GB2181311A - Dual voltage electric heating apparatus - Google Patents

Abstract

The apparatus, particularly a hair dryer, has switching means 28, 40 which operate automatically to connect heating elements 8, 9 in parallel when the supply voltage is below a predetermined level and in series when the voltage is above that level. With a voltage of 100 to 120V, bias 40 is turned on and changeover switch 28 is at contact 28a to effect the parallel connection, whenever with a voltage of 200 to 240V bias 40 is turned off and relay coil 27 is energised to change switch 28 to contact 28b to effect the series connection. An alternative embodiment uses two relays to effect the switching, the relays being operated sequentially to avoid short circuiting the supply. (Figures 6, 7, 9). One or more protection resistors (44), (Figure 8), (83), (Figure 9), (95), (96), (Figure 10), may be connected in series with various switch contacts to limit any short circuit current that may flow in the event of arcing between switch contacts-such as 28a, 28b- caused by vibration of movable contact plastic when the apparatus is subjected to physical shock. The protection resistors may contribute to the heating effect. A further switch (67), (Figure 6), may be provided having a "low" heat position in which the heaters are energised via a diode (72) in parallel with a resistor (73), and a "high" heat position in which this diode and resistor are shorted. <IMAGE>

Description

SPECIFICATION Electric heating apparatus The present invention relates to an electric heating apparatus.

There are a lot of voltages in the world, which are roughly divided into a voltage of 1 00V and a voltage of 200V. An electric heating apparatus such as a hair dryer which travellers carry as a necessary article operates at a predetermined voltage. When an electric heating apparatus is used in a country having a different supply voltage, the situation occurs where heaters are overheated or underheated. Thus, some measures must be taken to enable use of an electric heating apparatus in a different-voltage country.

To deal with the above case, an electric heating apparatus has been suggested which comprises two heaters and two changeoverswitches, whereby to connect the heaters in parallel on a voltage of 100V and connecting the heaters in series on a voltage of 200V by manuallyturning the switches (see Japanese Patent Publication No.50365/1981, Japanese Utility Model Publication No.18804/1983).

Referring now to Figure 11 which illustrates a circuit of a heating apparatus ofthe prior art mentioned above, one end of a first load 101 is connected to one terminal 105a of a power source 105thorough a main switch 107, and one end of a second load 102 is con nectedto anotherterminal 105b ofthe powersource 105.A common contact 1 of a first switch 103 is connected to anotherend ofthefirst load, and a first contact 1 03a is connected to another contact 1 05b of the powersource.Asecond contact 103b ofthefirst switch 103 is connected to a second contact 1 04b of a second switch 104, and a first contact 1 04a is connected to the terminal 105a ofthe power source 105 through the main switch 107. Acommon contact 104c ofthe second switch 104 is connected to another end ofthe second load 102.The first and the second switches are interlocked each other; when the first contact 1 03a ofthe first switch 103 is closed, thefirst contact 1 04a ofthe second switch 104 is same ultaneously closed, when the second contact 1 03b of the first switch 103 is closed, the second contact 1 04b ofthe second switch 104 is simultaneouslyclosed (see Japanese Patent Publication No.50365/1981).

In the above configuration, by manually flipping the switches to the first contact-sides, both loads are connected in parallel, and by mannuallyflipping the switches to the second sides, both loads are connected in series.

However, in the heating apparatus of the prior art, an operator sometimes flips the switch to a different voltage-position from the applied voltage of power source by mistake, when sufficient voltage is not supplied to the loads or an excess voltage is supplied to the loads to overheatordamagethe loads. This is the reason why an operator must confirm the posi tionoftheswitch before using the apparatus.

To overcome the above disadvantage, it is prop osed to turn switches in automatic response to a supplied voltage. In such an apparatus, a voltagedropping resistance is connected in series to a load.

When a voltage of power source is below a specific value, the both ends ofthe voltage-dropping resistance is short-circuited by a switching device e.g. a transistor, so thatthetotal voltage is applied to the load. a When avoltageofpowersourceisabovethe specific value, the shorted circuit mentioned above is opened,so thatthe voltage is applied both tothevol- tage-dropping resistance and the load (Japanese Patent Laid-Open Publication No.82980/1981). Discrimination whether a voltage ofthe power source is above or below the prescribed value is automatically done by a voltage-regulator diode orzenerdiode.According to the above configuration, an automatic changeoverto a supplied voltage can be realized.

However, the configuration involves a disadvantage that the voltage-dropping resistance wastes electric power. Even if heat generated from the resistance could be effectively utilized, a generated electric powerata highvoltageandawastedelectricpower art a low voltage are different, thus same amount of heat cannot be obtained. So this configuration is pre ferablyadoptedtoan apparatus wasting small amount of power such as a watch, but is unsuitable for an apparatus wasting large amount of power such as an electric heating apparatus.

Further, in the electric heating apparatus ofthe prior art shown in Figure 11, the switches are turned at a time, so thatthe movable contact plates of both switches can be respectively positioned between the first contact and the second contact in an instant. The arcs can breakout between thefirstcontact and the second contact ofthe first switch and also between the first contact and the second contact of the second switch, so that bringing about a short-circuit in the electric heating apparatus. If such occurrence of arc is repeated, lifetime of the switches is shortened.

Particularly, a compact electric heating apparatus such as a hair dryer has very small space for attaching switches, so that a small-sized switch must be used. In such a case, an interval between a first contact and a second contact is rather small and an arc breaks out more easily.

Notonlywhen turning a switch, an arc between the contacts also breaks out due to a vibration of movable contact plates of the switches by applying a strongshocktoa bodyoftheelectricheatingappar- ratus which is being plugged in an electrical outlet.

This also brings about a short-circuitin the electric heating apparatus.

An objectofthe invention isto provide an automatic-changeovertype electric heating apparatus which overcomes the problems and deficiences of the above-described known apparatus.

Another object of the invention is to provide an automatic changeover type electric heating appar- atuswherein an arc doers not breakout between contacts or a circuit is not shorted when switches are turned.

Still another object of the invention isto provide an electric heating apparatus wherein an arc does not break out between contacts ora circuit is notshorted when a contact plate is vibrated by receiving a strong shockwhile plugging in an outlet.

The above object is accomplished by providing an electric heating apparatus including a first heater, a second heater and a plurality ofswitchesto connect the heaters in parallel orin series to an outlet, being characterized in that the electric heating apparatus further includes a switch turning meansforturning the switches to connectthe heaters in series when a voltage of power source is above a prescribed value, and turning the switches to connectthe heaters in parallel when a voltage of an outlet is below a prescribed value.

In accordance with the above electric heating apparatus, when a voltage of supplied power is above the prescribed value, the turning meansturns each ofthe switches and connect the heaters in series. When a voltage of supplied power is below the prescribed value, the turning means turns each of the switches and connect the heaters in parallel.

The above object is accomplished by providing an electric heating apparatus including a first heater, a second heater and a plurality of switches to change a connection of the heaters from parallel to series or from series to parallel, being characterized in that the plurality of switches comprise a normal close switch and a changeover switch having a normal close first contact and a normal open second contact, and one end ofthe first heater is connected to one contact of the power source and another end ofthefirst heater is connected to anotherterminal ofthe power source through the normal close switch. An end ofthe second heater is connected to another terminal of the power source, and another end ofthe second heater is connected to a common contact of the changeover switch.The first contact ofthe changeover switch is connected to the one terminal ofthepowersource and the second contact thereof is connected to the other end of the first heater. The electric heating apparatus further includes a turning means to turn offthe normal close switch andthereafterturnthe changeover switch to the second contact, when a voltage ofthe power source is above the prescribed value.

In accordance with the above electric heating apparatus, when a voltage of supplied power is below the prescribed value, both heaters are maintained in a parallel connection, and when a voltage of supplied power is above the prescribed value, the turning meansfirstturns off the normal close switch.

At that state, the first heater is not energized and both ends of the first heater is in the same electric potential. Thereafter, the turning means turns the chan- geover switch from the first contact to the second contact. Therefore, an arc does not breakoutwhen the switches are turned.

The above object is accomplished by providing an electric heating apparatus including a first heater a second heater and a plurality of switches to connect the heaters in parallel or in series to an outlet, being characterized in that a protective resistor is seriesly inserted at least in one interval selected from the group consisting of an interval between each switch and a terminal of power source in which a heater is not inserted, and an interval between switches in which a heater is not inserted.

In accordance with the electric heating apparatus, current-flow is limited by the protective resistance seriesly inserted in the interval even if an arc breaks out due to a vibration of a movable contact plate of the switch.

The advantages of the invention will become further apparent from the following description taken in conjunction with the accompanying drawings, in which: Figure 1 is a sectional view of a hair dryer; Figure2 is an enlarged perspective view of a part; Figure3isa circuit diagram; Figure4 is another embodiment of the circuitdiagram; Figure 5 is a sectional view of a hair dryer of another embodiment; Figures 6and 7are circuit diagrams; Figures 8to 10 are circuit diagrams wherein a prot ective resistance is inserted, and Figure 11 is a circuit diagram of an electric heating apparatus ofthe prior art.

The first preferred embodiment of the electric heating apparatus of the invention is described below in detail with reference to Figures 1 to 3. The embodiment relates to a hair dryer, and more particularlyto a so-called curl dryerto curl hair by connecting an attachment of comb, brush, orthe like to a stickshaped main bodyofthe dryer. Figure list cross sectional view ofthe hair dryer, and Figure 2 is an enlarged perspective view of a part thereof. The hair dryer 1 has a slender cylindrical case 5, and an air outlet2 and air inlet3 are providedonthefrontface and the rearface ofthe case 5, respectively. On the inlet-side ofthe cylindrical case 5, a grip section 4 is formed. On the outlet-side within the cylindrical case 5, heater unit6 is disposed.The heater unit 6 includes an insulated boards 7 which are combined crosswise, a first heater 8 and a second heater 9 which are wound on the insulated boards 7, and a device ora limiter lotto preventtemperaturefrom rising. Between the air inlet3 ofthe cylindrical case 5 and the heater unit 6, a motor holder 11 is disposed, to which a motor 12 is fixed. On the axle ofthe motor 12 is mounted afan 13. To one end ofthe motor 12 atthe heater unit 6-side, an attaching board 14 is secured.

On the board 14, as illustrated in Figure 2, capacitors 15for noise suppression, diode 16 for rectifying the supplied power and triac 40 are attached. The diame teroftheattaching board 14isequaltoorslightly smaller than the outerdiameterofthe motor 12for reduction of resistance from a wind pass. Almost part ofthe triac 40 is projected from an outer edge of the attaching board 14, so thatthe triac is cooled with wind flowing along the outer body of the motor 12.

The numeral 1 4a denotes a screw to secure the attaching board.

In the cylindrical case 5, a dividing wall 19 integrally provided on the case forms a space 20.An edge 21 on the motor 12-side ofthe dividing wall 19 is inclined toward the interior wall of the cylindrical case 5, to focus the wind which is conducted from the air inlet3 and force the wind to blow on thetriac40. inside the space 20, a printed board 22 is disposed, on which electric parts are attached.

Referring to Figure 3 which illustrates an electric circuit diagram, an electric circuit ofthe hair dryer will be explained. An ac-electric voltage of 1 00Vto 120Vor200Vto 240V is supplied from powerinput terminals 24a and 24b ofthe ac-power source. The supplied ac-voltage is applied to a motor 12, heaters 8and 9 and aturning circuit 25 as aturning means through a limiter 10 and a power switch 30.

A broken line 18 encloses a motor 12 and motor driving circuit. The motor driving circuit consists of smoothing capacitors 15 and rectifying bridge dio des l6connectedto a partofafirst heater8 in parallel for supplying a dc-powerto the motor 12.

A half-wave rectifying diode 32 isto supply the turning circuit 25 a dc-voltage.

A broken line 17 enclosestheturning circuit 25, a triac 40 as the normal close switch, and a relay switch 28 as the changeoverswitch.

To an inputterminal 29a oftheturning circuit25,a voltage-regulator diode 43 is connected through a voltage-dropping resistance 41. In parallel to the voltage-regulator diode 43, a relay coil 27 and SCR (thyristor) 26 are connected both of which are con nectedeachotherinseries. Bytheconnection,the relay coil 27 is actuated under a constant voltage. The numeral 42 denotes a smoothing capacitor.

To the input terminals 29a, 29b of the turning cir cu it 25, there is connected voltage-dividing resistors 33 and 34 both of which are connected each other in series. A dividing point ofthe dividing resistors 33 and 34 is connected to a gate of the SCR 26 th rough anothervoltage-regulator diode 35. A dividing ratio ofthe dividing resistors 33 and 34 is instituted so that the voltage-regulator diode 35 reaches a zenervoltagewhen the supplied voltage becomes 1 50V. A junction point ofthe realy coil 27 to the SCR 26 is connected to a base of a transistor36 as the electrical switching means th rough a bias resistor 37. Between a base ofthetransistor36 and the inputterminal 29b, a resistor 31 is provided.In a normal state, current flows through the relay-coil 27, resistor 37 and resis tor 31, wherein resistance of the resistors 37 and 31 are instituted so that the above current does not reach a value enough to actuate the relay coil 27. A collector of the tra nsistor 36 is connected to an input terminal 29a through resistor 38 and 39. An emitterof the transistor 36 is connected to the input terminal 29b. Ajoint of the resistors 38 and 39 is connected to a gate of the triac 40.

An end ofthe heater 9 is connected to the power input terminal 24b, and another end thereof is con nectedto a power input terminal 24a through the triac 40. An end of the heater 8 is connected to the powerinputterminal 24a, and anotherend thereof is connected to a common contact 28e of the relay switch 28,Anormal closefirstcontact28a oftherelay switch 28 is connected to the power input terminal 24b, and normal open second contact 28b is connected to the other end of the heater9.

An operation of the above-mentioned electriccircuit configuration will be described hereinafter. In the case that ac-100V is supplied to the power input terminals 24a and 24b, a potential of dividing point of the resistors 33 and 34 is low, and the voltage regulatordiode35 does not reach a zenervoltage.

Thus, the gate potential ofthe SCR 26 is lowandthe SCR 26 is in an off-state. The relay coil 27 is not actuated, and the common contact 28c is still connected to the first contact 28a, as shown in the solid line of Figure 3. On the other hand, thetransistor36 is turned on through the relay coil 27 and bias resistor 37, and a potential of dividing point ofthe resistors 38 and 39 is lowered. Therefore, a trigger potential is applied to the gate ofthetriac40 and thetriac40 is turned on. In conclusion,thefirst heater8 and the second heater9 are connected each other in parallel to the power input terminals 24a and 24b, and ac 1 00V is supplied to each heater.

In case th at ac-200V is supplied, a potential of the dividing point ofthe dividing resistors 33 and 34 becomes relatively high in comparison with the case of ac-1 00V, and both ends ofthe voltage-regulator diode 35 reaches a zenervoltage. Thus,thevoltage- regulator diode 35 is turned on, and the gate potential ofthe SCR 26 rises to turn on the SCR. In response to turn-on of the SCR, a base potential ofthe transistor36 is low and thetransistor36 still remains in an off-state. Therefore, a potential ofjunction point ofthe resistors 38 and 39 is high, and trigger potential of the triac 40 is not applied, and the triac 40 is kept in an off-state.On the other hand, the relay coil 27 is actuated and a movable contact plate ofthe relay switch 28 is, as shown by a broken line in Figure 3,connected to the second contact28b. Bythe oper- ation mentioned above,the heater9 and8 arecon- nected each other in series relationship.

Resistances ofthe heaters are, for example, 61 ohms in thefirst heater8, and 77 ohms in the second heater 9. Where a supplied voltage is 1 00V, shunt re- sistanceofboth heaters becomes 34 ohms, and a waste of electric power is about 290W. Where a supplied volatage is 200V, series resistance of both heaters becomes 138 ohms, and a waste of electric power is about 290W. Both wastes of power are nearly the same.

The above embodiment can be modified as in Figure4 in which the heaters 8,9 and the switch 28 are connected in a different way. One end ofthefirst heater 9 is not connected to the terminal 24b ofthe powersource butto the first contact 28a ofthechan- geoverswitch28,thecommoncontact28cofthe changeover switch 28 is not connected to the other end ofthe second heater8 buttotheterminal 24b of the power source, and the first contact 28a ofthe changeover switch is not connected to the terminal 24b but to the other end of the second heater 8.

The heaters 8,9 are normally connected each other in parallel, and when ac-200V is supplied, the triac40 remains in an off-state, and the switch 28 is closed to the second contact 28b-side to connect the heaters 8, 9 each other in series,just in the same manner as in the embodimentdesribed referring to Figure 3.

In the embodiments in Figures 3 and 4, a condition of connection of the first heater9 and the second heater8 can be automatically changed depending on a voltage supplied, so that it is not necessaryto checka service voltage before using a hair dryer, as has been done in the prior art. Therefore, there is no danger of overheating the heater by a miss operation, so one can use the hair dryer with safety.

In the case of supplying 200V, the triac 40 still remains in the off-state afterturning on the power switch 30, and a short-circuit does not take place by an occurrence of arc between the contacts 28a and 28b in the momentthatthe relay switch 28 isturned, because the electric potentials ofthe contacts 28a and 28b arethesame atthat moment. This realizes a longer lifetime ofthe relay switch.

Further, byonce supplying atriggersignal to the gate of the SCR 26 from the dividing resistors 33 and 34, the SCR 26 maintains the initial state until the power is cut off. Therefore, even in a district where a voltage of 1 50V is used e.g. in Colombia, a chattering does nottake place in the heating apparatus ofthe embodiment. There is also no problem of fusing the contact ofthe switch.

Still further, the heating apparatus ofthe embodiment includes one relay and a triac, although two relatively large relays are necessary in the con ventional heating apparatus. That is to say, the triac in the instantembodiment replaces one of the relays ofthe conventional heating apparatus. Thus, a smal lerspace for housing electric components is necessarythan in the conventional case. This is because the embodiment can be preferably applied to an electric heating apparatus such as the curl dryer whose peripheral diameter is small enough to be gripped, so that a housing space for electric components is very narrow.

In the next, a second preferred embodiment of the electric heating apparatus of the invention will be described with reference to Figures 5 and 6.

The embodiment relates to a hair dryer of so-called pistol-shaped,wherein a handle is disposed per pendiculartoadirection of blowing the wind.

Figure5 is a sectional view of the above-mentioned hairdryer5l,wherein the hair dryer 51 has a cylindrical case 52, and an air outlet 57 and air inlet 55 are disposed on thefront and on the lateral side ofthe case 52, respectively. In the lower part of the case 52, a handle 54 is formed. On the outlet-side ofthe interior ofthe case 52, a heater unit 60 is disposed. The heater unit 60 is composed of heaters and a device or a limiter to preventtemperature from rising. The numeral 58 denotes a motor disposed on the outlet 55-side, and a fan 59 is mounted on the axle of the motor. Atthe upper part of a space 53 insidethe handle 54, a printed circuit board 63 is fixed.Atthe lower part ofthe space 53, a first relay 64and the second relay 65 are secured to an attaching board 64a. Between the printed board 63 and the attaching board 64a, there is disposed a power switch 67. The numeral 68 denotes a push button ofthe power switch 67.

An electric circuit of the hair dryer wil I be explained. Figure 5 is a circuit diagram. From power input terminals 70a and 70b, ac-voltage of 1 00to 1 20V or 200 to 240V is supplied to a motor 58, heaters 61 and 62 which constitute the heaterunit60, and a turningcircuit78,throughthelimiter75,thermal fuse 74, power switch 67, and the like. Reference 71 is a noise suppressing capacitor.

The power switch 67 is composed of switches 67a and 67b. The switch 67a turns on/off of the power and the switch 67b changes "high"/"low" ofatem- perature of wind. One depression of the push button 68 makes the switch 67a turn on and another depression of the push button 68 makes the switch 67bturn on. Where the switch 67a is closed and the switch 67b is opened, a rectifying diode 72 and a resistor 73 is inserted in the circuit for half-wave rectification, and an effective value of voltage supplied to the heaters is reduced. The wind temperature becomes "low".

Where the switches 67a and 67b are both closed,the rectifying diode 72 and the resistor 73 are shortcircuited and an ac-voltage is applied to the heaters as it is, so that an effective value of voltage is not reduced. The wind temperature becomes "high".

The broken line 79 encloses a motor 58 and a motor driving circuit. The motordriving circuitconsists of a rectifying bridge diodes 76 connected to a part ofthe first heater 61 in parallel, to supply dc-powerto the motor 58.

The broken line 77 encloses a diode 77b, a resistor 77a both of which constitute a half-wave rectifying circuit. A dc-voltage is supplied to theturning circuit through a diode 77b.

The broken line 78 encloses a turning circuit as a turning means. To the input terminal 80a oftheturn- ing circuit, a voltage-regulator diode 79c is connected through voltage dropping resistor 79a. To both ends ofthe voltage-regulator diode 79c, a twoterminal network consisting of relay coils and SCR 81 are connected each other in series. The two-terminal network is so constituted that seriesly connected relay coil 65 and relay switch 65b are connected to the relay coil in parallel relationship. The relay switch 65b is interlocked with the relay coil 64, and is turned offwhen not actuating the relay coil 65 and isturned on when actuating the same. In the connection mentioned above, the SCR 81 operates at a regularvoltage. 79b denotes a smoothing capacitor.

Totheinputterminals80aand80boftheturning ci rcu it 78, dividing resistors 78a and 78b both of which are in series are connected. The dividing point ofthe dividing resistors 78a and 78b is connected to a gate ofthe SCR 81 through avoltage-regulatordiode 78c. A dividing ratio ofthe dividing resistors 78a and 78b is so instituted thatthe voltage-regulator diode 78c reaches a zener voltage when the service voltage becomes 150V.

An end of the heater 61 is connected to the power input terminal 70b and another end thereof is connected to the power input terminal 70a through a normal close relay switch 65c as the normal close switch. The relay switch 65c is interlocked with the relay coil 65, and is turned on when not actuating the relay coil 65 and is turned offwhen actuating the same. An end of the heater 62 is connected to the power inputterminal 70a, and another end thereof is connected to a common contact 66e of the relay switch 66. The first contact 66a ofthe relay switch 66 is connected to the power input terminal 70b, and the second contact 66b is connected to the other end of the heater 61. When the relay coil 64 is not actuated, the common contact 66c is closed to the first contact 66a, and when the relay coil 64 is actuated,the common contact 66c is closed to the second contact 66b.

An operation ofthe electric circuit mentioned above will be described hereinafter. In the case that ac-1 00V is supplied to the power input terminals 70a and 70b, a potential ofthe dividing point of the dividng resistors 78a and 78b is low, and the voltageregulator diode 78c does not reach a zenervoltage.

Then a gate potential ofthe SCR 81 is low and the SCR 81 is kept in an off-state. The relay coil 65 is not actuated and the normal close relay switch 65b is closed. On the other hand, the relay switch 65b is opened and the relay coil 64 is not actuated, so thatthe relay switch 66 is closed to the normal close first con tact66a. Thus,the first heater61 and the second 62 are connected in shuntand ac-100Vis respectively supplied to the heaters.

In the case that ac-200V is supplied, a potential of the dividing point of the dividing resistors 78a, 78b is raised up, and the voltage-regulator diode 78c reaches a zenervoltage and isturned on, sothata gate potential ofthe SCR 81 rises to make the SCR 81 on. Responsive to theturn-on ofthe SCR 81,the relay coil 65 is energized, and the relay switch 65b is closed and the relay switch 65c is opened. After closing of the relay switch 65b, the relay coil 64 is energized, and the relay switch 66 is connected to the second contact 66b. Thus, the first heater 61 and the second heater 62 are connected in series.

The above embodiment can be modified as in Figure 7 in which the heaters 61,62 and the switch 66 are connected in a different manner. One end ofthe heater61 is not connected to the terminal 70b butto the first contact 66a of the changeover switch 66, the common contact66c of the changeoverswitch 28 is not connected to the other end ofthe second heater 62 but to the terminal 70b, and the first contact 66a is not connected to the terminal 70b but to the other end ofthe second heater 62.

The heaters 61,62 are normally connected each other in parallel, but when ac-200V is supplied, the rel ay switch 65c is first opened a nd su bsequently the relay switch 66 is closed to the second contact 66bside by the relay coil 64which is actuated by turn-on ofthe relay switch 65b, so that the heaters 61,62 are connected each other in series, just in the same manner as in the embodiment in Figure 6.

In accordancewith the embodiments in Figures6 and 7, connection ofthefirst and the second heaters can be automatically changed, and one need not make sure of a service voltage before using the hair dryer, as in the conventional case. Thus, there is not a danger of overheating the heater by a missoper- ation, so one can use the dryer with safety.

Further, the relay switch 66 operates only afterthe relay switch 65c is entirely opened. Therefore, an arc is not generated between the contacts 66a and 66b of the relay switch 66, and the supplied power is not short-circuited. This is because the electric potentials ofthe contacts 66a and 66b are the same when the switch 66 is turned.

Still further, by once supplying a trigger signal to the gate ofthe SCR 81 from the dividing resistors78a and 78b,the SCR 81 maintains the initial state until the power is cut off. Therefore, even in a district where a voltage of 1 50V is supplied e.g. in Colombia, a chattering does nottake place in the heating apparatus ofthe embodiment. There is also no problem of fusing the contacts of the switch.

Referring now to Figures 8 and 9, otherembodiments are described hereinafter where a protective resistor is added to the electric circuit ofthe embodiments in Figure 3 and Figure 6 ofthe invention, respectively.

Figure 8 depicts a circuit where a protective resistor 44 is inserted between the first contact 28a ofthe relay switch 28 and the power input terminal 24b in the circuit ofthe embodiment in Figure 3.

The hair dryer is used at a service voltage of 1 00V, when the hair dryer is dropped by mistake. Then, a movable contact plate of the relay switch 28 vibrates and moves between the contacts 28a and 28b. The contacts 28a and 28b are shorted by an arc generated through the movable contact plate. If the protective resistor 44 is not inserted, an over current flows in a short instance (during a few milliseconds corresponding to a half cycle of the ac-current), but the protective resistor 44 is inserted to limit the flow of current.

A value of the protective resistor 44 is, for example, about 3 ohms, while the first heater 9 is 61 ohms and the second heater is 77 ohms.

It is preferable that the protective resistor is disposed in the wind pass from a view point of effective use ofthe generated heat from the resistor. For instance, the protective resistor 44 is attached to the insu lated board 7 of a cross shape, as shown by twodott chain line in Figure 1.

Figure 9 depicts a circuit wherein a protective resistor 83 is inserted between the relay contact 66b and the heater 61, and another protective resistor 84 is inserted between the relay switch 65c and the heater 61, in the circuit ofthe embodiment in Figure 6.

As in the same manner in Figure 8, the protective resistors 83 and 84 prevent a short of circuit caused by an arc current generated by a vibration ofthe movable contact plate of the relay switch 66.

The protective resistor is preferably disposed in the wind pass to effectively use the generated heat as in the embodiment mentioned above.

In addition, the inserted position of the protective resistor is not limited to the above embodiments, and the protective resistor can be seriesly inserted in either interval through the power input terminal 24a, triac40 and relay contact28b, or interval through the relay contact 28a and powerinputterminal 24b, in Figure 8, and can also beseriesly inserted in either interval through the power input terminal 70a, relay switch 65c and relay contact 66b, or interval through the relay contact 66a and power input terminal 70b, in Figure 9.

Another embodiment for inserting the protective resistor can be schematically illustrated in Figure 10, wherein one end ofthe first heater 91 is connected to oneterminal 97b ofa powersource,a switch 93 is inserted between the other end of thefirst heater91 and the otherterminal 97a ofthe powersource,one end ofthe second heater92 is connected to the other end ofthe first heater 91, the other end of the second heater92 is connected to the common contact 94c of the changeover switch 94, the first contact 94a ofthe changeover switch 94 is connected to theterminal 97b, and the second contact 94b ofthe changeover switch 94 is connected to the other term ina 97a of the power source. Protective resistors 95 and 96 are respectively inserted in an interval between the powerterminal 97a and the second contact 94b, and in an interval between the powerterminal 97b and the first contact 94a. However, in this embodiment, a protective resistor is not necessarily inserted between the switch 93 and the terminal 97a,though none of the heater is inserted between the switch 93 and the terminal 97a.

The protective resistors 95 and 96 prevent a short of circuit caused by an arc current generated bya vibration of a movable contact plate of the switch 94, just in the same manner in Figure 8.

While the present invention has been described hereinabove in detail with reference to the accompanying drawings, it should be understood that the present invention is not limited to the specific embodiments, and various changes and modifications are possible without departing from the scope and spirit of the invention.

Claims (15)

1. An electric heating apparatus having a first heater, a second heater, a plurality of switches to connectthe heaters to each other in series orin par allel to a power source and a controlling means including a voltage-detecting means, to control the switches so that the heaters are automatically connected to each other in series when the voltagedetecting means detect the voltage ofthe power source to be above a prescribed value, and control the switches so that the heaters are automatically connected to each other in parallel when the voltagedetecting means detects the voltage ofthe power source to be below the prescribed value.

2. An electric heating apparatus having a first heater one end ofwhich is connected to one terminal of a power source, a normally closed switch inserted between the other end of the first heater and the otherterminal ofthe power source, a second heater, one end of which is connected to the otherterminal ofthe powersource, and a changeover switch hav ing afirstcontact, a second contact, and acommon contact which is normally closed to the first contact, said common contact being connected to the other end of the second heater, said first contact being connected to said one terminal of the power source, and said second contact being connected to said other end ofthe first heater and a turning means including a voltage-detecting means, to turn off said normally closed switch and thereafter turn said changeover switch to the second contact when the voltage-detecting means detects the voltage ofthe power source to be above a prescribed value.

3. An electric heating apparatus according to claim 2, wherein said normallyclosed switch is constituted by a triac and said changeover switch is constituted by a relay switch, and wherein said turning means includes a relay coil to turn the relay switch, and an electronic switching means to turn the triac.

4. An electric heating apparatus according to claim 3, wherein said electronic switching means is constituted by a transistor.

5. An electric heating apparatus according to claim 2, wherein said normally closed switch is a first relay switch and said changeover switch is a second relay switch, and said turning means includes a first relay coil to turn the first relay switch and a second relay coil to turn the second relay switch, said second relay coil being actuated after the first relay coil is actuated.

6. An electric heating apparatus according to claim 5, wherein said first relay coil turns a third relay switch, and said second relay coil is actuated when the third relay switch is turned.

7. An electric heating apparatus including a changeover switch having a first contact, a second contact, and a common contact which is normally closed to the first contact, a first heater one end of which is connected to thefirstcontact ofthe changeover switch, a normally closed switch inserted between the other end of the first heater and oneterminal of a powersource, a second heater one end of which is connected to said one terminal of the power source, said common contact being connected to the otherterminal ofthe power source, said first contact being connected to said other end ofthe second heater and said second contact being connected to said otherendofthefirstheater;; and wherein said elec tricheating apparatus further comprises a turning meansincludingavoltage-detecting means,toturn off said normally closed switch and thereafterturn said changeover switch to the second contact when the voltage-detecting means detects the voltage of the power source to be above a prescribed value.

8. An electric heating apparatus according to claim 7, wherein said normally closed switch is constituted byatriacand said changeover switch is constituted by a relay switch, and said turning means includes a relay coil to turn the relay switch, and an electronic switchig means to turn the triac.

9. An electric heating apparatus according to claim 8, wherein said electronic switching means is constituted by a transistor.

10. An electric heating apparatus according to claim 7, wherein said normally closed switch is a first relay switch and said changeover switch is a second relay switch, and said turning meansincludesafirst relay coil to turn the first relay switch and a second relay coil to turn the second relay switch, said second relay coil being actuated afterthefirst relay coil is actuated.

11. An electric heating apparatus according to claim 10, wherein said first relay coil turns a third relay switch, and said second relay coil is actuated when the third relay switch is turned.

12. An electric heating apparatus having a first heater, a second heater, and a plurality of switches to connect the heaters to each other in series or in parallel to a power source and wherein a protective resistoris serially inserted at least in an interval selected from the group consisting of an interval between each switch and aterminal of the power source in which none ofthe heaters is inserted, and an interval between the switches in which one of the heaters is inserted.

13. An electric heating apparatus according to claim 12, wherein said protective resistor is used as a heater.

14. An electric heating apparatus according to claim 12, wherein said switches are constituted by a switch and a changeover switch havingt a first contact, a second contact, and a common contact; and one end of said first heater is connected to oneterminal of the power source, said switch is inserted between the other end of the first heater and the other terminal of the power source, one end ofthe second heater is connected to said other end of the first heater, the other end ofthe second heater is connected to said common contact ofthe changeover switch, said first contact of the changeover switch is connected to said one terminal of the power source, and said second contact of the changeover switch is con nectedtotheotherterminal of the power source; and said protective resistor is serially inserted at least in an interval selected from the group consisting of an interval between the first contact ofthe changeover switch and said one terminal ofthe power source, and an interval between the second contact ofthe changeover switch and the other end ofthe power source.

15. An electric heating apparatus substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 10 of the accompanying drawings.