EP3143189B1 - Locking device for locking a door of a domestic appliance, domestic appliance, and corresponding method - Google Patents

Description

The invention relates to a locking device for locking a door of a household appliance, with a locking element for mechanically locking the door, with a drive element, in particular an electric coil, for moving the locking element from an unlocking position in which the door is unlocked, in a locking position, in wherein the door is locked, depending on a current flow through the drive element, wherein the drive element is electrically coupled to a DC voltage source, with a switching element electrically coupled to the drive element for controlling the flow of current through the drive element, and with a control device for driving the switching element, wherein the control device is designed to emit a series of drive pulses to the switching element for controlling the current flow through the drive element. The invention also relates to a household appliance with such a locking device and a method for locking a door of a household appliance.

The interest is directed in the present case in particular to a locking device which serves to lock a door of a washing machine. Such locking devices for washing machines are already known from the prior art in various embodiments. Usually, an electromagnet in the form of an electric coil is used, which acts as a drive element for the actual locking bar, which consists for example of a soft magnetic material. If the coil is subjected to electrical voltage, the locking bar or the locking element is moved to a locking position and held in this position in which the door is mechanically locked and therefore can not be opened by the user.

The coil is usually coupled in the prior art directly to a mains connection of the household appliance and thus operated with the mains voltage. As a switching element for controlling the flow of current through the coil, a triac is used, which with Help a corresponding microcontroller is controlled. Such a locking device with a triac is for example from the document EP 0 808 935 A2 known. A disadvantage of using a triac is the fact that the control terminal (gate) of the triac must be at the reference potential of the supply network or at the neutral potential (N). This in turn means that the microcontroller is also at this reference potential. In modern home appliances then a galvanic isolation between the microcontroller and other control devices must be ensured, which have a different electrical reference potential. For example, a motor control device for driving a three-phase brushless DC motor or a synchronous machine must be at a reference potential, which is different from the said neutral potential of the supply network. In order to allow mutual communication between the various control devices, therefore, for example, optocouplers are used, which provide the required galvanic isolation.

A remedy here creates the use of a DC voltage source for locking the door, as for example in the document DE 10 2012 010 929 A1 is described. The coil is incorporated here in an electrical circuit, which is supplied with a DC voltage. This DC voltage is provided for example by a machine-internal power supply, which generates the DC voltage by rectification from a mains AC voltage. The DC voltage is provided in particular by full-wave rectification from the mains AC voltage and may be an approximately amplitude-constant voltage generated by smoothing. In the supplied with the DC voltage circuit are one or more electrical consumers, which may include, for example, an electric drive motor for the washing drum of the washing machine. The current flow through the coil is controlled by means of a transistor which is driven by drive pulses. In order to obtain the desired course of the coil current, the duty cycle of the voltage pulses is varied such that the voltage pulses have a low duty cycle at the beginning of an excitation phase and an increasingly larger duty cycle towards the middle of the excitation phase until a maximum value of the coil current occurs.

If a DC voltage source is used to supply a locking device, however, the problem arises that the DC voltage provided by the DC voltage source can break when the energy is delivered to electrical consumers. This is the case in particular when a capacitor is used as the DC voltage source and the energy for the coil or other load is taken from the capacitor. This removal of electrical energy then causes a voltage drop across the capacitor, which then needs to be recharged. The charging of the capacitor is in turn dependent on the current phase position of the mains voltage. This problem is exacerbated even if additional consumers are supplied with the electrical energy from the DC voltage source. If the transistor is now driven with drive pulses of a predetermined and constant pulse length, then it may happen that the electrical energy or power delivered to the coil is altogether insufficient to bring the locking element into the locking position or to hold it there. Optionally, a reliable operation of the household appliance is then not possible.

It is an object of the invention to provide a solution, as in a locking device of the type mentioned an always reliable locking of the door can be made possible.

This object is achieved by a locking device, by a household appliance and by a method with the features according to the respective independent claims. Advantageous embodiments of the invention are the subject of the dependent claims, the description and the figures.

A locking device according to the invention is designed for locking a door of a household appliance. The locking device comprises a locking element for mechanically locking the door. For example, the locking element is formed of a soft magnetic material. The locking device further comprises a drive element for moving the locking element from an unlocking position, in which the door is unlocked and thus can be freely opened by the user, in a locking position in which the door is locked by the locking element, depending on a Current flow through the drive element. The drive element is electrically coupled to a DC voltage source. The drive element may be, for example, a coil or an electromagnet. The locking device further comprises a switching element - in particular a semiconductor switch - for controlling the current flow through the drive element. The switching element is electrically coupled to the drive element, in particular electrically connected in series with the drive element. A control device is used to control the switching element and thus to control the current flow while delivering at least one drive pulse to the switching element. According to the invention, the locking device has means for detecting a current voltage value of a DC voltage provided by the DC voltage source, and the control device sets a pulse length of the drive pulse as a function of the current voltage value during operation of the locking device.

By setting the pulse length of the current drive pulse and thus the length of the output to the drive element voltage pulse in response to the current voltage level of the DC voltage is achieved that the pulse length of the drive pulse always adapted to the current voltage value and thus to the drive element, a constant electric power and Therefore, sufficient electrical energy can be delivered, with which the locking element can be safely and reliably brought into the locked position. Thus, the pulse length of the drive pulse at a lower voltage value can be set greater than at a higher voltage value in order to always be able to ensure that a sufficient power is delivered to the drive element. Overall, a reliable and reliable operation of the locking device is thus possible, and this using a DC voltage source, which in particular has the advantage over operation of the drive element with mains voltage that the control device does not have to be at the neutral potential of the mains voltage and thus no additional measures need to be taken for a galvanic isolation between multiple controllers.

In the present case, a household appliance is understood to be a device which is used for household management. In particular, the household appliance is a device for the care of laundry items, such as a washing machine, a washer-dryer or a tumble dryer. The door is in particular a device component with which the opening of a laundry drum of the household appliance can be closed.

As already stated, a semiconductor switch whose control terminal is coupled to the control device is used in particular as a switching element. As a semiconductor switch, for example, a PNP bipolar transistor and / or an NPN bipolar transistor and / or an N-MOSFET and / or a P-MOSFET and / or an IGBT can be used.

In one embodiment, it may be provided that the control device controls the switching element with a sequence of pulses using pulse width modulation. Then, the control device can set a current duty cycle "of the drive pulses depending on the current voltage value." The drive pulses can therefore be a periodic sequence of pulses in which the ratio of the pulse duration to the cycle duration is set as a function of the current voltage value on the one hand has the advantage that the sequence of drive pulses can thus be synchronized with the mains voltage without much effort; on the other hand, such a pulse width modulation can be implemented with a periodic sequence of pulses without much effort since the frequency of the drive pulses always remains constant and only the current one Duty cycle dependent on the current voltage value needs to be adjusted.

It proves to be particularly advantageous if the DC voltage source is an intermediate circuit capacitor, which is coupled to an output of a rectifier, in particular a full-bridge rectifier, which is designed to rectify a mains voltage. The rectifier is in particular coupled, on the one hand, to mains connections of the household appliance and, on the other hand, to the intermediate circuit capacitor, so that the rectified rectifier voltage is delivered to the intermediate circuit capacitor and then smoothed with the intermediate circuit capacitor. At the DC link capacitor is thus a rectified and smoothed mains voltage (rms value), which can be used for the control of the locking device. This embodiment has the advantage that an already present in today's household appliances already existing electrical DC link can be used to move the locking element in the locked position or to keep in this locked position. It is therefore unnecessary to use separate DC sources with the associated disadvantages.

In addition, an inverter, which is designed to provide a supply voltage for an electric drive motor of the domestic appliance from the DC voltage, is additionally coupled to the same DC voltage source, in particular to the named DC link capacitor. So it is specifically used the DC voltage source, which otherwise serves to supply the drive motor. In this context, it may in particular be provided that the drive motor is a multiphase-in particular three-phase brushless DC motor (BLDC) or a synchronous machine, so that the inverter generates a plurality of alternating voltages from the DC voltage for the operation of the drive motor. Preferably, this drive motor is used to drive a laundry drum of the household appliance.

It is preferably the relationship that the smaller the DC voltage at the DC voltage source, the greater the pulse length of the one or more drive pulses is set. In other words, at a lower voltage value of the DC voltage, the control device can set a larger pulse length of the drive pulses than at a higher voltage value, so that the pulse length is inversely proportional to the DC voltage at the DC voltage source. By means of such a procedure it can always be ensured that a sufficient electric current flows through the drive element on average, which ensures safe operation of the locking device.

In order to keep constant the mean value of the electric current through the drive element and thus the power delivered to the drive element, the control means may adjust the pulse length of the drive pulses inversely proportional to the square of the voltage value of the DC voltage. Unintentional unlocking the door during operation of the household appliance can thus be safely prevented.

In the control device, for example, a lookup table can be stored, which indicates the dependence of the pulse length of the DC voltage of the DC voltage source. The controller may then adjust the current pulse length using the lookup table. The setting of the pulse length depending on the current voltage value can thus be realized without much computing power.

In addition to said switching element (hereinafter referred to as first switching element), the locking device may also comprise at least one further switching element (hereinafter referred to as second switching element), which is coupled to the drive element and in particular electrically connected in series with the first switching element and the drive element. The control device can be designed so that at least during the drive pulse of the first switching element, the second switching element is opened, so that this second switching element is driven with one or a series of drive pulse having at least the same pulse length as the drive pulses of the first switching element. The or the drive pulses of the second switching element thus overlap with the Antsteuerimpuls or the drive pulses of the first switching element. The use of the second switching element has advantages in terms of reliability: In case of failure of one of the switching elements can be prevented with the other switching element, that the drive element - in particular a coil - is acted upon by a DC voltage. If a defect or an error of one of the switching elements is detected by the control device, a corresponding warning signal can be output.

It can also be provided that a current phase position of the mains voltage is detected by the control device and the sequence of the drive pulses is generated as a function of the phase of the mains voltage. In this way, the sequence of drive pulses can be synchronized with the mains voltage and, for example, it is possible to generate the drive pulses when the DC voltage source is fully charged. Excessive discharge of the DC voltage source can thus be reliably prevented.

An inventive household appliance, in particular for the care of items of laundry, comprises a door and a locking device according to the invention for locking the door.

An inventive method is used to lock a door of a household appliance by means of a locking element which is moved depending on a current flow through a drive element from an unlocked position into a locking position, wherein the drive element is acted upon by a DC voltage of a DC voltage source and the current flow through the drive element by means of a Switching element is controlled, which is controlled by a control device with a sequence of drive pulses. A current voltage value of the DC voltage is detected, and a pulse length of the drive pulses is set by the control device as a function of the current voltage value.

The preferred embodiments presented with reference to the locking device according to the invention and their advantages apply correspondingly to the domestic appliance according to the invention and to the method according to the invention.

Further features of the invention will become apparent from the claims, the figures and the description of the figures. All the features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the respectively indicated combination but also in other combinations or alone.

The invention will now be described with reference to a preferred embodiment and with reference to the accompanying drawings.

Fig. 1

in schematischer Darstellung ein Haushaltsgerät gemäß einer Ausführungsform der Erfindung;

Fig. 2

in schematischer Darstellung eine Schaltungsanordnung mit einer Verriegelungseinrichtung gemäß einer Ausführungsform der Erfindung; und

Fig. 3

eine Abhängigkeit einer Pulslänge von Ansteuerimpulsen von einer Zwischenkreisgleichspannung.

Show it:

Fig. 1

a schematic representation of a household appliance according to an embodiment of the invention;

Fig. 2

a schematic representation of a circuit arrangement with a locking device according to an embodiment of the invention; and

Fig. 3

a dependence of a pulse length of drive pulses of a DC link voltage.

An in Fig. 1 illustrated household appliance 1 is for example a washing machine. The household appliance 1 comprises a laundry drum 2 for receiving items of laundry, which is rotatably mounted about a horizontal axis of rotation 4 in a housing 3 of the household appliance 1 - and also in a non-illustrated tub. The items of laundry can be introduced through an opening in the laundry drum 2, which can be closed by means of a door 5. In a manner known per se, the user of the household appliance 1 can open and close the door 5. If the door 5 is closed, it can then be locked by means of a locking device 6 relative to the housing 3 and thus mechanically fixed relative to the housing 3, so that an opening of the door 5 is no longer possible.

For driving the laundry drum 2, the household appliance 1 includes an electric drive motor 7. The user can select the desired operating program of the household appliance 1 with the aid of an operating device 8 and activate the operating process. If the operating process - here a washing process - activated, the door 5 is locked by means of the locking device 6 and unlocked only after completion of the operating process. Opening the door 5 during the operating process is thus prevented.

A circuit arrangement, as in Fig. 2 is shown, is part of the household appliance 1 and includes the locking device 6. The household appliance 1 has power connections 9, 10, which are based on electrical potentials L, N of the supply network. Between the power terminals 9, 10, a mains voltage U _{N is} provided, which is an AC voltage. The AC voltage U _N is rectified by means of a rectifier 11, which is formed in the embodiment as a full-bridge rectifier with four diodes. On the output side, the rectifier 11 is coupled to a DC link capacitor 12, which smoothes the rectified voltage. A _DC link DC voltage V _ZK is connected between a DC link node 13 on the one hand and a reference potential 14 on the other hand. The reference potential 14 is an electric different from the neutral potential N. Potential. The intermediate circuit capacitor 12 represents a DC voltage source in the context of the present invention.

In the embodiment, the drive motor 7 is a three-phase synchronous machine or a brushless DC motor (BLDC) with permanent magnets. Alternating voltages for the stator winding are provided by means of an inverter 15, which comprises an inverter with a plurality of transistors and is coupled to the intermediate circuit node 13. The alternating voltages for the drive motor 7 are thus provided from the DC link DC voltage V _ZK or the inverter 15 is supplied with electrical energy from the DC link capacitor 12. The inverter 15 is controlled by an engine control unit 16. This engine control unit 16 is located on the reference potential 14 and is supplied with a supply voltage, which is provided by means of a DC-DC converter 17, for example, a switching power supply, from the intermediate circuit DC voltage V _ZK . This supply voltage can be 3.3 V or 5 V, for example.

The locking device 6 comprises an electrical switch 18 of a door contact, which is closed only when the door 5 is closed and open when the door 5 is open. The current can thus flow through the switch 18 only when the door 5 is closed. On the one hand, the switch 18 is connected to a node 19, on which the same electrical potential as on the DC link node 13 is applied. On the other hand, the switch 18 is connected to a drive element 20 in the form of a coil or an electromagnet. On the other hand, this drive element 20 is coupled to the reference potential 14 via a node 21 and via a series connection of a first and a second switching element 22, 23. So there is a series circuit of the drive element 20 and the switching elements 22, 23 before. A freewheeling diode 24 connects the node 21 to the node 19 and thus allows a current flow through the drive element 20 when the switching elements 22, 23 lock.

In the exemplary embodiment, the switching elements 22, 23 are formed as NPN bipolar transistors. The collector of the first switching element 22 is connected to the node 21; the emitter of the switching element 22 is connected to the collector of the second switching element 23. The emitter of the second switching element 23 is connected to the Reference potential 14 is connected and thus lies on the reference potential 14. To control the switching elements 22, 23, a control device 25 is provided in the form of a microcontroller, which is connected to the base terminals of the switching elements 22, 23 and also located on the reference potential 14. A supply voltage (for example 3.3 V or 5 V) for the control device 25 is generated by means of a DC-DC converter 26, for example a switched-mode power supply. Optionally, the control device 25 may also be formed integrally with the engine control unit 16, so that the control device 25 and the engine control unit 16 are formed by a single control device, in particular a microcontroller.

A current flow I through the drive member 20 causes a locking member 27 is moved from an unlocked position in which the door 5 is unlocked in a locking position in which the door 5 is fixed by the locking member 27 relative to the housing 3 and thus locked. The locking element 27 may comprise, for example, a soft-magnetic locking bolt. In order to be able to check the current position of the locking element 27, the locking element 27 is associated with a switch 28, which is coupled on the one hand to the node 19 and on the other hand to the control device 25. If the locking element 27 is moved into the locking position, then the switch 28 is closed, which can be detected by the control device 25 accordingly. In addition, a node 29 between the emitter of the first switching element 22 and the collector of the second switching element 23 is coupled to the control device 25, so that a current flow through the switching elements 22, 23 and thus the proper operation of the switching elements 22, 23 can be checked ,

The switching element 22 is controlled by the control device 25. The second switching element 23 serves as a safety switching element and can be controlled independently of the switching element 22. In order to enable the activation of the drive element 20, the safety switching element 23 can be driven in synchronism with the first switching element 22 with a same sequence of drive pulses or alternatively with longer pulses, so that the second switching element 23 is opened at least during the entire duration of the drive pulses 30 of the first switching element 22 is. Alternatively, the second switching element 23 can also be switched through constantly.

In order to prevent the activation of the drive element 20 and thus prevent the switching of the solenoid valve 28, the safety switching element 23 can be locked. The actuation of the safety switching element preferably takes place independently of the switching element 22 by a safety circuit realized in the control device 25 or a software function. Thus, a safety function can be realized independently of the function commands for the shut-off element 27.

In an alternative embodiment, the switching element 22 may also be driven with a sequence of drive pulses 30 using pulse width modulation. Advantageously, a coupling capacitor can then be provided between the output of the control device 25 and the switching element 22, which prevents the switching element 22 is continuously activated in the event of malfunction.

The locking device 6 also includes means for detecting the current voltage value of the intermediate circuit _DC voltage V _ZK . The current voltage value is detected by the control device 25, which for this purpose, for example, a separate measuring input (not shown) may have, which - is connected to the intermediate circuit node 13 - for example via a voltage divider or other DC-DC converter. Additionally or alternatively, the voltage value of the DC link voltage V _ZK can be detected via a supply input 31, via which the control device 25 is supplied with the above-mentioned supply voltage.

It is provided that the temporal pulse length of the drive pulse 30 is set as a function of the current voltage value of the DC link voltage V _ZK . For this purpose, the control device 25 continuously or continuously detects the respective instantaneous voltage value of the DC link voltage V _ZK and controls the pulse length of the drive pulse 30 which is delivered to the first switching element 22 to bring the locking element 27 in the operation of the household appliance 1 in the locked position. In this case, the electric power delivered to the drive element 20 is adjusted by means of the pulse length of the drive pulse 30 as a function of the current voltage value of the DC link voltage V _ZK or adapted to the current voltage value. This adaptation looks like that Pulse length of the drive pulse 30 is inversely proportional to the square of the DC link voltage V _ZK .

This dependence is in Fig. 3 displayed. The _{DC link voltage} V _ZK in volts _{dc is} plotted on the y-axis; on the x-axis, a pulse length w of the drive pulse 30 is plotted in milliseconds. How out Fig. 3 As can be seen, the _DC link voltage V _ZK can vary between a lower limit of about 150V and an upper limit of about 400V. With a DC link voltage V _ZK of 400 V, a pulse length w of approximately 4.4 ms is set. With a DC link voltage V _ZK of 150 V, a pulse length w of approximately 32.6 ms is set. The following lookup table can be stored in the control device 25: _{DC link} voltage [V _DC ] Pulse length [ms] 400 4.4 350 5.9 300 7.8 250 11.1 200 18.5 150 32.6

wobei E die durch das Antriebselement 20 aufgenommene elektrische Energie, V den Spannungswert der Zwischenkreisgleichspannung V_ZK, I den maximalen Strom durch das Antriebselement 20 und w die Pulslänge in Sekunden bezeichnen. Wird eine sehr kleine Induktivität des Antriebselements 20 angenommen, so wird der Strom I grundsätzlich durch den Innenwiderstand bestimmt: $$\mathrm{I}=\mathrm{V}/\mathrm{R},$$

wobei R den Innenwiderstand des Antriebselements 20 bezeichnet. Daraus ergibt sich folgende Abhängigkeit der Pulslänge w von dem Spannungswert V: $$\mathrm{w}=\mathrm{ER}/{\mathrm{V}}^2.$$

The above dependency is based on the following consideration: Although a coil is used as the drive element 20, this drive element 20 also has an internal ohmic resistance (internal resistance), which also influences the energy supplied to the drive element 20. The energy absorbed by the drive element 20 is given by the following equation: $$\mathrm{e}=\mathrm{Viw}.$$

where E denotes the electrical energy absorbed by the drive element 20, V the voltage value of the DC link voltage V _ZK , I the maximum current through the drive element 20 and w the pulse length in seconds. If a very small inductance of the drive element 20 is assumed, then the current I is always determined by the internal resistance: $$\mathrm{I}=\mathrm{V}/\mathrm{R}.$$

where R denotes the internal resistance of the drive element 20. This results in the following dependence of the pulse length w on the voltage value V: $$\mathrm{w}=\mathrm{HE}/{\mathrm{<figure-callout\; id="2"\; label="V"\; filenames="imgf0001.png"\; state="\{\{state\}\}">V</figure-callout>}}^2,$$

If a constant energy E and a constant resistance R are assumed, this equation can be represented by a constant k as follows: $$\mathrm{w}=\mathrm{k}/{\mathrm{<figure-callout\; id="2"\; label="V"\; filenames="imgf0001.png"\; state="\{\{state\}\}">V</figure-callout>}}^2,$$

This dependence corresponds to the course according to Fig. 3 ,

The control device 25 can also detect the current phase position of the mains voltage U _N and synchronize the sequence of the drive pulses 30 with the mains voltage U _N such that the drive pulses 30 are generated when the intermediate circuit capacitor 12 is charged. This prevents that the intermediate circuit DC voltage V _ZK breaks too much and, for example, falls below the lower limit of 150 V.

In a drive with a train of pulses using a pulse width modulation, the controller 25 continuously and continuously detects the respective instantaneous voltage value of the intermediate circuit DC voltage V _ZK and continuously controls the pulse length of the drive pulses 30, which are continuously supplied to the first switching element 22 to the locking element Keep 27 in the operation of the household appliance 1 continuously in the locked position. In this case, according to the above considerations, the output to the drive element 20 electrical power is kept constant over time by the pulse length of the drive pulses 30 depending on the current voltage value of the DC link voltage V _ZK adjusted or adapted to the current voltage value. This adaptation looks such that the pulse length of the drive pulses 30 is inversely proportional to the square of the DC link voltage V _ZK .

LIST OF REFERENCE NUMBERS

1

household appliance

2

washing drum

3

casing

4

axis of rotation

5

door

6

locking device

7

electric drive motor

8th

operating device

9, 10

mains connection

11

rectifier

12

Link capacitor

13

Intermediate circuit node

14

reference potential

15

inverter

16

Engine control unit

17

DC converter

18

electrical switch

19

node

20

driving element

21

node

22, 23

switching element

24

Freewheeling diode

25

control device

26

DC converter

27

locking element

28

switch

29

node

30

drive pulses

31

supply input

I

current flow

L

electrical potential

N

electrical potential

U _N

mains voltage

V _ZK

Intermediate circuit voltage

w

pulse length

Claims (11)

1. Locking device (6) for locking a door (5) of a domestic appliance (1), having:

   - a locking element (27) for mechanically locking the door (5),

   - a driving element (20) for moving the locking element (27) from an unlocking position, in which the door (5) is unlocked, into a locking position, in which the door (5) is locked by the locking element (27), as a function of a current flow (I) through the driving element (20), wherein the driving element (20) is electrically coupled to a DC voltage source (12),

   - a switching element (22, 23) electrically coupled to the driving element (20) for controlling the current flow (I) through the driving element (20), and

   - a control device (25) for controlling the switching element (22, 23), wherein the control device (25) is designed to output at least one control pulse (30) to the switching element (22, 23) in order to control the current flow (I) through the driving element (20),

   characterised in that
   the locking device (6) has means for detecting a present voltage value (V) of a DC voltage (V_ZK) provided by the DC voltage source (12) and the control device (25) is designed to set a pulse length (w) of the at least one control pulse (30) as a function of the present voltage value (V).
2. Locking device (6) according to claim 1, characterised in that the DC voltage source (12) is an intermediate circuit capacitor, which is coupled to an output of a rectifier (11), in particular a full bridge rectifier, which is embodied to rectify a mains voltage (U_N).
3. Locking device (6) according to one of the preceding claims, characterised in that an inverter (15) which is embodied to provide a supply voltage for an electrical drive motor (7) of the domestic appliance (1) from the DC voltage (V_ZK) is coupled to the same DC voltage source (12).
4. Locking device (6) according to one of the preceding claims, characterised in that the control device (25) is designed to set a larger pulse length (w) with a lower voltage value (V) of the DC voltage (V_ZK) than with a higher voltage value (V).
5. Locking device (6) according to one of the preceding claims, characterised in that the control device (25) is designed to set the pulse length (w) inversely proportional to the square of the voltage value (V).
6. Locking device (6) according to one of the preceding claims, characterised in that a look-up table is stored in the control device (25), said look-up table specifying the dependency of the pulse length (w) on the DC voltage (V_ZK), wherein the control device (25) is designed to set the pulse length (w) by using the look-up table.
7. Locking device (6) according to one of the preceding claims, characterised in that in addition to the one switching element (22, 23), at least one further switching element (22, 23) is coupled to the driving element (20) and the control device (25) is designed to open the at least one further switching element (22, 23) at least during the control pulses (30) of the one switching element (22, 23).
8. Locking device (6) according to one of the preceding claims, characterised in that the control device (25) is designed to control the switching element (22, 23) with a sequence of pulses using a pulse width modulation and to set a present pulse duty factor of the control pulses (30) as a function of the present voltage value (V).
9. Locking device (6) according to claim 8, characterised in that the control device (25) is designed to detect a present phase position of a mains voltage (U_N) and to generate the sequence of control pulses (30) as a function of the phase of the mains voltage (U_N).
10. Domestic appliance (1), in particular for the care of laundry items, having a door (5) and a locking device (6) as claimed in one of the preceding claims.
11. Method for locking a door (5) of a domestic appliance (1) with the aid of a locking element (27), which is moved from an unlocking position, in which the door (5) is unlocked, into a locking position, in which the door (5) is locked, as a function of a current flow (I) through a driving element (20), wherein the driving element (20) is applied with a DC voltage (V_ZK) from a DC voltage source (12) and the current flow (I) through the driving element (20) is controlled by means of a switching element (22, 23), which is controlled by means of a control device (25) with a least one control pulse (30), characterised in that a present voltage value (V) of the DC voltage (V_ZK) is detected and a pulse length (w) of the control pulse (30) is set by the control device (25) as a function of the present voltage value (V).

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