GB2314702A

GB2314702A - Detecting overheating of printed circuit boards

Info

Publication number: GB2314702A
Application number: GB9613667A
Authority: GB
Inventors: Andrew Hutchinson; Thomas Paul Groves
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1996-06-28
Filing date: 1996-06-28
Publication date: 1998-01-07
Also published as: GB9613667D0

Abstract

A printed circuit board includes an internal printed circuit planar pattern 23 of electrical conductors forming a continuous track. A test voltage 24 is applied to one end of the track and the voltage at the other end 25 of the track is detected by a window comparator 26. If the detected voltage lies outside preset limits, an alarm signal is generated to shut down or reduce the power supplied to the board.

Description

Method and Apparatus for Detecting Overheating of Printed Circuit Boards.

Field of the Invention The present invention relates to a method and apparatus for detecting damage through overheating of one or more printed circuit boards which constitute an electrical load receiving power from an electrical power supply.

Background of the Invention Conventional over-current devices used to protect electrical printed circuit boards from a current overload are ineffective to prevent damage because the energy required to damage or destroy a printed circuit board may be as low as a few tens of watts. The trip characteristics of the overcurrent device are such as to be insensitive to this low level of overload. Such conventional devices are also ineffective in protecting current carrying conductors in electrical distribution systems in the presence of a loose connection. In these cases localised overheating occurs due to arcing at the point of the poor connection, but the overcurrent device cannot protect against this condition because the mean current being drawn may not exceed the rated current under normal conditions.

A heat detector may not be effective in equipment that has forced air cooling or a large number of components because the detector has to be in close thermal contact with the source of the heat. This would require the use of a large number of detector devices distributed throughout the equipment to guarantee satisfactory detection of overheating at a large number of sites. Thermal detector devices are also difficult to specify if they are expected to detect temperatures over a wide temperature range of operation, e.g.

-300C to +550C. A device that gives reliable indications at high ambient temperatures may not detect a problem in time at a low ambient temperature. A heat detecting device sized for the early detection of overheating at low ambient temperatures may operate falsely at high ambient temperatures. Furthermore, a heat detector would nor be practical for detecting loose connections because a separate device would be required at every connection point.

Smoke detectors can be used to detect the smoke caused by overheating at a loose connection, but they have many disadvantages associated with them. They require routine maintenance (annual cleaning), they are of large size relative to the circuit boards to be protected, they have a limited operating temperature range, they can provide false alarms through detecting dust particles, and it is difficult to position them effectively in a forced air environment.

Finally it may be necessary to install more than one detector.

Summary of the Invention The present invention seeks to provide an improved method and apparatus to detect the presence of overheating in a printed circuit board constituting an electrical load receiving power from an electrical power supply.

According to the present invention there is provided a method of detecting overheating of an electrical load comprising a printed circuit board receiving power from an electrical power supply, the method comprising; monitoring the integrity of an electrically conductive sense track linking a planar pattern of conductors in the circuit board and, generating an alarm signal representing the onset of overheating when the integrity of the sense track is broken.

Further according to the present invention there is provided apparatus to detect overheating of an electrical load comprising a printed circuit board connected to receive power from an electrical power supply, the apparatus comprising; an electrically conductive sense track linking a planar pattern of conductors in the circuit board, sensing means to monitor the electrical integrity of the sense track, and alarm signalling means to generate an alarm signal representing overheating when the sensing means senses a break in the integrity of the sense track.

Brief Description of the Drawings The invention will now be described, by way of example, with reference to the accompanying drawings in which; FIG. 1 is an illustration of an apparatus embodying the invention to detect the overheating of a printed circuit board load.

FIG.2A and FIG.2B are illustrations of the physical construction of a circuit board including a sense track which is incorporated into the load in the apparatus of FIG. 1.

FIG. 3 is an illustration in flow diagram form of an operation of the apparatus of FIG.1, FIG. 2A and FIG. 2B.

Description of a Preferred Embodiment In FIG. 1, a source of electrical power 20 is connected to supply an electrical load 21. The load 21 is constituted by a number of printed circuit boards although it is to be understood that the invention is applicable to a load which consists of only one such board. Monitoring means 22 monitor the integrity of a sense track included in the construction of each of the circuit boards.

As shown more particularly in FIG. 2A, the circuit boards are each constructed to include an internal printed circuit planar pattern 23 of electrical conductors that is used exclusively to detect overheating of the circuit board. The pattern of conductors constitute a grid that is concentrated over the areas of the circuit board that are deemed to be most at risk from overheating, such as high current power planes. The conductors are linked so as to form a continuous sense track which at one end receives a test or trace voltage from a supply 24. The voltage supplied by the supply 24 is set at a level that is different from any other power supply and d.c. signal levels present during operation of the printed circuit board. The end of the sense track remote from the supply 24 is connected to an input 25 of a sensing means 26 in the form of a window comparator which compares the voltage supplied by the sense track to an auxiliary voltage from an auxiliary supply 27. In the event that the voltage from the sense track deviates outside the limits set by the window comparator, an alarm signal is generated on the alarm terminal 28. As an alternative, the sensing means 26 could be in the form of a current detector used to monitor deviations from a constant current supplied by the sense track and operable to generate an alarm signal if the current deviates outside preset limits.

Referring now to FIG. 2B, the alarm signal from the sensing means 26 is used to operate a switch 29, either in electromechanical or solid state form, which connects power to the circuit board. In the event of overheating to the point where the board decomposes, the sense track will become broken or shorted to a power, ground or signal track to de-activate the power switch 29 so as to remove power from the faulty circuit board or reduce the power to a safe level. The integrity of the sense track will therefore be evidenced either by a break in the continuity of the track resulting in no signal voltage or current being supplied by the sense track or by the supply of a voltage or current which deviates outside the preset limits because of a short.

Referring now to FIG. 3, the power supply is powered up as represented by the block 30. The sensing means 26 senses the trace voltage to determine if the voltage is zero representing a break in the continuity of the sense track or a short to ground as represented by the block 31. The sensing means also tests whether the trace voltage is at a preset voltage v representing a short to a power track as represented by the block 32 and whether the trace voltage is at a signal level representing a short to a signal line, as represented by the block 33. In the event that the sensing means detects the condition represented by any one of the blocks 31, 32 or 33, an alarm signal is generated as represented by the block 34 to effect shut down of the circuit board power.

Claims

1. A method of detecting damage through overheating of an electrical load comprising a printed circuit board receiving power from an electrical power supply, the method comprising; monitoring the integrity of an electrically conductive sense track linking a planar pattern of conductors in the circuit board and, generating an alarm signal representing the onset of overheating when the integrity of the sense track is broken.

2. A method as claimed in claim 1, wherein the monitoring step is effective to sense a break in the continuity of the sense track.

3. A method as claimed in claim 1 or 2, wherein the monitoring step is effective to sense a short in the sense track.

4. A method as claimed in claim 1, 2 or 3, wherein the monitoring step is effected by means of a window comparator to sense deviations in a trace voltage supplied through the sense track to the comparator.

5. A method as claimed in any one of the preceding claims, wherein the alarm signal is used to switch off the power supply.

6. A method as claimed in any one of claims 1 to 4, wherein the alarm signal is used to reduce the supply of current from the power supply.

7. Apparatus to detect damage through overheating of an electrical load comprising a printed circuit board connected to receive power from an electrical power supply, the apparatus comprising; an electrically conductive sense track linking a planar pattern of conductors in the circuit board, sensing means to monitor the electrical integrity of the sense track and, alarm signalling means to generate an alarm signal representing overheating when the sensing means senses a break in the integrity of the sense track.

8. Apparatus as claimed in claim 7, wherein the sensing means is adapted to sense a break in the continuity of the sense track.

9. Apparatus as claimed in claim 7 or 8, wherein the sensing means is adapted to sense a short in the sense track.

10. Apparatus as claimed in claim 7, 8 or 9, wherein the sensing means comprises a window comparator to sense deviations in a trace voltage supplied through the sense track to the comparator.

11. Apparatus as claimed in claim 7, 8, 9 or 10 wherein the power supply has means responsive to the alarm signal to switch off the power supply.

12. Apparatus as claimed in claim 7, 8, 9 or 10 wherein the power supply has means responsive to the alarm signal to reduce the current supplied by the power supply.