Electronic signal filter with surge protection mechanism

An electronic signal filter is provided that includes a metal housing which has been adapted to be electrically grounded, and a circuit board positioned within the housing. The circuit board includes a first area and a second area, and has an electrically conductive trace formed on a portion of a top surface of the circuit board that provides electrical communication between the first and second areas of the circuit board. The filter further includes a metal shield connected to the housing, located between the first and second areas of the circuit board. The metal shield extends in a direction substantially perpendicular to the plane of the circuit board and has a slot formed therein for receiving the portion of the circuit board on which the electrically conductive trace is formed. The slot is dimensioned to provide a space between the metal shield and the conductive trace. The dimension of the space is selected to shunt current passing through the conductive trace to ground in the event of a voltage surge passing through the filter.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3A , read in connection with the corresponding view shown in FIG. 4 , is a cross sectional view of a metal shield of a filter in accordance with one embodiment of the present invention. The grounded metal shield 104 (or 105 as shown in FIG. 1 ) includes a slot 106 . The slot 106 includes a first section 106 A, providing intimate contact with bottom surface 100 b of circuit board 100 and with a portion of top surface 100 a of circuit board 100 .

Slot 106 further includes a second section 106 B defining a space between the top surface 100 a of circuit board 100 and shield 104 . A conductive trace 200 is positioned on top surface 100 a of circuit board 100 within second section 106 B of slot 106 . A space of dimension d is formed between the surface of conductive trace 200 and the top of slot 106 in the region of 106 B. The dimension d of the space relates to the dielectric constant of the material, or air, located within the space. If a voltage surge passes through the filter along conductive trace 200 , a spark is generated within the space, and the current is shunted to ground via the grounded metal shield 104 .

It is believed that a space having the dimension d of about 0.013 inches will spark at a voltage surge of about 1000 volts passing through the conductive trace. Accordingly, d should be selected within a range of about 0.010 to 0.020 inches to provide a 1000-2000V surge protection rating.

FIG. 3A also shows that the metal shield preferably includes a raised boss member 110 , which increases the effective thickness of the metal shield to more closely match the width of the shield-receiving slot 101 ( FIG. 1 ) cut through the circuit board 100 . It is difficult to cut a slot through the circuit board that matches the relatively small thickness of the metal shield.

The metal shield 104 is made from a conductive metal, such as tin plated steel, which is grounded to the housing of the filter. The circuit board 100 is made from an electrically insulating material, an example of which is glass-epoxy composite. The conductive trace 200 is made from a conductive material, an example of which is solder-covered copper.

FIG. 3B , read in connection with the corresponding view shown in FIG. 5 , is a cross sectional view of the metal shield of a filter in accordance with another embodiment of the present invention. The metal shield 104 (or 105 as shown in FIG. 1 ) includes a slot 206 . Slot 206 includes a first section 206 A, providing intimate contact with a portion of bottom surface 100 b of circuit board 100 and a portion of top surface 100 a of circuit board 100 .

Slot 206 also includes a second section 206 B defining a space between top surface 100 a of circuit board 100 and shield 104 . Slot 206 further includes a third section 206 C defining a space between bottom surface 100 b of circuit board 100 and shield 104 .

A conductive trace 200 is located on top surface 100 a of the circuit board 100 within the second section 206 B. A space of dimension d 2 is formed between the surface of the conductive trace 200 and slot 206 in the region of 206 B. An electrically conductive plated through-hole 201 passes from the top of conductive trace 200 , through circuit board 100 , and through a conductive contact pad 202 located on bottom surface 100 b of circuit board 100 . A space having the dimension d 3 is formed between the surface of conductive contact pad 202 and slot 206 in the region of 206 C.

The dimensions of the spaces d 2 and d 3 relate to the dielectric constant of the material, or air, located within the respective spaces. If a voltage surge passes through the filter along conductive trace 200 , a spark is generated within the space, and the current is shunted to ground via the grounded metal shield 104 . The through-hole 201 is plated with a conductive material, an example of which is copper.

It is believed that a space having the dimension d 2 (or d 3 ) of about 0.013 inches will spark at a voltage surge of about 1000 volts passing through the conductive trace. Accordingly, d 2 (or d 3 ) should be selected within a range of about 0.010 to 0.020 inches to provide a 1000-2000V surge protection rating.

FIG. 3B also shows that metal shield 104 preferably includes a raised boss member 110 , which increases the effective thickness of the metal shield to more closely match the width of the shield-receiving slot 101 ( FIG. 1 ) cut through the circuit board 100 .

Although not shown in the present drawings, the interior of the housing can be filled with a potting compound, such as polyurethane foam.