Heating cable control system

The present invention provides a heating cable control system. The system is configured with an optical coupling circuit, a NTC break-off detection circuit, and a fourth comparator circuit. The optical coupling circuit has an input terminal, a first control terminal, and a second control terminal. The first and second control terminals are electrically connected to first terminals of a NTC resistive layer and a PTC resistive wire of the heating cable, respectively. A second terminal of the PTC detection circuit is electrically connected to a silicon-controlled switch circuit. A second terminal of the NTC resistive layer is electrically connected to a negative input terminal of the fourth comparator circuit through the NTC break-off detection circuit, and is compared against a third reference voltage circuit. As such, when the NTC resistive layer becomes open-circuited, the heating to the PTC resistive wire is stopped reliably, thereby enhancing usage safety.

BACKGROUND OF THE INVENTION

(a) Technical Field of the Invention

The present invention is generally related to heating devices, and more particular to a heating cable control system having an optical coupling circuit, a NTC break-off detection circuit, and a fourth comparator circuit.

(b) Description of the Prior Art

As shown inFIG. 1, U.S. Pat. No. 8,164,035, titled Heating Device Having Dual-core Heating Cable, teaches a dual-core heating cable control system containing a dual-core heating cable30and a control device20. The control device20contains a control circuit6, a DC voltage circuit22, a first comparator circuit40, a second comparator circuit41, a synchronous signal input circuit23, a first reference voltage circuit24, an adjustment circuit27, a NTC (negative temperature coefficient) detection circuit75, a PTC (positive temperature coefficient) detection circuit72, a switch circuit, a load detection circuit28, a protection circuit, a function selection circuit29, and a status indicator circuit74. The DC voltage circuit22provides DC voltage Vccto power the control device20and, through the switch circuit, to activate the gate of a second silicon-controlled regulator so that the second silicon-controlled regulator conducts its anode and cathode, and that the AC is conducted to a PTC resistive wire of the dual-core heating cable to heat up. The PTC detection circuit obtains a load current through the PTC resistive wire and converts the load current to a voltage compared to the first reference voltage through the second comparator circuit. When a high level is detected, the heating up to the dual-core heating cable continues whereas a low level is detected, the heating up to the dual-core heating cable stops, thereby achieving constant temperature. Additionally, the dual-core heating cable is configured with a NTC resistive layer and electrical current flows through the PTC resistive wire, the NTC resistive layer, and then to the NTC detection circuit. The NTC detection circuit coverts the current to a voltage compared against the first reference voltage through the first comparator circuit. When a high level is detected, the heating up to the PTC resistive wire continues whereas a low level is detected, the heating up to the PTC resistive wire stops, thereby achieving a second over-temperature protection.

However, even though with the constant temperature and the second over-temperature protection, the heating cable control system still suffers the following disadvantage. When the NTC detection circuit stops heating up due to the second silicon-controlled regulator becomes open-circuited. The protection circuit could still trigger a first silicon-controlled regulator to conduct and the PTC resistive wire is still heated up. The NTC detection circuit then cannot accurately detect the breaking off of the NTC resistive layer and top the heating up to the PTC resistive wire. The dual-core heating cable then would be over-heated and damaged, or the user could be burned.

SUMMARY OF THE INVENTION

Therefore, the present invention provides a heating cable control system to obviate the foregoing shortcoming. The system is configured with an optical coupling circuit, a NTC break-off detection circuit, and a fourth comparator circuit. The optical coupling circuit has an input terminal, a first control terminal, and a second control terminal. The first and second control terminals are electrically connected to first terminals of a NTC resistive layer and a PTC resistive wire of the heating cable, respectively. A second terminal of the PTC detection circuit is electrically connected to a silicon-controlled switch circuit. A second terminal of the NTC resistive layer is electrically connected to a negative input terminal of the fourth comparator circuit through the NTC break-off detection circuit, and is compared against a third reference voltage circuit. As such, when the NTC resistive layer becomes open-circuited, the heating to the PTC resistive wire is stopped reliably, thereby enhancing usage safety.

The heating cable control system contains an AC source, a control device, and a dual-core heating cable.

The AC source has a first terminal and a second terminal. The first terminal is connected to ground.

The control device contains a fuse, a DC voltage circuit, a synchronous signal input circuit, a first reference voltage circuit, a control circuit, a NTC detection circuit, an adjustment circuit, a load detection circuit, a silicon controlled switch circuit, a silicon controlled short-circuit detection circuit, a function selection circuit, a PTC detection circuit, a second reference voltage circuit, a NTC break-off detection circuit, a third reference voltage circuit, a status indicator circuit, an optical coupling circuit, a first comparator circuit, a second comparator circuit, a third comparator circuit, and a fourth comparator circuit. The control circuit contains a microchip.

The dual-core heating cable contains the NTC resistive layer and the PTC resistive wire inside. The PTC resistive wire has a first terminal and a second terminal, and the NTC resistive layer has a first terminal and a second terminal. The first terminals of the PTC resistive wire and the NTC resistive layer are electrically connected to the first and second control terminals of the optical coupling circuit, respectively; The second terminals of the PTC resistive wire and the NTC resistive layer are electrically connected to the load detection circuit's another terminal and a terminal of the NTC break-off detection circuit, respectively.

The optical coupling circuit has an input terminal, a first control terminal, and a second control terminal. The first and second control terminals are electrically connected to first terminals of the NTC resistive layer and the PTC resistive wire, respectively. The second terminal of the PTC detection circuit is electrically connected to the silicon-controlled switch circuit. The second terminal of the NTC resistive layer is electrically connected to a negative input terminal of the fourth comparator circuit through the NTC break-off detection circuit, and is compared against the third reference voltage circuit. As such, when the NTC resistive layer becomes open-circuited, the heating to the PTC resistive wire is stopped reliably, thereby enhancing usage safety.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown inFIGS. 2 and 3, a heating cable control system according to an embodiment of the present invention mainly contains a control device and a dual-core heating cable30. The control device contains a fuse21, a DC voltage circuit22, a synchronous signal input circuit23, a first reference voltage circuit24, a control circuit6, a NTC detection circuit75, an adjustment, circuit27, a load detection circuit28, a silicon controlled switch circuit70, a silicon controlled short-circuit detection circuit71, a function selection circuit29, a PTC detection circuit72, a second reference voltage circuit25, a NTC break-off detection circuit73, a third reference voltage circuit26, a status indicator circuit74, an optical coupling circuit50, a first comparator circuit40, a second comparator circuit41, a third comparator circuit42, and a fourth comparator circuit43. Please also refer toFIG. 3-1, the control circuit6contains a microchip61having a first pin6101, a second pin6102, a third pin6103, a fourth pin6104, a fifth pin6105, a sixth pin6106, a seventh pin6107, an eighth pin6108, a ninth pin6109, a tenth pin6110, an eleventh pin6111, a twelfth pin6112, a thirteenth pin6113, a fourteenth pin6114, a fifteenth pin6115, a sixteenth pin6116, a seventeenth pin6117, an eighteenth pin6118, a nineteenth pin6119, a twentieth pin6120, a twenty first pin6121, a twenty second pin6122, a twenty third pin6123, a twenty fourth pin6124, a twenty fifth pin6125, a twenty sixth pin6126, a twenty seventh pin6127, and a twenty eighth pin6128. The second pin6102is electrically connected to a second terminal222of the DC voltage circuit22. The third pin6103is electrically connected to a second terminal752of the NTC detection circuit75. A first terminal751of the NTC detection circuit75is electrically connected to a first terminal H3of a NTC resistive layer31of the dual-core heating cable30. A third terminal753of the NTC detection circuit75is electrically connected to a negative input terminal402of the first comparator circuit40. The fourth pin6104is connected to ground. The fifth pin6105is electrically connected to a second terminal232of the synchronous signal input circuit23. A first terminal231of synchronous signal input circuit23is electrically connected to a first terminal221of the DC voltage circuit22. The sixth, seventh, eighth, and ninth pins6106,6107,6108, and6109are electrically connected to an terminal of the adjustment circuit27, respectively, and another terminal of the adjustment circuit27is electrically connected the first reference voltage circuit24. The tenth, eleventh, and twelfth pins6110,6111, and6112are electrically connected to the status indicator circuit74, respectively. The thirteenth pin6113is electrically connected to a terminal of the function selection circuit29whose another terminal is connected to ground. The fifteenth pin6115is configured with a thirty ninth resistor R39whose one terminal is connected to the fifteenth pin6115and another terminal is connected to ground. The sixteenth pin6116is configured with a twentieth resistor R20whose one terminal is connected to the sixteenth pin6116and another terminal is connected to ground and the thirty ninth resistor R39's another terminal. The eighteenth pin6118is electrically connected to a terminal of the silicon-controlled short-circuit detection circuit71. The nineteenth pin6119is electrically connected to a third terminal703of the silicon-controlled switch circuit70. A first terminal701of the silicon-controlled switch circuit70is electrically connected to the load detection circuit28's another terminal. A second terminal702of the silicon-controlled switch circuit70is electrically connected to a terminal of the PTC detection circuit72. The twentieth pin6120is electrically connected to an input terminal503of the optical coupling circuit50further having a first control terminal501and a second control terminal502. The twenty-first pin6121is electrically connected to an output terminal423of the third comparator circuit42. A negative input terminal422of the third comparator circuit42is electrically connected to the second reference voltage circuit25. A positive input terminal421of the third comparator circuit42is electrically connected to the silicon-controlled short-circuit detection circuit71. The twenty-second pin6122is electrically connected to a terminal of the load detection circuit28. Another terminal of the load detection circuit28is electrically connected to a second terminal H2of a PTC resistive wire32, and the first terminal701of the silicon-controlled switch circuit70. The twenty third pin6123is electrically connected to an output terminal433of the fourth comparator circuit43whose positive and negative input terminals431and432are electrically connected to the third reference voltage circuit26and the NTC break-off detection circuit73, respectively. The twenty fourth pin6124is electrically connected to an output terminal403of the first comparator circuit40whose positive and negative input terminals401and402are electrically connected to the first reference voltage circuit24and the third terminal753of the NTC detection circuit75, respectively. The twenty fifth pin6125is electrically connected to an output terminal413of the second comparator circuit42whose positive and negative terminals411and412are electrically connected to the PTC detection circuit72and the first reference voltage circuit, respectively. The twenty seventh pin6127is configured with a thirteenth resistor R13whose one terminal is connected to the DC voltage Vccand another terminal is connected to the twenty seventh pin6127. The twenty seventh pin6127is also configured with a fifth capacitor C5whose one terminal is connected to the thirteenth resistor R13's another terminal, and whose another terminal is connected to the twenty seventh pin6127.

The dual-core heating cable30contains the NTC resistive layer31and the PTC resistive wire32inside. The PTC resistive wire32has a first terminal H1and the second terminal H2. The NTC resistive layer31has the first terminal H3and a second terminal H4. The first terminals H1and H3of the PTC resistive wire32and the NTC resistive layer31are electrically connected to the first and second control terminals501and502of the optical coupling circuit50, respectively. The first control terminal501is also electrically connected to AC voltage. The second terminals H2and H4of the PTC resistive wire32and the NTC resistive layer31are electrically connected to the load detection circuit28's another terminal and a terminal of the NTC break-off detection circuit73, respectively.

As shown inFIG. 3, the optical coupling circuit50of the present invention has the input terminal503, the first control terminal501, and the second control terminal502. The first and second control terminals501and502are electrically connected to the first terminals H3and H1of the NTC resistive layer31and the PTC resistive wire32, respectively. As also shown in FIGS.4and4-1, when the PTC resistive wire32is to be heated up, the switch SW1is closed and AC voltage is conducted through the PTC resistive wire32to ground. The PTC resistive wire32has resistance Rband the NTC break-off detection circuit73has resistance Rawhere both Raand Rbare 100Ω. The NTC resistive layer31has resistances Rc, Rdwhich are both 800KΩ. Then, the voltage between the points A and B (i.e., the voltage between the PTC resistive wire32and the NTC resistive layer31) can be obtained from the following equation where Rais ignored as it is relatively small compared to Rcand Rd:

Vab=VA⁢⁢C*RcRa+Rc+Rd=VA⁢⁢C*800⁢⁢K100+800⁢⁢K+800⁢⁢K=VA⁢⁢C*800⁢⁢K800⁢⁢K+800⁢⁢K=VA⁢⁢C2
The voltage between the NTC resistive layer31and the PTC resistive wire32is one half of the source voltage. Then, by electrically connecting the second terminal H4of the NTC resistive layer31to the NTC break-off detection circuit73, a terminal of the NTC break-off detection circuit73to the negative input terminal432of the fourth comparator circuit43, the positive input terminal431of the fourth comparator circuit43to the third reference voltage circuit26, and the third reference voltage circuit26to the DC voltage Vcc, the fourth comparator circuit43produces a comparison result between its positive and negative input terminals431and432when the PTC resistive wire32is heated up, as shown inFIG. 5. By electrically connecting the first and second control terminals501and502of the optical coupling circuit50to the first terminals H3and H1of the NTC resistive layer31and the PTC resistive wire32, the voltage between the NTC resistive layer31and the PTC resistive wire32could be comparable to the AC voltage. As the second terminal H4of the NTC resistive layer31, through the NTC break-off detection circuit73, is compared against the third reference voltage circuit26by the fourth comparator circuit43, the comparison result (i.e., a high- or low-level signal) is delivered to the microchip61of the control circuit6for examination through the output terminal433of the fourth comparator circuit43. If the comparison result is a high-level signal, the NTC resistive layer31or the NTC detection circuit75is considered as not breaking off. The microchip61of the control circuit6issues signals to the optical coupling circuit50to conduct the first and second control terminals501and502, so that the PTC resistive wire32is continuously heated up. On the other hand, as shown inFIG. 3, if the comparison result is a low-level signal, the NTC resistive layer31or the NTC detection circuit75is considered as breaking off. The microchip61of the control circuit6turns off the silicon-controlled switch circuit70and the PTC resistive wire32, so that the PTC resistive wire32is stopped from heating up. Therefore, both user and operation safety is enhanced.