(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 in FIG. 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 cable 30 and a control device 20. The control device 20 contains a control circuit 6, a DC voltage circuit 22, a first comparator circuit 40, a second comparator circuit 41, a synchronous signal input circuit 23, a first reference voltage circuit 24, an adjustment circuit 27, a NTC (negative temperature coefficient) detection circuit 75, a PTC (positive temperature coefficient) detection circuit 72, a switch circuit, a load detection circuit 28, a protection circuit, a function selection circuit 29, and a status indicator circuit 74. The DC voltage circuit 22 provides DC voltage Vcc to power the control device 20 and, 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.