Method and apparatus for protecting monitor circuit from fault condition

• US 20030202310A1
• Filed: 04/29/2002
• Published: 10/30/2003
• Est. Priority Date: 04/29/2002
• Status: Active Grant

• Current Assignee: Won-Door Corporation
• Original Assignee: Won-Door Corporation
• Inventors: Goodman, E. Carl, Field, D. George, Banta, Kevin D.
• Granted Patent: US 7,050,283 B2
• Time in Patent Office: Days
• US Class Current: 361/106
• CPC Class Codes: H02H 9/026 Current limitation using PTC resistors, i.e. resistors with a large positive temperature coefficient

Abstract

A method and apparatus for improving the fault protection of a monitor circuit is provided by coupling an input protection circuit to an output section. The input protection circuit may include a fusible device that limits or removes a fault condition present at an input to the input protection circuit. The fusible device may be, for example, a resettable positive temperature coefficient (“PTC”) device configured to limit the current passing through it to a predetermined level once it reaches a predetermined temperature. A resistive element may be thermally coupled to the PTC device to assist it reaching the predetermined temperature. The monitor circuit may further be configured to generate a sensory signal in response to a fault condition.

Claims

1. A monitor circuit comprising:

an input protection circuit comprising;

a first input;

a second input coupled to ground, and

a fault interrupt device coupled between the first input and a control node, the fault interrupt device configured to protect the monitor circuit from a fault condition on the first input; and

an isolated output section coupled to the input protection section via the control node, the isolated output section configured to produce an output signal upon the occurrence of a triggering event at the first input, wherein, the monitor circuit is configured to produce the output signal even while in the fault condition.

2. The monitor circuit of claim 1, wherein the fault interrupt device is further coupled to the second input.

3. The monitor circuit of claim 1, wherein the fault interrupt device is further configured to limit the current at the control node to a predefined level.

4. The monitor circuit of claim 1, wherein the fault interrupt device is further configured to limit the voltage at the control node to a predefined level.

5. The monitor circuit of claim 1, wherein the monitor circuit is configured to generate a sensory signal in response to the presence of the fault condition on the first input.

6. The monitor circuit of claim 5, wherein the sensory signal comprises a pulsed output signal.

7. The monitor circuit of claim 6, wherein the fault interrupt device is further configured to trip and reset at a predetermined rate to produce the pulsed output signal.

8. The monitor circuit of claim 7, wherein the predetermined rate is determined by the periodic oscillation rate of the fault condition on the first input.

9. The monitor circuit of claim 1, wherein the fault interrupt device comprises a fusible element.

10. The monitor circuit of claim 9, wherein the fusible element comprises a positive temperature coefficient (“

PTC”

) device configured to set the current passing through it to a predetermined limit upon reaching a defined temperature.

11. The monitor circuit of claim 10, wherein the fault interrupt device further comprises a first diode and a resistive element coupled in series with the PTC device, the diode configured to isolate the PTC device from the fault condition during a first half cycle of the fault condition.

12. The monitor circuit of claim 11, wherein the fault interrupt device further comprises a second diode coupled between the control node and the second input, the second diode configured to limit the voltage at the control node during a second half cycle of the fault condition.

13. The monitor circuit of claim 10, wherein the fault interrupt device further comprises a resistive element coupled in series with the PTC device, wherein the resistive element is also thermally coupled to the PTC device.

14. The monitor circuit of claim 13, wherein the fault interrupt device further comprises a breakdown diode coupled between the control node and the second input, the diode configured to limit the voltage at the control node at a first level during a first half cycle of the fault condition and at a second level during a second half cycle of the fault condition.

15. The monitor circuit of claim 1, wherein the event trigger includes the completion of the external dry connection between the first input and the second input.

16. The monitor circuit of claim 1, wherein the event trigger is in response to the detection of a fire.

17. An alarm system comprising a controller, the controller comprising:

a processor;

a memory device; and

a monitor circuit comprising;

an input protection circuit having a first input, a second input coupled to ground, and a fault interrupt device coupled between the first input and a control node, the fault interrupt device configured to protect the monitor circuit from a fault condition on the first input; and

an isolated output section coupled to the input protection section via the control node, the isolated output section configured to produce an output signal upon the occurrence of a triggering event at the first input, wherein, the monitor circuit is configured to produce the output signal even while in the fault condition.

18. The monitor circuit of claim 17, wherein the monitor circuit is configured to generate a sensory signal in response to the presence of the fault condition on the first input.

19. The monitor circuit of claim 18, wherein the sensory signal comprises a pulsed output signal.

20. The monitor circuit of claim 19, wherein the fault interrupt device is further configured to trip and reset at a predetermined rate to produce the pulsed output signal.

21. The monitor circuit of claim 20, wherein the predetermined rate is determined by the periodic oscillation rate of the fault condition on the first input.

22. The monitor circuit of claim 17, wherein the fault interrupt device comprises a fusible element.

23. The monitor circuit of claim 22, wherein the fusible element comprises a positive temperature coefficient (“

PTC”

) device configured to set the current passing through it to a predetermined limit upon reaching a defined temperature.

24. The monitor circuit of claim 23, wherein the fault interrupt device further comprises a first diode and a resistive element coupled in series with the PTC device, the diode configured to isolate the PTC device from the fault condition during a first half cycle of the fault condition.

25. The monitor circuit of claim 24, wherein the fault interrupt device further comprises a second diode coupled between the control node and the second input, the second diode configured to limit the voltage at the control node during a second half cycle of the fault condition.

26. The monitor circuit of claim 24, wherein the fault interrupt device further comprises a resistive element coupled in series with the PTC device, wherein the resistive element is also thermally coupled to the PTC device.

27. The monitor circuit of claim 26, wherein the fault interrupt device further comprises a breakdown diode coupled between the control node and the second input, the diode configured to limit the voltage at the control node at a first level during a first half cycle of the fault condition and at a second level during a second half cycle of the fault condition.

28. A method of fault protecting a monitor circuit having an input protection circuit coupled to an output section at a control node, the method comprising:

during a first half cycle of a fault condition present on a first input of the input protection circuit, blocking a voltage at the first input; and

during a second half cycle of the fault condition, limiting a current between the first input and the control node to a predetermined current level and setting the voltage at the control node to a predetermined voltage level.

29. The method according to claim 28, wherein blocking the voltage at the first input comprises blocking the voltage with a diode.

30. The method according to claim 28, wherein limiting the current comprises tripping a positive temperature coefficient (“

PTC”

) device.

31. The method according to claim 28, wherein setting the voltage at the control node comprises passing a ground potential from a second input of the input protection circuit to the control node through a diode.

32. The method according to claim 28, further comprising generating a sensory signal in response to the presence of the fault condition on the first input.

33. The method according to claim 32, wherein generating the sensory signal comprises setting an output to a first voltage level during the first half cycle of the fault condition and setting the output to a second voltage level during the second half cycle of the fault condition.

34. A method of fault protecting a monitor circuit having an input protection circuit coupled to an output section at a control node, the method comprising:

during a first half cycle of a fault condition present on a first input of the input protection circuit, setting a voltage at the control node to a first predetermined voltage level; and

during a second half cycle of the fault condition, setting the voltage at the control node to a second predetermined voltage level and limiting a current between the first input and the control node to a predetermined current level.

35. The method according to claim 34, wherein setting the voltage at the control node to the first predetermined voltage level comprises operating a breakdown diode coupled between the control node and a second input of the input protection circuit in the breakdown region.

36. The method according to claim 35, wherein setting the voltage at the control node to a second predetermined voltage level comprises passing a ground potential from the second input to the control node through the breakdown diode.

37. The method according to claim 34, wherein limiting the current comprises tripping a positive temperature coefficient (“

PTC”

) device.

38. The method according to claim 37, wherein tripping the PTC device comprises transferring thermal energy to it from a resistive element.

39. The method according to claim 36, further comprising generating a sensory signal in response to the presence of the fault condition on the first input.

40. The method according to claim 39, wherein generating the sensory signal comprises setting an output to a first voltage level during the first half cycle of the fault condition and setting the output to a second voltage level during the second half cycle of the fault condition.