GB2092842A

GB2092842A - Power supply circuit

Info

Publication number: GB2092842A
Application number: GB8137884A
Authority: GB
Original assignee: Braun GmbH
Current assignee: Braun GmbH
Priority date: 1981-02-05
Filing date: 1981-12-16
Publication date: 1982-08-18
Also published as: DE3133325A1; GB2092842B; FR2499332B1; DE3133325C2; FR2499332A1

Abstract

In a fan heater, hair dryer (etc), tapped heater resistor 4, 7, dioder 12, 13, 17, 18, a blower motor 20 and independently operable switcher 3, 6, 9, are connected to one another and to the poler 1, 2, of an a.c. power source according to the circuit of Figure 3. If resistor 4 is of higher heating output rating than resistor 7, four stages of operation are possible with successively higher heating power outputs, and different motor speeds the four stages corresponding respectively to closure of a switch 6, b switcher 6, 9, c switcher 3, 9, d switcher 3, 6, 9. In alternative embodiments (Figure 1, 2, not shown), switch 9 is omitted, the anode of diode 13 being directly connected to pole 2, and resistor 7 may be connected to switch 6 via a diode (19) which may be selectively shorted by a switch (19, Fig. 2). <IMAGE>

Description

SPECIFICATION Power supply circuit This invention relates to a circuit arrangement for the power supply of two heating resistors arranged in parallel manner between the poles of an a.c. voltage source and switchable by means of, in each case, a contact and a blower motor connected by means of a rectifier circuit between a voltage tap to at least one of the heating resistors, particularly for hair driers, fan-forced heaters or the like.

Such circuits are known and are frequently used with domestic appliances, e.g. in the case of hair driers, air humidifiers, fan-forced heaters, etc. In all these appliances, an air flow delivered by the blower motor is to be heated, different heating stages being reached in that the heating resistor is 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 and consequently no air would be delivered. This would lead to overheating and destruction of the appliance.

In normal two-stage operation, it is a considerable disadvantage that the speed of the motor and therefore the quantity of air conveyed is constant and as a result the expelled air is heated to a greater or lesser extent. In the prior art, it is conceivable to obviate this disadvantage by varying the supply voltage for the blower motor for speed change purposes, as a function of the momentary operating stage, so that the delivered airflow is adapted to the particular calorific power. This can be brought about by a second voltage tap on the first heating resistor to which the motor is switched over by an additional contact corresponding to the operating stage. It is also conceivable to provide the motor with a series resistor, which is switched on for the lower heating stage.

However, with both these solutions, the constructional expenditure is considerable because at least one additional contact is required. This additional contact must necessarily be operable with the contacts for the heating resistors, so that no defective operating states can be switched.

Apart from the above disadvantages, there is gen eral ly also a requirement for such appliances to have further operating stages. The speed of the blower motor must then always be in the correct relationship to the calorific power to as far as possible ensure a desired air outlet temperature. For this purpose, an electric circuit has been proposed in which the two parallel heating resistors are jointly connected to the rectifier circuit and across a third heating resistor to the second pole of the voltage supply.

In this circuit, the two parallel heating resistors of different thickness can be individually or jointly operated enabling three different operating states to be achieved with the two contacts. In this particularly simple circuit, the blower motor speed is also adapted to the theoretical calorific power. A disadvantage of this circuit is that the third heating resistor which considerably contributed to the overall calorific power and arranged parallel to the rectifier circuit is traversed by varying the powerful currents as a function of the operating stage. Thus, in the case of this circuit which is very advantageous per se, very narrow limits are set in fixing the stages. The reasons for this are that a heating resistor can only be used in an optimum manner in such appliances for a particular electrical power.If only a very low current flows through it, its heating and that of the air flowing by it are inadequate. In the case of high speed currents, there is a natural limit where the heating resistor burns through or the appliance of the parts immediately adjacent to the heating resistor over-heat. Thus, with this circuit the stages can only be fixed within the limits within which at least a certain minimum current flows through the third heating resistor, whilst not exceeding a maximum permitted current.

The problem of the present invention is to provide a circuit arrangement of the aforementioned type with a minimum number of contacts, a minimum making and/or breaking capacity and a maximum number of possible variations in which the two heating resistors can be dimensioned and operated completely independently. The power of the blower motor should be automatically adaptable in a manner which is substantially freely determinable beforehand to the overall heating power of the appliance without additional contacts. In addition, the heating resistors can have widely varying dimensions and can be individually or jointly operated without harmful secondary effects. As a result, at least three different operating stages can be obtained at which the blower motor operates with in each case different speeds.

According to the invention, there is provided a circuit arrangement for the power supply of two heating resistors arranged in parallel manner between the poles of an a.c. voltage source and switchable by means of, in each case, a contact and a blower motor connected by means of a rectifier circuit between a voltage tap to at least one of the heating resistors, particularly for hair driers, fan-forced heaters or the like, wherein the blower motor is connected both to the cathode-side connection of first and second diodes connected on the anode-side with the voltage tap of one heating resistor or to the second pole of the a.c. voltage source, and to the anode-side connection of third and fourth diodes connected on the cathode-side with the voltage tap of the other heating resistor or the second pole of the a.c. voltage source, so that the blower motor can be supplied with a first half-wave of the a.c. voltage applied across the first and fourth diodes or across the second and third diodes with a half-wave of the a.c.

voltage of the same level or of an oppositely directed different level, as a function of the position of the voltage tap.

The solution according to the invention provides a power supply circuit for an electric load with two parallel heating resistors and a blower motor where, with respect to the number of calorific power stages and the blower speed, a maximum number of possible variations are provided for a minimum number of contacts and a minimum making and/or breaking capacity. The two heating resistors can be dimensioned and operated completely independently of one another, the blower motor power being automatically adapted in a manner which is substantially freely determinable beforehand to the overall heating power or the appliance without requiring additional contacts. The heating resistors can have widely varying dimensions and can be individually or jointly operated without harmful side-effects.As a result, at least three different operating stages can be obtained in which the blower motor operates with in each case different speeds.

If, in the case ofthe circuit arrangement according to the invention, the first heating resistor is switched on, only the positive half-wave of the a.c. voltage flows from the voltage tap across the blower motor to the second pole of the a.c. voltage source, said operating voltage from the voltage tap being independently and freely selectable at the first heating resistor. If now the second heating resistor is switched on, only the negative half-wave of the a.c.

voltage flows from the second voltage tap across the blower motor to the second pole of the a.c. voltage source. The operating voltage is dependent on the freely selectable voltage tap at the second heating resistor, which can also be completely differently dimensioned in order to obtain a second heating stage of the appliance. Athird heating stage is obtained through the joint operation of both heating resistors, the calorific power resulting from the sum of the calorific powers of both heating resistors. An operating voltage is thereby also obtained for the blower motor, which is also formed from the sum of the positive half-way at the voltage tap of the first heating resistor and the negative half-wave at the voltage tap of the second heating resistor.In other words, the total calorific power in the third operating stage is the sum of the calorific powers from the first and second stages and the effective operating voltage for the blower motor is the effective or r.m.s.

value of the effective operating voltage of the first and second stages. Thus, the operating voltage for the blower motor varies in two freely determinable ways and a third way necessarily following therefrom with respect to the calorific power.

Preferably, there is a first line from the voltage tap of said one resistor to the second pole of the a.c.

voltage source, and which is passed across the blower motor, in which in said first line upstream and downstream of the blower motor is connected in each case a diode poled in the forward direction to the second pole of the voltage source and from the second pole of the a.c. voltage source to the voltage tap of the said other resistor there is a second line leading in the same direction across the blower motor in which upstream and downstream of said blower motor there is in each case a diode poled in the forward direction to the second voltage tap of the other resistor.

According to a preferred embodiment of the invention, a third contact is connected in series with the contact connected in series with said one or said other heating resistor, said third contact being also connected in parallel with a further diode with a cathode-side connection to the said one heating resistor or an anode-side connection to the said other heating resistor.

As is apparent from the above variant of the circuit arrangement according to the invention, as a result of the three active switching combinations of the two contacts in series with the heating resistors three different speeds for the motor are obtained, because both voltage half-waves can be made varyingly high as a result of the different voltage taps at the two heating resistors. In connection V :th the three different speeds for the motor, there are 4three corresponding heating stages and to these a fourth heating stage can be added by providing the third contact in series with one of the contacts connected in series with the heating resistors. The combination of the third contact with the parallel-connected diode can be inserted, as required, in one or other heating line.

According to another embodiment of the invention the second pole of the a.c. voltage source is connected to the end ofthe said one heating resistor opposite to the associated contact and a third contact is provided in the connection of the second pole of the a.c. voltage source to the anode of the second diode or the cathode of the fourth diode.

This embodiment of a circuit arrangement according to the invention is particularly suitable for electrical hot air appliances with a so-called cold stage where the contact in series with one heating resistor, as well as the additional contact connected in parallel to the diode, are opened and only the contact connected in series with the further contact is closed.

However, in this position, the complete heating current does not flow across the blower motor, so that the air given off by the electric hot air appliance is not completely cold. Furthermore, in the case of this embodiment, an additional diode parallel to the further contact is required. The diodes of the diode bridge which are in any case needed for the operation of the blower motor are used in the so-called cold stage to pass a half-wave through the heating iine. At the same time, the complete heating current is passed through the blower motor, so that four different operating stages with an in each case different motor speed are obtained, whilst only requiring a minimum number of components and heating taps.

Preferably, the two heating resistors are designed for different calorific powers.

The invention will now be further described, by way of example, with reference to the drawings, in which: Fig. lisa circuit diagram of one embodiment of a circuit according to the invention; Fig. 2 is a circuit diagram of a second embodiment of a circuit according to the invention; Fig. 3 is a circuit diagram of a third embodiment of a circuit according to the invention; and Figs. 4 to 7 show the load current pattern for the different combinations of contacts of the embodiment of the circuit arrangement according to the invention shown in Fig. 3.

In the drawings, like parts are denoted by like reference numerals.

The circuit diagram of Fig. 1 shows the two poles 1 and 2 of an a.c. voltage source. A line 8 passes from pole 1 across a contact 3 and a first heating resistor 4 to pole 2. In addition, a line 5 passes from pole 1 across a second contact 6 and a second heating resistor 7 to the second pole 2.

On the first heating resistor 4 is provided a first voltage tap 10 to which is connected a line 11 leading to the pole 2. The line 11 contains two diodes 12, 13 interconnected on the cathode side. A second voltage tap 15 is provided on the second heating resistor 7 and to it is connected a line 16 leading to the pole 2. The line 16 contains two diodes 17, 18 interconnected on the anode side. A blower motor 20 is connected on the one hand to the line 11 between the diodes 12 and 13 and on the other hand to the line 16 between the diodes 17 and 18.

The following operation pattern is obtained with this circuit. On operating the contact 3, current flows from the pole 1 across the first heating resistor 4to the pole 2. There is a voltage drop between the voltage tap 10 and the pole 2, so that during the positive half-wave of the alternating current at poles 1 and 2 an operating current decisive for the blower motor 20 flows from the first voltage tap 10 across the diode 12, the blower motor 20 and the diode 18. During the negative half-wave of the alternating current, such an oppositely poled current flow is prevented by the diodes 12 and 18.

On only operating the contact 6, current flows from the pole 1 across the second heating resistor 7 to the pole 2. There is also a voltage drop between the voltage tap 15 and the pole 2 and during the negative half-wave of the alternating current to the second pole 2, an operating current decisive for the blower motor 20 flows across the diode 13, the blower motor 20 and the diode 17. During the positive half-wave, such an oppositely poled current flow is prevented by the diodes 13 and 17.

In all cases, the operating voltage for the blower motor 20 is directly dependent on the voltage drop on the particular heating resistor 4 or 7 which is in operation. Thus, the two heating resistors 4 and 7 can be completely differently dimensioned and in each case the blower motor 20 receives in each case an operating current freely determinable with respect to the connected calorific power. There is no reciprocal influencing of the heating resistors. This even applies if the two resistors 4 and 7 are operated simultaneously, as will be explained herein before.

If the contacts 3 and 6 are simultaneously closed for the joint operation of both heating resistors 4 and 7, current flows across both the resistors, so that the total calorific power corresponds to the sum of the two individual calorific powers. In this case, the blower motor 20 is supplied with operating current by the first heating resistor 4 during the positive half-wave of the alternating current and by the second heating resistor 7 during the negative half-wave.

The average voltage obtained corresponds to the effective sum of the two average voltages when operating the resistors 4 or7 alone. Thus, here again, the blower motor 20 is supplied with an operating voltage, which is in a predetermined relationship to the calorific power of the appliance.

In the circuit shown in Fig. 2, the same elements are given the same reference numerals as in the circuit shown in Fig. 1 so that a detailed description of the connection between the individual elements is rendered superfluous and instead reference should be made to the description of the circuit arrangement shown in Fig. 1.

In this embodiment, the two heating lines once again supply the blower motor 20, the heating line 8 contributing the positive half-wave and the other heating line the negative half-wave of the alternating voltage applied to the poles 1 and 2 for the purpose of full-wave operation. As the two half-waves can be of varying levels owing to the different voltage taps 10 or 15 on the two heating resistors 4 or 7 and also different rated powers of the individual heating resistors 4 and 7, three different speeds for the motor and also three corresponding heating stages are obtained from the three active switching combinations of the contacts 3 and 6. Afurther contact 14 is connected in series with the second contact 6 and a diode 19 is connected in parallel with the contact 14.The diode 19 has an anode-side connection to the second heating resistor 7 or a cathode-side connection to the second contact 6. This arrangement adds a fourth heating stage to the three heating stages previously described. The further contact 14 can alternatively be connected in series with the first contact 3, the parallel diode 19 being provided with a reverse forward direction, i.e. with a cathode-side connection to the first heating resistor 4.

This further, fourth, heating stage corresponds to a stage in which the first contact 3 and the further contacts 14 are open and only the second contact 6 is closed. The negative half-wave of the a.c. voltage current then passes from the second pole of the a.c.

voltage source 2 across the second diode 13, the blower motor 20, the third diode 17, the voltage tap 15 and part of the heating resistor 7, the diode 19 and the closed contact 6 to the first pole 1 of the voltage source. By means of a secondary path, the negative half-wave passes across the heating resistor 7, the diode 19 and the closed contact 6 to the first pole of the voltage source 1. The drawing makes it clear that in this embodiment of the invention, the complete heating current does not flow across the blower motor 20 and in fact partly flows across the second heating resistor 7, so that the desired cold stage is not obtained to the required extent. To a certain extent, the second heating resistor 7 emits calorific power, so that the air flow given off is not as cool as is desired. In this circuit, an additional diode parallel to the further contact switch 14 is required.

In the embodiment shown in Fig. 3, the second pole of the a.c. voltage source 2 is connected to one end of the first heating resistor 4. A further switch 9 is inserted in the connection of the two heating resistors 4 and 7. Other than this modification, this circuit diagram of Fig. 3 corresponds to that of Fig. 1, so that here again the same components are designated by the same reference numerals, making a further detailed description unnecessary and reference should once again be made to Fig. 1.

In this embodiment of the invention, the diodes 12, 13 and 17, 18 of the diode bridge, which are already provided for operating the blower motor 20, are utilized for ensuring that only one half-wave of the a.c.

voltage applied passes through the heating line in the so-called cold stage. At the same time, the com plete heating current is supplied through the blower motor 20. Here again, a maximum number of switching variants are obtained with a minimum number of contacts and the minimum making and/or breaking capacity. The operation of this circuit arrangement will now be explained in connection with the combinations shown in Figs. 4 to 7.

To make the different operating stages more readily comprehensible, a rated power of 700 Watt is used as the basis of the first heating resistor 4 and 500 Watt for the second heating resistor 7. The voltage taps 10 and 15 are here applied at half the resistance value of the two resistors 4 and 7. In the first of the four different operating stages the, in each case, different speeds of the blower motor 20 shown in Fig. 4, the first and further contacts 3 and 9 are open, whilst only the second contact 6 is closed. In this so-called cold stage position, the negative half-wave of the a.c. voltage applied, the current flows to the first pole 1 of the voltage source across part of the first heating resistor 4, the first diode 12, the blower motor 20, the third diode 17, over part of the second heating resistor 7 and the closed second contact 6.In the above numerical example, the electrical appliance absorbs a power of 250 Watt and gives off an outlet temperature of approximately 41 C. As can be clearly gathered from the drawing, the complete half-wave current passes through the blower motor, which is the explanation of the low outlet temperature in the cold stage.

In the second operating stage shown in Fig. 5, only the first contact 3 is open, whilst the second contact 6 and further contact 9 are closed. In this switch position, the negative half-wave of the feeding a.c. voltage, the current flows from the second pole 2 of the voltage source across the closed further contact 9, the second diode 13, the blower motor 20 and the third diode 17 to the voltage tap of the second heating resistor 7 and also directly from the contact 9 to the termination of the heating resistor 7 and from the resistor 7 across the closed second contact 6 to the first pole 1 of the voltage source.In this position, the electrical appliance has a power consumption of 500 Watt and at its nozzle gives off air at a temperature of approximately 60"C. As can be clearly gathered from the drawing, in this case only part of the heating current passes through the blower motor 20.

In the third operating position shown in Fig. 6, the first contact 3 and the further contact 9 are closed, whereas the second contact 6 is open. When in this switch position, the positive half-wave of the a.c.

voltage applies, the current flows from the first pole 1 of the voltage source across the closed first contact 3 both across the first heating resistor 4 to the second pole 2 of the voltage source, and across the voltage tap of the first heating resistor 4, the first diode 12, the blower motor 20 and the third diode 18 of the diode bridge and the closed further contact 9 to the second pole 2 of the voltage source. Here again, only part of the positive half-wave current flows through the blower motor 20. In this position, according to the aforementioned numerical example the electrical appliance consumes a power of 700 Watt and gives off an outlet temperature of approximately 62"C.

In the fourth operating position, which is also the last position other than the variant with all the contacts open, shown in Fig. 7, all the contacts are closed, so that both in the positive and negative half wave of the a.c. voltage applied current flows across both heating resistors 5 and 7 and across the blower motor 20. The current flow in the positive half-wave of the a.c. supply voltage applied is indicated by the continuous-line arrows, whilst the -un-ent flow in the negative half-wave of the a.c. supply. > voltage applied is indicated by the broken-line arrows. In this position, the electrical appliance consumes the full elec- trical power of 1200 Watt corresponding to the summated output of the two heating resistors and gives off an airoutlettemperature of 80"C.

The described operation of the circuit according to the invention shows how it is possible with simple means to obtain considerable possible variations with a minimum of components used and only two heating taps. By a simple, but surprising measure, a fourth operating position is obtained in the so-called cold stage, in which all the heating current is supplied through the blower motor, so that a correspondingly low outlet temperature of the heated air is obtained.

Claims

1. A circuit arrangement for the power supply of two heating resistors arranged in parallel manner between the poles of an a.c. voltage source and switchable by means of, in each case, a contact and a blower motor connected by means of a rectifier circuit between a voltage tap to at least one of the heating resistors, particularly for hair driers, fan-forced heaters or the like, wherein the blower motor is connected both to the cathode-side connection of first and second diodes connected on the anode-side with the voltage tap of one heating resistor or to the second pole of the a.c. voltage source, and to the anode-side connection of third and fourth diodes connected on the cathode-side with the voltage tap of the other heating resistor or the second pole of the a.c. voltage source, so that the blower motor can be supplied with a first half-wave of the a.c. voltage applied across the first and fourth diodes or across the second and third diodes with a half-wave of the a.c. voltage of the same level or of an oppositely directed different level, as a function of the position of the voltage tap.

2. A circuit arrangement according to claim 1, wherein there is a first line from the voltage tap of said one resistor to the second pole of the a.c. voltage source, and which is passed across the blower motor, in which in said first line upstream and downstream of the blower motor is connected in each case a diode poled in the forward direction to the second pole of the voltage source and wherein from the second pole of the a.c. voltage source to the voltage tap of the said other resistor there is a second line leading in the same direction across the blower motor in which upstream and downstream of said blower motor there is in each case a diode poled in the forward direction to the second voltage tap of the other resistor.

3. A circuit arrangement according to claim 1 or claim 2, wherein a third contact is connected in series with the contact connected in series with said one or said other heating resistor, said third contact being also connected in parallel with a further diode with a cathode-side connection to the said one heating resistor or an anode-side connection to the said other heating resistor.

4. A circuit arrangement according to claim 1 or claim 2, wherein the second pole of the a.c. voltage source is connected to the end of the said one heating resistor opposite to the associated contact and a third contact is provided in the connection of the second pole of the a.c. voltage source to the anode of the second diode or the cathode of the fourth diode.

5. A circuit arrangement according to preceding claim, wherein the two heating resistors are designed for different calorific powers.

6. A circuit arrangement substantially as described herein with reference to the drawings.