Heat detector having an increased accuracy alarm temperature threshold and improved low temperature testing capabilities

• US 6,288,638 B1
• Filed: 04/19/2000
• Issued: 09/11/2001
• Est. Priority Date: 05/06/1999
• Status: Expired due to Term

• Current Assignee: Maple Chase Company (Schneider Electric SE)
• Original Assignee: Ranco Incorporated of Delaware (Schneider Electric SE)
• Inventors: Tanguay, William P., Corsello, Keith
• Primary Examiner(s): Pope, Daryl C.
• Application Number:
  US09/552,380
• Time in Patent Office: 510 Days
• Field of Search: 340/511, 340/514, 340/515, 340/577, 340/578, 340/584, 340/587
• US Class Current: 340/514
• CPC Class Codes: G01K 15/00 Testing or calibrating of thermometers
• CPC Class Codes: G01K 7/20 in a specially-adapted circuit, e.g. bridge circuit
• CPC Class Codes: G08B 17/06 Electric actuation of the alarm, e.g. using a thermally-operated switch
• CPC Class Codes: G08B 21/20 responsive to moisture
• CPC Class Codes: G08B 29/24 Self-calibration, e.g. compensating for environmental drift or ageing of components

Abstract

A heat detector that includes a temperature sensing and testing circuit that provides an increased accuracy temperature sensing range and a push-to-test circuit that is electronically functional at low ambient temperatures. The heat detector includes a sensing circuit consisting of a voltage divider formed from a supply resistor and a thermistor. The thermistor is connected directly between a detector voltage input to a control unit and ground. The push-to-test circuit includes a parallel combination of a test resistor and a transistor each connected between the supply resistor and a voltage source. During normal operating conditions, base current is supplied to the transistor such that the transistor functions as a short circuit connection between the supply resistor and the voltage source. Upon depression of a push button, the base of the transistor is connected to the emitter such that the transistor acts as an open circuit. When the push button is depressed, a test resistor is connected in series with the supply resistor to simulate an increase in the ambient temperature to test the heat detector.

Claims

1. A heat detector comprising:

a control unit for comparing a detector voltage to a reference voltage and generating an alarm signal based on the magnitude of the detector voltage relative to the magnitude of the reference voltage, the control unit including a detector input terminal that receives the detector voltage;

a temperature sensing component coupled to the detector input terminal, the resistance of the temperature sensing component being responsive to changes in temperature;

a supply resistance coupled to the detector input terminal to form an upper leg of a voltage divider, a bottom leg of the voltage divider including the temperature sensing component, wherein the voltage divider generates the detector voltage and the detector voltage varies based upon the resistance of the temperature sensing component; and

a testing circuit positioned in the upper leg of the voltage divider between the supply resistance and a voltage source, the testing circuit being operable to modify the detector voltage to simulate a change in temperature to test the heat detector.

2. The heat detector of claim 1 wherein the testing circuit comprises:

a transistor connected between the supply resistance and the voltage source, wherein the transistor is saturated during normal operating conditions to form an essentially short circuit between the supply resistance and the voltage source;

a test resistance connected in parallel with the transistor between the supply resistance and the voltage source; and

a push button depressible to simulate a change in temperature to test the heat detector, wherein when the push button is depressed, the transistor is turned off and forms an essentially open circuit between the supply resistance and the voltage source such that the upper leg of the voltage divider includes the series combination of the test resistance and the supply resistance.

3. The heat detector of claim 2 further comprising a bias resistance connected between the transistor and ground, wherein the bias resistance saturates the transistor during normal operating conditions.

4. The heat detector of claim 3 wherein the push button is connected between the base and the emitter of the transistor such that when the push button is depressed, the base and emitter of the transistor are connected such that the transistor is turned off.

5. The heat detector of claim 1 wherein the temperature sensing component is a negative temperature coefficient thermistor such that as the temperature increases, the resistance of the thermistor and the detector voltage decrease.

6. The heat detector of claim 5 wherein the thermistor is connected directly to ground and directly to the supply resistance.

7. The heat detector of claim 5 wherein the reference voltage is approximately 50% of the voltage source.

8. The heat detector of claim 1 wherein the temperature sensing component is a thermistor coated with a layer of insulation.

9. The heat detector of claim 7 wherein the layer of insulation is heat shrunk tubing formed from Kynar™

.

10. The heat detector of claim 2 wherein the value of the test resistance is selected such that when the push button is depressed, the upper leg of the voltage divider including the series combination of the test resistance and the supply resistance is greater than the resistance of the lower leg of the voltage divider including the temperature sensing component when the temperature is above a preselected lower limit.

11. The heat detector of claim 10 wherein the preselected lower limit is approximately −

20°

F. (−

29°

C.).

12. In a heat detector having a control unit for comparing a reference voltage to a temperature detector voltage generated by a voltage divider formed by an upper leg including a supply resistance and a lower leg including a temperature sensing component whose resistance is responsive to temperature changes, the improvement comprising:

a testing circuit positioned in the upper leg of the voltage divider between the supply resistance and a voltage source, the testing circuit being operable to simulate a change in temperature, the testing circuit comprising;

a transistor connected between the supply resistance and the voltage source, wherein the transistor is saturated during normal operating conditions to form an essentially short circuit between the supply resistance and the voltage source;

a test resistance connected in parallel with the transistor between the supply resistance and the voltage source; and

a push button depressible to simulate a change in temperature, wherein when the push button is depressed, the transistor is turned off and forms an essentially open circuit between the supply resistance and the voltage source such that the upper leg of the voltage divider includes the series combination of the test resistance and the supply resistance.

13. The improvement of claim 12 further comprising a bias resistance connected between the transistor and ground to saturate the transistor during normal operating conditions.

14. The improvement of claim 13 wherein the push button is connected between the base and the emitter of the transistor such that when the push button is depressed, the base and emitter of the transistor are connected such that the transistor is turned off.

15. The improvement of claim 12 wherein the value of the test resistance is selected such that when the push button is depressed, the resistance of the upper leg of the voltage divider including the series combination of the test resistance and the supply resistance is greater than the resistance of the lower leg of the voltage divider including the temperature sensing component when the temperature is above a preselected lower limit.

16. The improvement of claim 15 wherein the preselected lower limit is approximately −

20°

F. (−

29°

C.).

17. A heat detector comprising:

a control unit for comparing a reference voltage to a detector voltage generated by a voltage divider including an upper leg and a lower leg, the control unit generating an alarm signal based on the magnitude of the reference voltage relative to the magnitude of the detector voltage, the control unit including a detector input that receives the detector voltage;

a thermistor coupled to the detector input to form the lower leg of the voltage divider, the resistance of the thermistor being responsive to changes in temperature;

a supply resistance coupled to the detector input terminal to form a portion of the upper leg of the voltage divider;

a transistor connected between the supply resistance and a voltage source, wherein the transistor is saturated during normal operating conditions to form an essentially short circuit between the supply resistor and the voltage source such that the upper leg of the voltage divider includes only the supply resistance;

a test resistance connected in parallel with the transistor between the supply resistance and the voltage source; and

a push button depressible to simulate a change in temperature, wherein when the push button is depressed, the transistor is turned off and forms an essentially open circuit between the supply resistance and the voltage source such that the upper leg of the voltage divider includes the series combination of the test resistance and the supply resistance.

18. The heat detector of claim 17 wherein the thermistor is a negative temperature coefficient device such that as the temperature in the desired location increases, the resistance of the thermistor decreases such that the detector voltage also decreases.

19. The heat detector of claim 17 wherein the thermistor is coated with a layer of insulation.

20. The heat detector of claim 19 wherein the layer of insulation is heat shrink tubing formed from Kynar™

.

21. The heat detector of claim 17 wherein the value of the test resistance is selected such that when the push button is depressed, the resistance of the upper leg of the voltage divider including the series combination of the test resistance and the supply resistance is greater than the resistance of the lower leg of the voltage divider including the thermistor when the temperature is above a preselected lower limit.

22. The heat detector of claim 21 wherein the preselected lower limit is approximately −

20°

F. (−

29°

C.).

23. The heat detector of claim 17 further comprising a bias resistance connected between the base of the transistor and ground to saturate the transistor during normal operating conditions.

24. The heat detector of claim 23 wherein the push button is connected between the base and the emitter of the transistor such that when the push button is depressed, the base and emitter of the transistor are connected to turn off the transistor.