Patent Publication Number: US-6664516-B2

Title: Electric appliance with a PTC heating member and a method of operating same

Description:
This invention relates to an electric appliance with a positive temperature coefficient (PTC) heating member, and a method of operating such an appliance, which may, for example, be an electric hair curler. 
     PTC thermistors, a type of PTC heating member, are made of polycrystalline ceramic on a base of barium titanate by doping a small amount of rare earth element, e.g. yttrium (Y), lanthanum (La), etc. PTC thermistors of various shapes and specifications may, for example, be obtained from Ohizumi Manufacturing Co., Ltd. of Japan. 
     FIG. 1 of the accompanying drawings is a graph showing a typical electrical resistance/temperature relationship of a PTC thermistor. The electrical resistance of the PTC thermistor is measured at the ambient temperature at a voltage sufficiently low to avoid self-heating. The temperature at which the electrical resistance of PTC thermistor begins to increase rapidly is called the “curie temperature” (T c ), which is defined as the temperature at which the resistance value is twice that of the minimum resistance value (R min ). For the particular thermistor whose resistance/temperature relationship is shown in FIG. 1, the temperature coefficient ox, between any two temperatures (T 1 , T 2 ) is given by equation (1) below:              α   =     2.303          log          R   2       R   1             T   2     -     T   1            100        %        /   ∘          C   .                 (   1   )                         
     As the electric voltage applied to a PTC thermistor increases, the temperature of the PTC thermistor will rise slowly by self-heating. When the temperature approaches and eventually exceeds the curie temperature (T c ), the electric current will begin to decrease, as shown in FIG. 2, which shows the relationship between the electric current passing through the PTC thermistor relative to the applied voltage, at various ambient temperatures. As can be seen in FIG. 2, such a relationship is influenced by the ambient temperature. When the electric voltage is gradually increased, the temperature of the PTC will gradually increase by self-generated heat. When the temperature reaches around the curie temperature (T c ), it shows a negative current characteristic, namely that as voltage continues to increase, the electric current decreases. This is shown in more detail in FIG. 3, which shows the relationship between the electric current passing through the PTC thermistor with time. 
     It can be seen in FIG. 3 that when an electric voltage is applied to the PTC thermistor, there will be a attenuation of the current. Initially, a very large electric current will flow through the PTC thermistor. As the time of application of this voltage increases, the electric current will decrease sharply until it reaches a low level, whereupon it will remain relatively constant. This low level is well below the normal working current of a heat generating resistor, and therefore there is, in the long run, an advantage of using PTC thermistor for generating heat, in preference to a resistor. 
     However, the characteristic shown most clearly in FIG. 3 has hindered the use of PTC thermistors as heating elements in electric appliances with heating members, in particular those appliances with batteries (rechargeable or otherwise) for operating the heating members. As discussed above, when an electric voltage is applied to the PTC thermistor, a large electric current will initially be drawn from the power source to start up the PTC thermistor. In cases where the power source are batteries, each time of starting the electric appliance will significantly shorten the normal useful life of the batteries, as batteries are not designed to provide such a large flow of electric current. This cannot be adequately compensated, even if the electric current decreases with the passage of time to a low level. 
     It is thus an object of the present invention to provide an electric appliance with a PTC heating member, and a method of operating such an electric appliance, in which the aforesaid shortcoming is mitigated, or at least to provide a useful alternative to the public. 
     According to a first aspect of the present invention, there is provided an electric appliance with a positive temperature coefficient (PTC) heating member and at least a first electric power source, wherein said PTC heating member is adapted to be powered by said first electric power source and at least a second electric power source, characterized in that said PTC heating member is adapted to be powered by said second electric power source when said electric appliance is started, and to be subsequently powered by said first electric power source. 
     According to a second aspect of the present invention, there is provided a method of operating an electric appliance with a positive temperature coefficient (PTC) heating member and at least a first electric power source, including steps (a) of powering said PTC heating member by at least a second electric power source; (b) powering said PTC heating member by said first electric power source, characterized in powering said PTC heating member by said second electric power source when said electric appliance is started, and powering said PTC heating member by said first electric power source subsequently. 
     Embodiments of the present invention will now be described, by way of examples only, and with reference to the accompanying drawings, in which: 
    
    
     FIG. 1 shows a typical relationship between the electrical resistance and the temperature of a PTC thermistor; 
     FIG. 2 shows relationship between the electric current passing through a PTC thermistor relative to the electric voltage applied thereto, at various ambient temperatures; 
     FIG. 3 shows the relationship between the electric current passing through a PTC thermistor with time; 
     FIG. 4 is a diagram showing an electric appliance incorporating a PTC thermistor, according to a first embodiment of the present invention; 
     FIG. 5 is a diagram showing an electric appliance incorporating a PTC thermistor, according to a second embodiment of the present invention; 
     FIG. 6 is a diagram showing an electric appliance incorporating a PTC thermistor, according to a third embodiment of the present invention; 
     FIG. 7 is a diagram showing an electric appliance incorporating a PTC thermistor, according to a fourth embodiment of the present invention; 
     FIG. 8 is a diagram showing an electric appliance incorporating a PTC thermistor, according to a fifth embodiment of the present invention; 
     FIG. 9 is a diagram showing an electric appliance incorporating a PTC thermistor, according to a sixth embodiment of the present invention; 
     FIG. 10 is a diagram showing an electric appliance incorporating a PTC thermistor, according to a seventh embodiment of the present invention; 
     FIG. 11 is a diagram showing an electric appliance incorporating a PTC thermistor, according to an eighth embodiment of the present invention; 
     FIG. 12 shows the circuitry of a timing integrated circuit which may be used in the embodiments shown in FIGS. 6 and 7; and 
     FIG. 13 shows a block diagram of an integrated circuit for regulating the charging of the battery in the electric appliances of the embodiments shown in FIGS. 5,  7 ,  9  and  11 . 
    
    
     Referring to FIG. 4, such shows a circuit diagram of an electric appliance, e.g. an electric hair curler, according to a first embodiment of the present invention, generally designated as  100 . The electric appliance  100  includes a positive temperature coefficient (PTC) heater  102  electrically connected via an optional on/off switch  104  to one or more batteries  106 , and a power jack  108 . The batteries  106  may, for example, be disposable batteries, e.g. dry batteries, or car batteries. The power jack  108  is electrically connected to a receiver  110  designed for connection with a power plug  112  connectable, probably via a transformer, to an outside electric power source, which may, for example, be a municipal a.c. source at 220 v or a car battery. The basic principle is that the batteries  106  are of a voltage of not more than 50 volts, and the outside electric power source is of a higher electric power than the batteries  106 . 
     When the power plug  112  is connected with the power jack  108 , electric power is supplied to the PTC heater by the outside electric power source. Simultaneously, a movable contact arm  114  of the power jack  108  is pushed out of contact from a stationary contact arm  116 , whereby the power supply from the batteries  106  is disconnected. The electric appliance  100  then starts, and the PTC heater  102  heats up under the power supplied by the outside electric power source. A user may, when he so desires, manually remove the power plug  112  from the power jack  108 , to disconnect the electric appliance  100  from the outside electric source. Upon removal of the power plug  112  from the power jack  108 , the movable contact arm  114  returns, e.g. upon the action of a biasing force of a spring, to its normal state to contact and electrically connect the stationary contact arm  116 , so that the PTC heater  102  is now powered, and thus heated up or kept warm, by the batteries  106 , in place of the outside electric power source. 
     By way of such an arrangement, the very large initial electric power for starting the PTC heater  102  will be borne by the outside electric power source, and not by the batteries  106  in the appliance  100 . The on/off switch  104  may be operated to connect or disconnect the electrical connection between the PTC heater  102  and the batteries  106  and/or the power jack  108 , through which the PTC heater  102  is connected with the outside electric power source. 
     FIG. 5 shows a circuit diagram of an electric appliance according to a second embodiment of the present invention, generally designated as  200 . In this electric appliance  200 , when a power jack  208  is connected with a power plug  212 , a movable contact arm  214  will be moved to contact and electrically connect with a stationary contact arm  216 , so that if the power plug  212  is connected, e.g. via a transformer, to an outside electric power source, the electric appliance will be started, and a PTC heater  202  will be heated up, due to the passing of electricity therethrough. At the same time one or more rechargeable batteries  206  will be recharged by the outside electric power source, under the control of a charger integrated circuit (IC)  220 . The rechargeable batteries  206  are also protected against overcharging by a resettable device  222  produced and traded by Raychem Circuit Protection, a division of Tyco Electronics, under the trade name PolySwitch. This device is a kind of polymeric PTC non-linear thermistor that limits the magnitude of electric current that may pass threrethrough. 
     At this time, the entire circuit is powered by the outside electric power source, even after the PTC heater  202  is sufficiently heated up to its steady state. When the power plug  212  is removed from the power jack  208 , the movable contact arm  214  will disengage from the stationary contact arm  216  and return to its normal open position, whereupon the PTC heater  202  is then powered, and thus heated up or kept warm, by the rechargeable batteries  206  only, in place of the outside electric power source. 
     FIG. 13 shows a block diagram of a charger IC which may be used in the embodiment shown in FIG. 5 discussed above. This can be used as a protector for rechargeable Ni—Cd or Ni—MH batteries. Such an IC may be one traded by Ricoh Corporation, of USA, under its R5440N2xxA Series, which can detect over-voltage and halt a charging current. It is composed of Over-voltage detectors VD 1 , VD 3 , Low-voltage detectors VD 2 , VD 4 , an oscillator circuit, a reference unit, a delay circuit, and a logic circuit. 
     FIG. 6 shows a circuit diagram of an electric appliance according to a third embodiment of the present invention, generally designated as  300 . When the appliance  300  is connected to an outside electric power source (not shown), electric current flows through a coil  304  of a relay  306 , thus attracting a pole D 6  of the relay  306  to connect with T 61  position, thus breaking up the electrical contact between a PTC heater  302  with one or more batteries  308  in the appliance  300 . The PTC heater  302  starts to heat up under the power from the outside electric power source, and a timer integrated circuit (IC)  310  starts to count down. The time T to be counted down is determined by the value of a capacitor C 1  and resistors R 1 , R 2 , according to equation (2) below: 
     
       
           T= 0.693( R 1+2 R 2)* C 1  (2)  
       
     
     The values of C 1 , R 1  and R 2  are such that the resultant count down time T is of a sufficient duration to allow the PTC heater  302  to attain its relatively steady and low current state. 
     At the same time, the timer IC  310  triggers on a transistor TR 62 , so that electric current flows through a red light emitting diode (LED) L 62  and the transistor TR 62 , whereupon the LED L 62  lights up. When the timer IC  310  counts down to zero, the timer IC  310  resets the transistor TR 62  to off. As the transistor TR 62  is off, no electric current will flow through the LED L 62  and the transistor TR 62 . Electric current instead flows through a transistor TR 61 , and thereby to light up a green LED L 61 , indicating that the PTC heater  302  has attained its relatively steady and low current state, and is thus ready for use. 
     When the electric appliance  300  is disconnected from the outside electric power source, no electric current will flow through the coil  304 , whereupon the pole D 6  will return to its normally closed (NC) position to connect with T 62 . The PTC heater  302  is then electrically connected with and powered, and thus kept warm or heated up, by the batteries  308 . 
     An integrated circuit which may be used as the timer IC  310  may be one traded by Unisonic Technologies Co., Ltd., of Taiwan, under their serial No. UTC NE555, an exemplary block diagram of which is shown in FIG.  12 . When operated in an astable mode, the frequency and duty cycle of such an IC are controlled by two external resistors and one capacitor, i.e. R 1 , R 2  and C 1  in FIG.  6 . 
     A circuit diagram of an electric appliance, generally designated as  400 , made in accordance with a fourth embodiment of the present invention is shown in FIG.  7 . This electric appliance  400  differs from the third embodiment discussed above mainly in that there are provided in the electric appliance  400  a number of rechargeable batteries  406 . A charger integrated circuit (IC)  408  and a resettable device  410  are also provided to protect the rechargeable batteries  406  from being overcharged. When the electric appliance  400  is electrically connected with an outside electric power source (not shown), the rechargeable batteries  406  are recharged under the control of the charger IC  408 , and the protection of the resettable device  410 . 
     FIG. 8 shows a circuit diagram of an electric appliance according to a fifth embodiment of the present invention, generally designated as  500 . When this electric appliance  500  is started by being electrically connected with an outside electric source (not shown), electric current will flow through a coil  502  of a relay  504 , whereby a pole D 8  is attracted to contact and electrically connect with a position T 81 . A large inrush electric current is thus drawn from the outside electric power source to power and heat up a PTC heater  506 . As the circuit current is high, a transistor TR 82  is triggered to switch on, so that the electric current flows through a red LED L 82  and the transistor TR 82 , thus lighting up the red LED L 82 . 
     When the PTC heater  506  is sufficiently heated up to the steady state, the current becomes low. When a transistor TR 83  senses that the electric current flowing through a resistor R 9  decreases to below a predetermined reference level, the transistor TR 82  is switched off. The value of the predetermined reference level is determined by the value of the power of the PTC heater  506 , and the value of the input voltage of the outside electric power source. The values of resistors R 8  and R 9  may have to be changed in response to changes in the value of the power of the PTC heater  506 , and that of the input voltage of the outside electric power source. 
     Upon switching off of the transistor TR 82 , no current flows through the red LED L 82  and the transistor TR 82 . The electric current flows instead through a transistor TR 81 , and thereby to light switch on a green LED L 81 , signalling that the PTC heater  506 , and thus the electric appliance  500 , is ready for use. At this point, the whole electric appliance  500  is still powered by the outside electric power source. 
     When the electric appliance  500  is disconnected from the outside electric power source, no electric current will flow through the coil  502  of the relay  504 , whereupon the pole D 8  will return to its normally closed (NC) position to connect with T 82 . The PTC heater  502  is then electrically connected with and powered, and thus kept warm or heated up, by batteries  508 . 
     A circuit diagram of an electric appliance, generally designated as  600 , made in accordance with a sixth embodiment of the present invention is shown in FIG.  9 . This electric appliance  600  differs from the fifth embodiment discussed above mainly in that there are provided in the electric appliance  600  a number of rechargeable batteries  606 . A charger integrated circuit (IC)  608  and a resettable device  610  are also provided to protect the rechargeable batteries  606  from being overcharged. When the electric appliance  600  is electrically connected with an outside electric power source (not shown), the rechargeable batteries  606  are recharged under the control of the charger IC  608 , and the protection of the resettable device  610 . 
     A circuit diagram of an electric appliance, generally designated as  700 , made in accordance with a sixth embodiment of the present invention is shown in FIG.  10 . When the electric appliance  700  is electrically connected with an outside electric power source (not shown), an electric current flows through a coil  704  of a relay  706 , which attracts a pole D 10  to contact and electrically connect with a position T 101 . A PTC heater  702  then heats up under the power of the outside electric power source. At the same time, a negative temperature coefficient (NTC) thermistor  710  positioned adjacent to the PTC heater  702  is at a high resistance state. A transistor TR 102  is triggered to switch on, so that electric current flows through a red LED L 102  and the transistor TR 102 , whereupon the red LED L 102  lights up. 
     When the temperature of the PTC heater  702  is sufficiently high, as the NTC thermistor  710  is near to the PTC heater  702 , it will be heated up by the heat generated by the PTC heater  702 , so that its electric resistance decreases. When the temperature of the NTC thermistor  710  rises to a predetermined reference level, its electric resistance will fall to a level at which the transistor TR 102  will be switched off. Upon the transistor TR 102  being switched off, no electric current will flow through the red LED  102  and the transistor TR 102 . Electric current instead flows through a transistor TR 101 , whereupon a green LED  101  will light up, indicating that the PTC heater  702  is at a steady current state, ready to be used. In the meantime, the electric appliance  700  is powered by the outside electric power source. 
     When the electric appliance  700  is disconnected from the outside electric power source, no current flows through the coil  704 , whereupon the pole D 10  will return to its normally closed (NC) position to connect with T 102 . The PTC heater  702  is then electrically connected with and powered, and thus kept warm or heated up, by batteries  708 . 
     The distance between the PTC heater  702  and the NTC thermistor  710 , the power of the PTC heater  702 , and the input power voltage, will all affect the time duration before which the temperature of the NTC thermistor  710  rises to the predetermined reference level. The NTC thermistor  710  may be in direct contact with the PTC heater  702 , or be positioned adjacent to it without touching it. 
     NTC thermistors which may be used in the electric appliance  700  discussed above may be ones traded by Ohizumi Manufacturing Co., Ltd. of Japan under their NGR series, with an operating temperature range of −55° C. to 300° C., or NRC series, with an operating temperature range of −20° C. to 100° C. NTC thermistors are resistors with high negative temperature coefficient of resistance. The relationship between its electrical resistance and temperature may be approximated by equation (3) below:                R   1     =       R   0        exp                   B        (       1     T   1       -     1     T   0         )                 (   3   )                         
     in which R 0  is the initial electrical resistance of the NTC thermistor at temperature T 0  measured in Kelvin, and R 1  is the electrical resistance at temperature T 1 . B is a constant for a given thermistor, and may be approximated by equation (4) below:              B   =       (       ln                   R   1       -     ln                   R   0         )     /     (       1     T   1       -     1     T   0         )               (   4   )                         
     The temperature coefficient of resistance β of the NTC thermistor can be approximated by equation (5) below:                    β   =       1   R               R          T                     =       -     B     T   2         *   100      %                   (   5   )                         
     The relationship between the power P (which equals voltage (V) multiplies current (I)) applied to an NTC thermistor at ambient temperature (T 0 ) and the consequent temperature rise (T 1 −T 0 ) due to self-heating can be approximated by equation (6) below:                    P   =     V   *   I                 =     δ        (       T   1     -     T   0       )                     (   6   )                         
     in which δ is the dissipation constant, normally measured in mW/° C. 
     A circuit diagram of an electric appliance, generally designated as  800 , made in accordance with an eighth embodiment of the present invention is shown in FIG.  11 . This electric appliance  800  differs from the seventh embodiment discussed above mainly in that there are provided in the electric appliance  800  a number of rechargeable batteries  806 . A charger integrated circuit (IC)  808  and a resettable device  810  are also provided to protect the rechargeable batteries  806  from being overcharged. When the electric appliance  800  is electrically connected with an outside electric power source (not shown), the rechargeable batteries  806  are recharged under the control of the charger IC  808 , and the protection of the resettable device  810 . 
     It should be understood that the above only illustrates examples whereby the present invention may be carried out, and that various modifications and/or alterations may be made thereto without departing from the spirit of the invention. In particular, it should be understood that the value of the various electronic components given in the drawings are examples only, and may be changed with the change of the voltage of the outside electric power source and the power of the PTC heater in the electric appliance. 
     It should also be understood that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any appropriate sub-combinations.