Safety circuits for electric heating element

A system controller such as a microprocessor receives voltage signals produced from a resistor identifying the functional or nonfunctional operating state of a power switch which controls the amount of power provided to a load in an electrical system. The resistor is also used to provide voltage signals representing the temperature of a heating element of the type used in heating pads and heating blankets. The functional or nonfunctional operating state of a safety switch in a safety circuit capable of blowing a fuse and cutting power to the load is periodically tested to ensure the safety of the electrical system.

BACKGROUND AND SUMMARY

This disclosure describes circuits and associated software algorithms that help prevent safety-related thermal events such as overheating in electrically-heated products and particularly in heated pads and heated blankets.

Electronically-controlled products such as heating pads, heating wraps, heating blankets and the like that use household electricity as the source of power utilize an electronic power switch or “main power switch” that applies electric power to loads posing potential safety issues. Such loads include heating elements such as heating wires provided within heating pads and heating blankets.

In the case of electrically-heated products, the power is provided on an as needed basis and as directed by a microprocessor to maintain desired temperatures selected by a user. The main power switch turns on and off at varying intervals to produce and maintain a temperature level selected by a user. In practical terms, an electronic power switch as described herein can take the form of, for example, a triac or SCR.

To protect a user from thermal burns, it is critical that the main electronic power switch that controls the temperature of a heating product turns OFF when required in order to maintain a selected temperature and to prevent a potential thermal overheat condition. This is particularly important in those heating products which apply relatively high wattage to a heating element to quickly bring the heating product up to a temperature selected by a user.

To prevent an overheat condition or “thermal runaway”, a disabling circuit is provided to disconnect the electronic controls and heating elements from their power source in order to stop all heating when it is determined that the main electronic power switch fails to shut OFF when commanded to do so by a processor control system. This test of the disabling circuit can be carried out first as there is little point to testing the electronic main power switch if it is determined that it has failed but power to the heating element can't be terminated by the disabling circuit.

As described below, improvements over existing heating control systems include:

1. The virtual prevention of a false positive indication of failures of heating control circuits. Multiple fault indications during each test period must be present in order to prevent “false positive” results by any one of two tracked fault conditions. This eliminates the possibility that a perfectly good heating product will be destroyed or forced to discontinue operations unnecessarily.

2. The introduction of dwell time. A dwell time is introduced prior to the initiation of tests in order to allow the power lines to stabilize and for electrical transients to decay, thereby creating a more reliable background for the tests and associated results.

3. A test to determine that a disabling circuit is functional. A disabling circuit is provided to render a heated product such as a heated textile inoperable by blowing a fuse in the event that the main electronic switch, which controls power to a heating element, does not turn OFF when commanded. To ensure that the disabling circuit is capable of blowing the fuse, a test circuit and an associated test are provided in this disclosure to verify the proper operation of the disabling circuit.

Tests for two (2) fault conditions are integrated into a safety test period that is repeated throughout the operation of an electric powered product such as a heating product. These tests are listed as follows:

1. Test the disabling circuit to validate that it operates properly.

2. Test and determine whether the main power switch can turn OFF.

As noted above, an electronic power switch or “main power switch” as described herein can take the form of, for example, a triac or SCR, but other types of switches can be used.

DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

An example of a safety system constructed in accordance with this disclosure is shown inFIG. 1. This system can be used with any type of electrical load such as electrically-powered products which generate heat. In the example ofFIG. 1, the safety system outlined above is applied to a heating system such as the resistive heating system10. A power source, such as household alternating current is provided through power lines12and14to power the heating system10.

A power supply18provides direct current to a processor such as microprocessor20. Microprocessor20controls the operation of the heating system10, including the operation and testing of the safety circuits described below.

AC power is applied to a resistive load such as a resistive heating element24to provide heat to a heating product such as a heating pad26. The heating element24can take the form of a resistive heating wire R1. A user selects a desired level of heat by operating a heat selector switch30. A series of LEDs34lights up sequentially to indicate the level of heat selected.

The level of heat produced by the heating element24is controlled by the microprocessor20operating a main power switch36. In this example, the main power switch36is shown as a triac T1. Under normal operating conditions, the microprocessor20instructs a switch activation circuit40to open and close the main power switch36(triac T1) according to programmed duty cycles corresponding to heating levels selected by a user.

The microprocessor20also receives a signal44produced between the main power switch36and ground representing the operating temperature of the heating element24. A voltage drop across a sensor such as resistor48(R2) varies inversely with the temperature of the heating element24and is sent to the microprocessor as a voltage signal V2. The microprocessor compares the value of this signal, namely the value of V2, with an acceptable heating level set within the microprocessor.

In the event the microprocessor20detects an overheated heating element24due to V2falling below a preset value, the microprocessor20commands the switch activation circuit40to open the main power switch36based on the voltage signal44produced by sensor48. The sensor48is also used to test the operation of the main power switch36, as described more fully below.

The microprocessor20also operates a system disabling circuit which includes a fuse52, a safety switch54and a diode50or D1. In this example, safety switch54is represented by a triac T2. The operational condition or state of the safety switch54(that is whether the safety switch54opens and closes upon commands from the microprocessor20) is determined by a test circuit56which includes a diode60or D2and a first resistor64or R4and a second resistor66or R5.

The test circuit56produces a test signal72in the form of a voltage V3which serves as an input to the microprocessor20. Depending on the value of V3, the microprocessor20may turn off power to the heating element24by commanding, the main power switch36to open and stay open.

As further shown inFIG. 1, resistor R2is used to measure the current through the heating element24in the heating pad26and through the main power switch36(T1) and provide input signals to the microprocessor20in the form of voltage inputs V2. These measurements are taken under different operational states of the safety system10and serve two different purposes.

During normal operations, the voltage drop V2across R2decreases substantially linearly and inversely as the temperature of the heating element24in the heating pad26increases linearly. This signal, V2, when decreasing below a preset value stored in the microprocessor20, indicates to the microprocessor20when an overheated heating element condition has occurred. At this point the microprocessor20stops all heating by opening the main power switch (T1) via the switch activation circuit40and keeps the main power switch open until the temperature of the heating element24returns to allowable levels.

Resistor R2also provides a different signal via V2, to determine if the main power switch36(T1) has failed to shut off when commanded to do so by the microprocessor20.

The microprocessor20is programmed to carry out testing of the operational states of T1and T2during test periods which can be spaced apart such as described below. During each test period the following processes are followed:

1. To help ensure stable and accurate test results, a dwell time of about 200 milliseconds is provided after commanding T1to open or turn off and before tests can begin. This allows the power lines12and14and associated circuits to stabilize following any line transients due to activation/deactivation of power to the load (the heating element24).

2. At some fixed or variable intervals between one second and 30 minutes, for example every 2 minutes, the microprocessor20commands the main power switch T1to turn power off prior to initiation a of test cycle. This period of this test cycle is generally less than 1 second in length.

3. During this test cycle, a series of spaced or staggered tests are conducted to validate the operation of the disabling circuit and the ability of the main power switch36(T1) to turn off when so commanded by the microprocessor20.

4. Two flag registers, one for each type of test are set in the microprocessor memory and are used to record all test results.

The first test: Test the disabling circuit:

After the initial dwell time and during the time that the AC power line L1is positive relative to the AC power line L2, the microprocessor20turns on safety switch54(T2the disabling triac).

If triac T2is operating properly, it will turn ON and current will flow through T2, D2, R4and R5as referenced inFIG. 1. That will result in a voltage signal (marked V3) across R5. The microprocessor20, upon receiving a detectable V3voltage value, will then consider T2to be operable and capable of closing upon command. R4is set at a high resistance level value to limit the current through the fuse52and prevent the fuse52from blowing.

If no voltage appears across R5during the initial test of T2, the microprocessor20will consider that T2failed to turn on, and a flag will be set in the disabling circuit register. In either case the microprocessor20will then proceed to retest T2in the same manner after a predetermined time period such as 100 milliseconds to 200 milliseconds. That is, this initial test is conducted at least twice during each test cycle.

The second test: Test the main power switch (T1):

The test circuit for T1includes resistor R2. Resistor R2measures the current through the heating element24in the heating pad26and through the main power switch (T1), and serves two purposes. During normal operations, the voltage drop V2across resistor R2linearly decreases as the temperature of the heating element24linearly increases.

This signal44, which is input to the microprocessor20as voltage signal V2, indicates to the microprocessor20whether the heating element24is functioning properly or overheating. If V2falls below a set value indicating overheating, the microprocessor20commands T1to turn off and provides an indication to the user that the system10has been disabled. Once the heating element24cools down, the system10may be turned on once again. This is preferable to blowing the fuse52which prevents any further use of the system10.

To test the operation and proper function of T1, the microprocessor20also measures the voltage across R2(marked V2) while T1is commanded by the microprocessor20to turn off via switch activation circuit40. If NO voltage appears across R2, then the main power switch (T1) is OFF, as commanded prior to the first test above. This indicates that the microcontroller20can maintain proper operation of the system10and prevent overheating of the heating element24.

If however, there is a voltage V2across R2, a flag will set in main switch register. This measurement of V2is conducted twice during each test cycle with, for example, a period of 100 milliseconds to 200 milliseconds between each test of T1. At the end of each test cycle, ail flags in both registers are cleared.

From a timing perspective, the following tests will be conducted during each test cycle with the power to the heating element24turned off by T1at Time=0.

From 0-199 milliseconds: Dwell time

At 200 milliseconds: Disabling circuit test

At 300 milliseconds: Disabling circuit test

At 400 milliseconds: Main switch test

At 500 Milliseconds: Main switch test

The effect on the system10as a result of the above tests and during a test period is as follows:

Disabling circuit (T2) tests: A flag is set in the disabling circuit register to indicate that the disabling circuit test detected a suspected problem in this circuit. If more than 1 such flag is set during the test cycle, the microprocessor20issues a command to T1to turn off all heating and annunciate to the user that a system problem has been detected. Otherwise, reset the disabling circuit flag register.

Main Power Switch (T1): A flag is set in the main power switch register to indicate a suspected shorted main power switch36(T1) every time the main power switch (T1) is tested (commanded to open or turn off) and a voltage V2is measured across R2. If only one flag is set during a test cycle, the main power switch register is cleared.

The fuse52is blown when the AC cycle is such that power line L2is positive relative to L1and when triac T2is activated via the system disable activation circuit74. This creates a short circuit across the power lines L1and L2via diode D1and T2and the fuse52thereby blowing the fuse52.

An example of a logic flow chart for operating the microprocessor20and controlling the system10is shown inFIG. 2. This flow chart includes the measuring and testing of the system10as described above and is self-explanatory.

There has been disclosed heretofore the best embodiment of the disclosure presently contemplated. Obviously, numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure may be practiced otherwise than as specifically described herein.