BI-POLAR TRIAC SHORT DETECTION AND SAFETY CIRCUIT

• US 20160037582A1
• Filed: 03/14/2014
• Published: 02/04/2016
• Est. Priority Date: 03/14/2013
• Status: Active Grant

• Current Assignee: Multitech Medical Devices USA LLC
• Original Assignee: Weiss Controls, Inc.
• Inventors: NOVIKOV, Lenny, WEISS, John William
• Granted Patent: US 10,536,991 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01R 15/18 using inductive devices, e.g. transformers
• CPC Class Codes: G01R 31/42 AC power supplies
• CPC Class Codes: H02H 3/023 by short-circuiting
• CPC Class Codes: H02H 3/08 responsive to excess current responsive to abnormal temperature caused by excess current H02H5/04
• CPC Class Codes: H05B 1/0202 Switches
• CPC Class Codes: H05B 1/0227 Applications
• CPC Class Codes: H05B 1/0252 Domestic applications
• CPC Class Codes: H05B 1/0272 For heating of fabrics
• CPC Class Codes: H05B 3/0019 Circuit arrangements

Abstract

A safety circuit to disrupt power to a heating element of an appliance to be powered through a solid state switch (Tri-ac Q1), typically a triac, from an AC power source (V1) having a positive half cycle and a negative half cycle delivering power. A low resistance condition is sensed by detecting either the current through or voltage across the solid state switch during the positive half cycle and the negative half cycle of the AC power line, when the solid state AC switch is not actuated. A fault signal is generated to interrupt power to the heating element, preferably by a crowbar circuit opening a fuse, whenever the low resistance condition is detected.

Claims

1. A safety circuit for use in disrupting power to a heating element of an appliance to be powered through a solid state switch from an AC power source having a positive half cycle and a negative half cycle delivering power, including detection circuitry for detecting a low resistance condition in said solid state switch during the positive half cycle of the AC power through said solid state switch and detecting a low resistance condition in said solid state switch during the negative half cycle of the AC power through said solid state switch to generate a fault signal to interrupt power to said heating element whenever said low resistance condition is detected while said solid state switch is not activated.

2. A safety circuit as set forth in claim 1, said detection circuitry comprising an NPN transistor switch responsive to the positive voltage drop across said solid state switch and a PNP transistor switch responsive to the negative voltage drop across said solid state switch, said PNP transistor switch connected to the base of said NPN transistor switch to actuate said NPN transistor in response to either positive or negative voltage drop across said solid state switch,

said detection circuitry providing a high logical level output to a microcontroller whenever said solid state switch is conducting, said microcontroller being arranged to generate said fault signal to interrupt power to said heating element if said high logical level is received while said solid state switch is not actuated.

3. A safety circuit as set forth in claim 2, said solid state switch being a triac.

4. A safety circuit as set forth in claim 2, said microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.

5. A safety circuit as set forth in claim 1, said detection circuitry detecting the level of current through solid state switch during either half cycle of the AC power to generate said fault signal to interrupt power to said heating element when said current through said solid state switch reaches a predetermined level.

6. A safety circuit as set forth in claim 5, said solid state switch being a triac.

7. A safety circuit as set forth in claim 5, including a microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.

8. A safety circuit as set forth in claim 5, said detection circuit including a current transformer.

9. A safety circuit as set forth in claim 8, said solid state switch being a triac.

10. A safety circuit as set forth in claim 8, including a microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.

11. A safety circuit as set forth in claim 5, said detection circuitry including an optocoupler connected across a voltage divider for detecting the level of current through solid state switch during either half cycle of the AC power to generate a fault signal to interrupt power to said heating element when said current through said solid state switch reaches a predetermined level.

12. A safety circuit as set forth in claim 11, said solid state switch being a triac.

13. A safety circuit as set forth in claim 11, including a microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.

14. A safety circuit as set forth in claim 5, said detection circuitry including two NPN transistors connected to a load resistor in series with the solid state switch for detecting the level of current through solid state switch during either half cycle of the AC power to generate said fault signal to interrupt power to said heating element when said current through said solid state switch reaches a predetermined level.

15. A safety circuit as set forth in claim 14, said solid state switch being a triac.

16. A safety circuit as set forth in claim 14, including a microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.

17. A safety circuit as set forth in claim 5, said detection circuitry including inversely parallel diodes connected in series with said solid state switch for detecting the level of current through solid state switch during either half cycle of the AC power to generate said fault signal to interrupt power to said heating element when said current through said solid state switch reaches a predetermined level.

18. A safety circuit as set forth in claim 17, said solid state switch being a triac.

19. A safety circuit as set forth in claim 17, including a microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.

20. A safety circuit as set forth in claim 1, said detection circuitry including a voltage divider connected across said solid state switch through a high resistance to generate an analog signal detecting the voltage level of solid state switch during either half cycle of the AC power to generate said fault signal to interrupt power to said heating element when said voltage falls to a predetermined level, and a converter for said analog signal to deliver a signal to a microcontroller unit to generate said fault signal.

21. A safety circuit as set forth in claim 20, said solid state switch being a triac.

22. A safety circuit as set forth in claim 20, said microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.

23. A safety circuit as set forth in claim 1, said detection circuitry including a current divider connected to said solid state switch to generate an analog signal detecting the current level of solid state switch during either half cycle of the AC power to generate said fault signal to interrupt power to said heating element when said current reaches a predetermined level, and a converter for said analog signal to deliver a signal to a microcontroller unit to generate said fault signal.

24. A safety circuit as set forth in claim 23, said solid state switch being a triac.

25. A safety circuit as set forth in claim 23, said microcontroller unit receiving said fault signal to activate a crowbar circuit to open a fuse.

26. A safety circuit for use in disrupting power to a heating element of an appliance to be powered through a solid state switch from an AC power source having a positive half cycle and a negative half cycle delivering power, said solid state switch being triggered to supply power to said heating element during only a predetermined number of cycles of said AC power source, and a circuit interrupter in series with said heating element and solid state switch to disrupt power to said heating element when a larger current is established indicating said solid state switch is passing more than said predetermined number of cycles to said heating element.

27. A safety circuit as set forth in claim 26, said circuit interrupter being a fuse.

28. A safety circuit for use in disrupting power to dual heating elements each formed of a positive temperature coefficient flexible wire and powered through a solid state switch by a respective cycle of the AC power source having a positive half cycle and a negative half cycle delivering power, including circuitry sensing the voltage across said solid state switch through each cycle of the AC power source, and a micrcontroller comparing said voltage to a preset value to determine if either of said heating elements reaches a predetermined level to generate a fault signal to interrupt power to the heating element having reached the predetermined level.

29. A method for controlling temperature of an AC operated multiple circuit heating element, whereas at least one circuit is operated on a positive half cycle of the AC power line, and at least one circuit is operated on a negative half cycle of the power line, the circuits being operatively connected to the power line by at least one solid state AC switch, having a control terminal capable of turning said AC switch on for a duration of a half cycle of an AC power line, comprising:

inserting an impedance in series with the said at least one AC switch,

measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a positive half cycle of the AC line, while said AC switch is not actuated, and registering the received value as a positive bias voltage,

measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a negative half cycle of the AC line, while said AC switch is not actuated, and registering the received value as a negative bias voltage,

measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a positive half cycle of the AC line, while said AC switch is actuated, and registering the received value as a positive active voltage,

measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a negative half cycle of the AC line, while said AC switch is actuated, and registering the received value as a negative active voltage,

measuring AC line power voltage magnitude, and registering the received value as a line voltage,

calculating resistance of said at least one circuit of said heating element operated on a positive half cycle of the AC power line by taking a ratio of the line voltage to a result of subtraction of said positive bias voltage from said positive active voltage,

calculating resistance of said at least one circuit of said heating element operated on a negative half cycle of the AC power line by taking a ratio of the line voltage to a result of subtraction of said positive active voltage from said positive bias voltage,

supplying power to said at least one circuit of said heating element operated on a positive half cycle of the AC line if the calculated resistance value of said circuit is below a predefined value,

and

supplying power to said at least one circuit of said heating element operated on a negative half cycle of the AC line if the calculated resistance value of said circuit is below a predefined value.

30. A method for protecting of an AC operated appliance, whereas a load is operatively connected to the power line by at least one solid state AC switch having a control terminal capable of turning said AC switch on for a duration of a half cycle of an AC power line, said appliance further having a disconnect means for operatively disrupting power to the load, comprising the following:

inserting an impedance in series with the said at least one AC switch,

measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a positive half cycle of the AC line, while said AC switch is not actuated, and registering the received value as a positive bias voltage,

measuring a voltage drop across said impedance proportional to the magnitude of current passing through said AC switch during a negative half cycle of the AC line, while said AC switch is not actuated, and registering the received value as a negative bias voltage,

actuating said disconnect means when an absolute value of either the difference between said positive bias voltage and a preset positive bias value, or the difference between said negative bias voltage and a preset negative bias value, exceeds a predefined value.

31. A method as set forth in claim 30, whereas said preset positive bias value and said preset negative bias value are substantially equal.