  118  layers such that the holes  126 ,  128  and  130  are substantially coaxially aligned with the holes  132 ,  134  and  136 , respectively. 
     The holes  132 ,  134  and  136  in the lower layer  118  are small enough to fit tightly with the screws  108 ,  110  and  112 , thereby providing a secure electrical and mechanical connection. The holes  126  and  128  in the upper layer  116  are large enough to provide lateral electrical clearance when the busbar  100  is connected to the PCB  200  and when one of the screws  108 ,  110  is not present. The upper layer  116  is thick enough and sufficiently spaced from the lower layer  118  to provide electrical clearance between the lower layer  118  and the PCB  200  in a vertical direction. 
     The skilled reader will be familiar with how to calculate the required electrical clearance distance, which is a function of the voltage difference between the solder pads  210 ,  212 ,  214  on the PCB  200  and the busbar  100 . For example, recommended voltage spacings are laid down in the relevant standards documents for electrical circuits. Thus the skilled person could construct the busbar  100  disclosed herein with appropriate dimensions to provide the electrical clearance necessary for a particular application in which the busbar  100  is intended to be used. 
     In the embodiment described herein, each of the upper  116  and lower  118  layers are formed from zinc-plated, passivated steel. Each of the upper  116  and lower  118  layers has a thickness of between approximately 0.8 mm and approximately 1.6 mm. Each of the holes  126  and  128  in the upper layer  116  may have a diameter of approximately 8 mm. However, the diameters of the holes  126  and  128  may be greater or less than 8 mm, depending on the diameter of the screws  108  and  110 . The screws  108 ,  110 ,  112  are ISO standard M3 or M4×8 to 16. The PCB  200  is approximately 1.6 mm thick. 
     Whilst the above embodiment has been described as having screws  108 ,  110 ,  112  as forming the electrical and mechanical connections between the busbar  100  and the PCB  200 , any form of mechanical and/or electrical connection can be employed. Alternatively, the electrical connection and the mechanical connection between the busbar  100  and the PCB  200  may be achieved separately. Furthermore, there may be fewer than the three sets of solder pads  210 ,  212 ,  214 , screws  108 ,  110 ,  112 , holes  204 ,  206 ,  208  in the PCB  200 , holes  126 ,  128 , 130  in the upper layer  116  and holes  132 , 134 , 136  in the lower layer  118  depicted herein, or there may be more. Additional sets of solder pads, holes and screws may be used to removably connect electrical components on the PCB  200  other than the MOV or EMC capacitor described above to earth; these other electrical components may be provided in addition to or in place of the MOV and the EMC capacitor. 
     Optionally, the lip portions  138  surrounding the holes  132 ,  134 ,  136  in the lower layer  118  may be omitted. If the lip portions  138  are omitted, the portions of the lower layer  118  surrounding the holes  132 ,  134 ,  136  may be tapped to receive the threads of the screws  108 ,  110 ,  112 , respectively. 
     Although the folds  120  and  124  are depicted as having a curved profile, they may instead be angular or less curved. Alternatively, in place of the fold described above, the upper  116  and lower  118  layers may be joined using known welding or soldering techniques. Alternatively, the upper  116  and lower  118  layers may be integral, and the holes  126 ,  128 ,  130  in the upper layer and the holes  132 ,  134 ,  136  in the lower layer may comprise three single counterbored holes. It is not necessary to provide a gap between the upper  116  and lower  118  layers in a vertical direction. 
     The first end  102  of the busbar  100  may be different to the first end  102  described above, or it may not be present at all. 
     Whilst the device  100  has been described as a “busbar”, another name could be used for the device  100 . The device  100  is a conductor for connecting electrical components on a PCB to earth. 
     The relative terms “front”, “rear”, “right-hand-side”, “left-hand-side”, “upper”, “lower”, “vertical”, “lateral” and “horizontal” used herein relate to the orientation of the device as shown in the Figures and are employed to facilitate description of the device. They are not intended to be limiting. 
     An embodiment has been described herein by way of example only. It will be appreciated that variations of the described embodiment may be made which are still within the scope of the invention.