GB2091503A - Electric circuit for the multistage operation of heating resistors - Google Patents

Abstract

An electric circuit for the multistage operation of heating resistors comprises a pair of parallel- connected heating resistors 5a, 6a, a direct-current fan motor 11, powered via a diode bridge 10, and a pair of further heating resistors 5b, 6b connected in parallel across the input terminals of the diode bridge 10. Each heating resistor 5a, 6a, is connected in series with a respective one of the further resistors 5b, 6b by a respective voltage tap 7, 8 which are connected in parallel. Independently operable switches 3, 4 are provided for resistors 5a, 6a respectively. If the resistors 5a, 6a have different resistance values, three-stage operation is possible, i.e. with resistor 5a connected in the circuit, with resistor 6a connected in the circuit or with both resistors 5a, 6a connected in the circuit, the motor 11 receiving a corresponding voltage in each case, so that a constant air outlet temperature can be obtained for all three stages. Resistors 5b, 6b may be replaced by a single heating resistor (9, Fig. 2, not shown). <IMAGE>

Description

SPECIFICATION Electric circuit for the multistage operation of heating resistors This invention relates to an electric circuit for the multistage operation of at least two parallelconnected heating resistors, separately switchabli by means of contacts with a voltage tap on one of the heating resistors for supplying a low voltage direct current motor across a diode bridge.

Such circuits are known and are frequently used in domestic appliances, for example hair driers, air humidifiers, fan-forced heaters and the like. In all these appliances an air flow delivered ba a motor is to be heated, two different heating stages being attained in that the heating resistor i operated with the voltage tap only or together with the second heating resistor. The second heating resistor cannot be operated alone because the motor would then be without a power'supply, so that no air would be conveyed. This would lead to overheating and destruction of the appliance.

In normal two-stage operation it is an important disadvantage that the motor speed and consequently the air quantity conveyed is constan and the outflowing air is heated to a greater or lesser extent. For obviating this disadvantage it is known from the prior art to vary the supply voltage for the low voltage direct current motor as a function of the momentary operating stage for modifying the speed and consequently adapt the airflow delivered to the particular calorific power.

This is brought about by a second voltage tap on the first heating resistor to which the motor can b switched by an additional contact, as a function a the operating stage. It is also possible to provide the motor with a series resistor, which is switchec in by an additional contact in the case of the lowe heating stage. Both these solutions require an additional contact. This additional contact must necessarily be operable with the contacts for the heating resistors, so that no defective operating states can be switched.

However, in the case of such domestic appliances there is also generally a need for a thiri operating stage. This has already been obtained ir the prior art by supplying the complete alternatins current supply for the circuit across a blocking diode, which can be bridged by a further additions contact and therefore rendered inoperative. The electric power of the appliance can be halved by the blocking diode to provide a third operating stage, because it only permits the passage of one half-wave of the alternating current. Apart from the additional costs, this has further serious technical disadvantages. Thus, in this operating stage the motor is only supplied with the very considerably pulsating current of the past halfwave.In the case of the low voltage direct current motors used, this leads to considerable wear and this has hitherto only been accepted, because of the much higher costs which would otherwise be involved, where such a control is permitted.

Thus, the control leads to considerable interference in the power supply network, particularly if significant power levels are reached.

The latter may even occur with very small appliances if a sufficient number thereof are operated simultaneously in neighbouring households. For this reason many countries do not permit appliances with high consumption ratings.

In addition, high technical expenditure is required for interference suppression purposes on such appliances (anti-interference capacitors).

The present invention aims to overcome the abovementioned disadvantages by providing an electric circuit of the aforementioned type in which the motor output is automatically adapted to the particular heating stage, without an extra contact being necessary for this. A further aim is to permit three operating stages with two heating resistors having in each case one contact, the motor also operating in a three-stage manner.

According to one aspect of the invention, there is provided an electric circuit for the multistage operation of at least two parallel-connected heating resistors separately operable by means of a respective contact in each case, said circuit having a low voltage tap at one of the heating resistors for supplying a low voltage direct current motor across a diode bridge wherein a second voltage tap is provided on the second heating resistor which is connected to the diode bridge parallel to the first voltage tap.

Another aspect of the invention provides an electric circuit for the multistage operation of heating resistors, said circuit comprising at least two parallel-connected heating resistors, a directcurrent motor, a diode bridge having its output terminals connected across said direct-current motor and at least one further resistor connected across the input terminals of the diode bridge, wherein each of said parallel-connected heating resistors is connected to a, or a respective, voltage tap between the said further resistor(s) and the diode bridge and wherein each said heating resistor is separately operable by means of a respective contact switch.

As a result of the electric circuit according to the invention, the low voltage direct current motor is in each case supplied with an operating voltage corresponding to the overall calorific power of the appliance, without an extra contact being necessary. The operating voltage for the motor results from the voltage drop at the two heating resistors behind the voltage tap, the total resistance thereof corresponding to the second of Kirchhoff's Laws being decisive. However, this voltage drop, together with the calorific power of the appliance is dependent on the total heating resistance occurring in operation upstream of the voltage taps and which is only dependent on the two contacts. If only one of the heating resistors is switched on, the calorific power and simultaneously the voltage drop for the motor are low.If both heating resistors are switched on, the calorific power is high, as is the voltage drop for the motor. The supply voltage for the motor is therefore adapted to the momentary calorific power of the appliance without additional switching processes or the like being necessary and without having to incorporate into the circuit elements causing wear or additional expenditure.

Unlike in the prior art it is possible with the electric circuit according to the invention to operate each of the two heating resistors ' separately because the motor is supplied with power in each case. Thus, in the case of the circuit according to the invention, the two heating resistors can be given different resistance values so that a third operating stage is directly obtained without requiring additional components.

Thus, there are two operating stages when using in each case one of the two heating resistors, whilst the third operating stage occurs when the two resistors are jointly used. However, for all three operating stages the voltage drop decisive for the motor is directly proportional to the total calorific power of the appliance, so that a constant air outlet temperature is directly obtained because the speed and therefore the delivery efficiency of the low voltage direct current motor is directly dependent on its operating voltage resulting from the voltage drop.

It is particularly pointed out in this connection that half-wave operation does not take place in any of the three cases, as was required in the prior art for obtaining a third (and even a second) operating stage. The upstream connection of such a blocking diode would naturally also be possible with the circuit according to the invention. The number of operating stages would then be doubled to six, but half-wave operation would have to be accepted.

According to one embodiment of the invention, the two heating resistors comprise in each case two series-connected partial heating resistors, to whose electrical connection is connected in each case a respective voltage tap. The voltage taps need not be passed out of the heating resistors and can instead be directly available for the circuit, e.g. on a plate.

The constructional expenditure is further reduced if the two partial heating resistors provided parallel to the diode bridge are jointly formed by a single heating resistor which leads to a reduction in the number of elements, so that the appliance can therefore be made more cheaply.

This is possible without impairing operation because said two partial heating resistors are always only operated jointly in a parallel connection.

The invention will now be further described, by way of example, with reference to the drawings, in which: Fig. 1 is a circuit diagram of one embodiment of an electric circuit according to the invention; and Fig. 2 is a circuit diagram of a second embodiment of an electric circuit according to the invention.

Fig. 1 shows two terminals 1, 2 to which can be applied an a.c. voltage. A line leads from terminal 1 to switching contacts 3 and 4. A line is passed from the contact 3 across a heating resistor comprising two series-connected partial heating resistors 5a, 5b to the terminal 2. In the same way, a line leads from the contact 4 across a second heating resistor comprising partial heating resistors 6a and 6b to the terminal 2.

Between the partial heating resistors 5a, Sb on the one hand and 6a, 6b on the other is provided, in each case, a respective voltage tap 7, 8 jointly connected to a supply connection of a diode bridge 10. A second supply connection of the diode bridge 10 is connectd to the terminal 2. A low voltage direct current motor 11 is connected across the output connections of the diode bridge 10.

The following operation takes place with this circuit. On operating the first contact 3 current flows from the terminal 1 across the partial heating resistor 5a and partial heating resistor Sb and 6b to the terminal 2. Between the voltage taps 7, 8 and the terminal 2 there is a voltage drop which is applied to the diode bridge 10 and which is decisive as a supply voltage for the motor 11.

The calorific power of the appliance results from the resistance of the two parallel-connected partial heating resistors Sb, 6b in accordance with the second of Kirchhoff's Laws and the partial heating resistor 5a in series therewith. The voltage drop applied to the diode bridge is dependent on the partial heating resistors 5a and the total resistance of the partial heating resistors 5b, 6b located between the taps 7, 8 and the terminal 2.

The power consumption of the motor 11 across the diode bridge 10 is considered to be negligibly small in this connection. Thus, the voltage drop decisive for the supply of the motor 11 is in fixed relationship to the total resistance connected in the circuit by the contact 3 and therefore to the momentary calorific power of the appliance.

If, for a second operating stage, the contact 4 is additionally closed, the total resistance of the appliance is reduced by the partial heating resistor 6a arranged in parallel with the partial heating resistor 5a, so that the calorific power of the appliance correspondingly rises. However, the voltage drop between voltage taps 7, 8 and the terminal 2 also becomes higher because the resistance between them remains the same, whereas the resistance between the voltage taps 7, 8 and the terminal 1 is reduced by the switching in of the partial heating resistor 6a. The power of the motor 11 is directly adapted to the increased calorific power of the appliance by a correspondingly higher supply voltage without special measures being required for this.

A third operating stage with a different calorific power is obtained if the contact 4 is closed and the contact 3 is open and if the partial heating resistors 5a and 6a are given different values. This leads to a third total heating resistance for the calorific power of the appliance and to this is once again automatically and directly adapted the voltage drop between the voltage taps 7 and 8 and the terminal 2 for the motor because the change of the total resistance only takes place between the taps 7, 8 and the terminal 1.

Thus, with the circuit according to the invention and only two contacts 3, 4 three different operating values with different calorific powers are obtained in which the speed of the motor 11 and consequently its air delivery efficiency is adapted directly to the calorific power of the appliance. As a result, a constant air outlet temperature can be obtained without difficulty. It is also important that the motor 11 is always operated on a full-wave basis, which ensures a longer life and no additinal elements are required to prevent radio interference.

Fig. 2 shows a further embodiment of the invention in which the parts of the circuit which correspond to those of Fig. 1 are given the same reference numerals and will not be described again. The essential difference compared with the embodiment of Fig. 1 is that the partial heating resistors Sb and 6b are replaced by a single heating resistor 9. Thus, only a single electrical connection is required in the appliance between the partial heating resistors 5a, 6a and the resistor 9 and this simultaneously forms the voltage tap for diode bridge 10.

In principle, the operation is the same as for the embodiment of Fig. 1 because in the latter the two partial heating resistors Sb and 6b are only jointly operated. Thus, once again three operating stages are obtained with an automatic adaptation of the motor speed as a function of the positions of the two contacts 3, 4. One operating stage is obtained in each case by solely operating one of the contacts 3 or 4 and the third operating stage is obtained by the joint operation of contacts 3, 4. As opposed to this, it was not possible in the prior art for the contacts to be individually operated at random because the motor was only supplied by means of a single line branch and therefore the associated contact always had to be closed beforehand.

The invention is not restricted to the abovedescribed embodiments. In particular, more than two parallel-connected heating resistors may be provided if more than three-stage operation of the circuit is desired.

Claims (9)

1. An electric circuit for the multistage operation of at least two parallel-connected heating resistors separately operable by means of a respective contact in each case, said circuit having a voltage tap at one of the heating resistors for supplying a low voltage direct current motor across a diode bridge wherein a second voltage tap is provided on the second heating resistor which is connected to the diode bridge parallel to the first voltage tap.

2. An electric circuit according to claim 1, wherein the two heating resistors have different values, at least up to the voltage taps.

3. An electric circuit according to claim 1 or claim 2, wherein the two heating resistors in each case comprise two series-connected partial heating resistors to whose electrical connection is in each case connected a respective voltage tap.

4. An electric circuit according to claim 3, wherein the two partial heating resistors arranged parallel to the diode bridge are jointly formed by a single heating resistor.

5. An electric circuit for the multistage operation of heating resistors, said circuit comprising at least two parallel-connected heating resistors, a direct-current motor, a diode bridge having its output terminals connected across said direct-current motor and at least one further resistor connected across the input terminals of the diode bridge, wherein each of said parallel-connected heating resistors is connected to a, or a respective, voltage tap between the said further resistor(s) and the diode bridge and wherein each said heating resistor is separately operable by means of a respective contact switch.

6. An electric circuit according to claim 5, wherein a plurality of further resistors are provided, each said further resistor being connected with a respective one of said parallelconnected heating resistors by a respective voltage tap and said further resistors being connected in parallel

7. An electric circuit according to claim 5, wherein a single further resistor is provided which is connected to both or all of said parallelconnected heating resistors.

8. An electric circuit according to any one of claims 6 to 8, wherein the parallel-connected heating resistors have different resistance values.

9. An electric circuit substantially as described herein with reference to the drawings.