Abstract:
A heater control device in which a heater is made to double as a temperature sensor so as to eliminate the leading wires used exclusively for the temperature sensor. The heater control device includes a zero-crossing detector which generates a short duration pulse signal each time an AC power voltage is crossing the zero-level. The heater acts as a temperature sensor only for the short duration of the pulse signal.

Description:
This application is a continuation, of now abandoned application Ser. No. 499,428, filed May 31, 1983. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a heater control device, and more particularly to an improved heater control device in which a heater doubles as a temperature sensor for the purpose of eliminating the leading wires to be used exclusively for a temperature sensor. 
     There is shown in FIG. 1 a circuit diagram of a conventional heater control device. In the figure a heater 1p is power-supplied from an A.C power source 18p through a bidirectional thyristor 2p connected in series with the heater. The thyristor 2p is gated by a switching signal from a control circuit to which a temperature signal of a sensor 6p is input, the control circuit comprises a comparator, a zero-crossing detector and a variable standard voltage circuit which provides a standard voltage corresponding to a predetermined value of temperature. The comparator compares a temperature signal from the temperature sensor 6p with the standard voltage, and outputs a trigger signal to the gate terminal of the thyristor 2p. On the other hand the zero-crossing detector is a circuit device which produces a pulse signal each time the instantaneous absolute value of the A.C power source voltage become smaller than a certain definite value. A diode 17p and capacitor 14p constitute a D.C power source to be supplied to the temperature sensor 6p and the control circuit. The control circuit, as a whole is devised so as to put the comparator into action only for the duration of the pulse signal produced by the zero-crossing detector. The comparator thus outputs to the thyristor 2p a trigger pulse with the same width as that of the pulse signal produced by the zero-crossing detector, only while the temperature is lower than the predetermined temperature value. Therefore, the heater 1p is energized with the thyristor 2p made conducting until the temperature of an object to be heated rises up to the predetermined value of temperature. However, such a circuit construction has a disadvantage that, in case the heater and the temperature sensor must be set remote from the main part of the device, the device needs two pairs of leading wires: one pair is for the heater and the other for the temperature sensor. A multiplicity of leading wires is apt to make a source of trouble due to the breakage of a wire. Particularly the breakage of the sensor wire may cause an uncontrolled running of the heater. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a heater control device which is devised so as to need only one pair of leading wires by making a heater double as a temperature sensor. Another object of the present invention is to provide a heater control device in which a heat dissipation from the control circuit is reduced to a several-tenth in comparison with conventional devices, so as to enable the device to be assembled in a narrow space such as the inside space of the grip of an iron or a hairdryer. A further object of the present invention is to provide a heater control device which works sensitively and precisely with an operating D.C power source voltage kept much lower than a commercial A.C power voltage supplied to the heater. 
     According to the present invention a heater which is to be power-supplied from an A.C power source through an A.C power switching element constitutes a bridge circuit together with accompanying resistors. This bridge circuit is made active to enable the heater to act as a temperature sensor only for a short duration of a zero-crossing period of the A.C power instantaneous voltage. A temperature measured by the heater acting as a sensor is compared by a comparator with a reference voltage which corresponds to a predetermined temperature value. The reference voltage is also output from the bridge circuit. If the temperature is lower than the predetermined temperature value, the comparator outputs a trigger pulse to the gate of the A.C power switching element, and makes the heater energized with the A.C power. The present invention can be embodied with diodes incorporated in the bridge circuit to protect the control circuit more securely from the A.C power voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a circuit diagram of a conventional heater control device. 
     FIG. 2 shows a circuit construction of an embodiment of the present invention. 
     FIG. 3 shows a circuit construction of another embodiment of the present invention. 
     FIG. 4 shows the voltage and current wave forms observed at various points in the circuit shown in FIG. 3. 
     FIG. 5 shows an embodiment of a display device for use in connection with the present invention. 
     FIG. 6 shows a circuit construction of a further embodiment of the present invention 
     FIG. 7 shows a circuit example of a zero-crossing detector to be used in the present invention. 
     FIG. 8 shows the voltage and current wave forms observed at various points in the circuit shown in Fig. 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     There is shown in FIG. 2 a circuit construction of an embodiment of the present invention. A heater 1b is power-supplied from an A.C power source 18b. Resistors 7b, 8b, 9b and 10b and the heater 1b constitute a bridge circuit, to which a D.C operating current is supplied from a D.C power source 13b through a switching transistor 11b. A zero-crossing detector 12b outputs, a switching pulse signal to the transistor 11b each time the instantaneous voltage of the A.C power source 18 is crossing the zero level, and makes the transistor conducting only for a duration of the switching pulse. Comparator 4f and 4g constitute a wired NOR logic together with a resistor 32. A reference voltage which corresponds to a predetermined temperature value is input to the non-inverting input terminal of the comparator 4g from the variable resistor 9b, while a temperature signal is input to the inverting input terminal of the same comparator 4g from a point where the resistor 7b and the heater are connected to each other. A capacitor 33 and a resistor 34 constitute a differentiating circuit, whose output is led to the non-inverting input terminal of a comparator 4h. The comparator outputs a switching signal to a transistor 37 through which a trigger current is supplied to thyristor 2b from the D.C power source 13b. In such a circuit construction, while the zero-crossing detector 12 does not generate a pulse signal, that is, the bridge circuit is kept not supplied with a current the output of the wired NOR logic is kept low irrespective of the output of the comparator 4g, because the output of the comparator 4f is kept low. With the bridge circuit supplied with a current, the comparator 4f turns to a high level so that, if the output voltage of the heater 1b acting as a temperature sensor is higher than the reference voltage obtained from the resistor 9b, the output of the comparator 4g and, therefore, that of the entire wired NOR logic turns to a high level. The differentiating circuit consisting of the capacitor 33 and the resistor 34 makes a positive and a negative pulses from the output of the wired NOR logic correspondingly to the rise-up and the fall-down edges of the pulse signal of the zero-crossing detector. That negative pulse triggers the thyristor 2b through the comparator 36, transistor 37 and a diode 38, and makes the thyristor 2b conducting. Thus the heater 1b is kept energized only while a temperature measured is lower than the predetermined temperature value. The time charts shown in FIG. 8 show, in the order from the top down to the bottom, the voltage of the A.C power source, the output voltage of the wired NOR logic, the input of the differentiating circuit, the current through the transistor 37 or the diode 38 and the current supplied to the heater 1b, respectively. 
     In the following another embodiment of the present invention is described in conjunction with FIG. 3. In the figure all the circuit components which correspond to those shown in FIG. 2 are given the same reference number with the suffix changed to c from b. The description of these components are omitted. Resistors 31, 32, 7c and a heater 1c constitute a resistance bridge, while diodes 33, 35 and 36 are to protect the bridge from the A.C power voltage supplied from an A.C power source 18c. This embodiment is devised so as to obtain a temperature signal not directly from the heater 1c acting as a temperature sensor but from the resistor 34 whose resistance value is chosen much larger than the resistance of the heater 1c. The voltage appearing on the resistor 34 can be made exactly reflect the voltage developed on the heater 1c by choosing the forward voltage of the diode 35 equal to that of the diode 36. The general performance of this embodiment is similar to the case of the preceeding embodiment. A zero-crossing detector 12c operates a switching transistor 11c with its output pulse signal, while a comparator 4c compares a temperature signal e m  obtained from a point F with a reference voltage e s  given at a point E. If the temperature signal e m  is smaller than the reference voltage e s&#39;   the comparator output a negative trigger pulse and triggers bidirectional thyristor 2c through a diode 37 after the end of the zero-cross signal, as is the case of the embodiment shown in FIG. 2. The comparator 4c corresponds the circuit consisting of the wired NOR logic, the comparator 4g and relative components in FIG. 2. The time charts A to H shown in FIG. 4 correspond to the voltage at points A to H in FIG. 3, respectively. 
     There is shown in FIG. 6 a further embodiment of the present invention. In FIG. 6 all the components which correspond to those shown in FIGS. 2 and 3 are given the same reference number with the suffix changed to a. Diodes 20 and 21 are inserted to protect a bridge circuit consisting of resistors 7a, 8a, 9a, and 10a and a heater 1a from a A.C power source 18a. In this embodiment, when a A.C power voltage is crossing the zero level from the minus side toward the plus side, a pulse signal generated from a zero-crossing detector 12a makes a transistor 11 conducting to supply a current to the bridge circuit. At this time, if a temperature signal input to the non-inverting terminal of the comparator 4a is smaller than a reference voltage at the inverting terminal, the comparator output becomes low, and a capacitor 23 is discharged through a diode 25. After the pulse signal is over, the capacitor 23 is charged through a resistor 22, because a transistor provided at the output stage in the comparator 4a  is cut off. The current charging the capacitor triggers a directional thyristor 2a through a diode 24, so that the heater 1a is supplied with a positive half cycle of the A.C current from an A.C power source 18a. During a minus half cycle the current does not flow, because the thyristor 2a is of a uni-directional type. In case the temperature exceeds a predetermined temperature, the comparator output is kept high irrespective of the pulse signal from the zero-crossing detector 12a, so that the thyristor 2a is not triggered. 
     FIG. 5 shows an example of a display apparatus for displaying the temperature value e m  which is measured each time the pulse signal is output from the zero-crossing detector. In the figure operational amplifiers 51 and 64 constitute an impedance converting circuit 50 and an adding circuit 52, respectively. The output e m2  of the adding circuit 52 is given by an equation below, if e m1  and E REF  are input thereto: ##EQU1## where R 1 , R 2  and R 3  are the resistance value of resistors 53, 63 and 62, respectively. An operational amplifier 61 constitutes a circuit to output a positive standard voltage E REF  with a positive DC voltage E being input. A pulse signal from the zero-crossing detector is input to the gate of a FET 54 to make the FET conducting for the duration of the pulse signal. A sample holding circuit 55 holds the value e m2 , which is renewed for each pulse signal from the zero-crossing detector. An A-D converter 57 converts the output of the sample holding circuit to a digital value, which is visually shown on a display 56. By adjusting a resistance R 3  of the resistor 62 so that the output voltage may become 0 at a heater temperature of 0° C., the output is proportional to a measured temperature. 
     FIG. 7 shows a circuit example of a zero-crossing detector, for which a commercially available IC unit may be used. In this circuit an A.C voltage is input between terminals 70 and 72, and a negative D.C source is supplied to a terminal 77. A terminal 79 is the output terminal of the whole circuit. An integrating circuit 82 consisting of resistor 80 and a capacitor 81 is provided to delay the phase of an input A.C voltage. While the absolute value of the output of the integrating circuit is efficiently high, either transistor 73 or 74 is alternately made conducting, and a transistor 75 is kept inactive, because the potential of its base rises substantially up to the common line potential due to the voltage developed in a resistor 76. The transistor 75 is made conducting only in a short time-width in which both of the transistors 73 and 74 are kept non-conducting due to the falling down of the absolute output of the integrating circuit 82 to a level equal to or lower than the emitter-base cut-off voltage of both the transistors 73 and 74. The width of the pulse signal obtained by this zero-crossing detector circuit is about 100 μ sec, while that of a conventional zero-crossing detector is about 300 to 400 μ sec. 
     The present invention, as is understood from the description given above, has the following advantages: 
     Only one pair of leading wires is needed, because the heater doubles as a temperature sensor. Therefore a heater control device based on the present invention is advantageously applicable particularly to such a device as iron and a hair dryer in which the temperature of the heater itself is an object to be controlled; and 
     The electric power consumed by the control circuit is largely reduced, because the circuit is supplied with an operating D.C power only for a duration of the pulse signal generated by a zero-crossing detector.