Reduction of brightness differences when operating a lighting device of a household appliance with multiple light sources

Abstract

Method for operating a lighting device (11) of a household appliance (1) by: - Operating a first electric light source (14a, LED41) by means of a first driver circuit (15a) with a first electric current (I1) which has a first current amplitude, - Operating a second electric light source (14b, LED42) by means of a second driver circuit (15b) with a second electric current (I2) which has a second current amplitude, - Controlling the first driver circuit (15a) and the second driver circuit (15b) by means of a control unit (40), and - Acquisition of a first measured value (M1) correlated with the first current (I1) by a first measuring element and acquisition of a second measured value (M2) correlated with the second current (I2) by a second measuring element, characterized by: - Clocked control of the first driver circuit (15a) with a first switch-on time ratio (PWM1) and the second driver circuit (15b) with a second switch-on time ratio (PWM2) depending on the first measured value (M1) and the second measured value (M2) such that a brightness difference between the first light source (14a, LED41) and the second light source (14b, LED42) is reduced compared to operation with an identical first (PWM1) and second switch-on time ratio (PWM2), and determination of the first switch-on time ratio (PWM1) and the second switch-on time ratio (PWM2) depending on a sensitivity characteristic of the human eye, whereby an identical brightness effect can be achieved when using different colors.

Description

[0001] The invention relates to a method for operating a lighting device of a household appliance and to a lighting device in a household appliance according to the preambles of the independent claims. Furthermore, the invention relates to a household appliance with such a lighting device.

[0002] Nowadays, household appliances, especially large ones, are equipped with additional lighting effects that go beyond familiar display methods, such as signaling via a seven-segment display or a matrix display. A typical example of such an additional lighting effect is an illuminated door ring, commonly used in laundry appliances. This door ring can have various visual aspects, for which it is advantageous to install several independently controllable light sources, for example, in the form of light-emitting diodes (LEDs). However, even in this case, it may be necessary in certain operating situations to operate all light sources at a similar brightness to achieve homogeneous illumination of the ring along the door opening.Typically, light sources, especially LEDs, are connected in series when the same brightness is required for all light sources, so that the same current flows through them. Depending on the design of the appliance in question, this is not always possible. Consequently, a constant current source is often used to drive a specific current through the different light sources.

[0003] Especially when using simple and inexpensive power supply circuits, the following limitations can occur: the respective currents may differ due to component parameter tolerances, such as those of the transistors used, or current deviations may occur due to different component temperatures, particularly affecting transistors or diodes. Similarly, the dynamic properties of other components can influence the resulting electrical current due to interactions. These limitations can negatively impact the desired setpoint of the electrical current. Consequently, the brightness of individual light sources may vary.

[0004] In this context, a current regulator is known from JP 2014-226006 A, which enables a variable adjustment of the magnitude of a constant current through two or more LEDs connected in series by means of a combined buck / boost DC voltage converter.

[0005] US 2015 / 0076999 A1 discloses a modification of duty cycles of pulse width modulation frequency drive signals to compensate for LED driver mismatches in a multi-channel LED system.

[0006] German patent application DE 10 2010 041 227 A1 discloses a light dimming module comprising a dimming unit coupled to a digital input interface and an output interface. The dimming unit is configured to generate a section-wise linear exponential digital control signal with N segments, and the output interface is configured to control the intensity of a light source.

[0007] The object of the present invention is to provide a method, a lighting device and a household appliance which enable a more uniform brightness adjustment of separate electric light sources in a lighting device of a household appliance.

[0008] This problem is solved by a method, a lighting device, and a household appliance according to the independent claims. Advantageous embodiments of the present invention are the subject of the dependent claims.

[0009] The invention relates to a method for operating a lighting device of a household appliance by operating a first electric light source belonging to the lighting device by means of a first driver circuit also belonging to the lighting device with a first electric current which has a first current amplitude, operating a second electric light source belonging to the lighting device by means of a second driver circuit also belonging to the lighting device with a second electric current which has a second current amplitude, controlling the first driver circuit and the second driver circuit by means of a control unit belonging to the lighting device, and detecting a first measured value correlated with the first current by a first measuring element as well as detecting a second measured value correlated with the second current by a second measuring element.

[0010] According to the invention, the method is further developed by clocked control of the first driver circuit with a first switch-on time ratio and the second driver circuit with a second switch-on time ratio depending on the first measured value and the second measured value in such a way that a brightness difference between the first light source and the second light source is reduced compared to operation with an identical first and second switch-on time ratio.

[0011] The inventors recognized that, despite being dimensioned for the same operating current, deviations of up to 10 percent or more can occur in the achieved current amplitudes of the driver circuits commonly used in household appliances, which are cost-effective to implement. Furthermore, the invention is based on the realization that the aforementioned limitations, which lead to different brightness levels between the first and second electric light sources with identical control, can be overcome by operating the first and second light sources in a pulsed manner, with each light source being controlled with a correspondingly modified duty cycle. The duty cycle is also referred to as the duty factor or duty cycle.This specifies the ratio of the pulse duration (i.e., the duration of the on-time) to the period for a periodic sequence of pulses. According to the invention, during each on-time, a first electric current with a constant current intensity equal to the first current amplitude is passed through the first light source, while during the off-time following the on-time, the flow of the first electric current through the first electric light source is interrupted. The second electric light source is controlled in a corresponding manner.

[0012] By recording a first measurement correlated with the first current and a second measurement correlated with the second current, the effective electric current flowing through the respective light source can be determined metrologically. As a first approximation, the electric current flowing through the respective light source is considered an indicator of its actual brightness. More refined approaches, taking into account further influencing factors, are discussed in more detail below.

[0013] According to the invention, if the mean or RMS value of the electric current flowing through the first light source is greater than the RMS value or mean value of the second electric current flowing through the second light source, the first duty cycle can be reduced and / or the second duty cycle can be increased. Similarly, if the RMS value or mean value of the first electric current is less than the RMS value or mean value of the second electric current, the first duty cycle can be increased and / or the second duty cycle can be reduced. This makes it easier to compensate for component tolerances, such as a voltage drop across a transistor or diode and / or the gain of a transistor (especially an active current source).Furthermore, deviations in current amplitudes caused by temperature differences can be compensated for.

[0014] Furthermore, the invention enables the brightness to be adjusted across the boundaries of a single household appliance. This is particularly advantageous when, for example, a first household appliance in the form of a washing machine and a second household appliance in the form of a tumble dryer are installed together and operated at least temporarily, and especially when both appliances belong to the same product family and have a uniform appearance.

[0015] Of course, the invention is not limited to a first electric light source and a second electric light source; rather, any number of additional electric light sources can be controlled using this method.

[0016] The procedure involves determining the first and second on-time ratios based on the sensitivity characteristics of the human eye. For example, an evaluation can be performed using a logarithmic scale. This allows for a better adaptation to subjective brightness perception. This is particularly important when using different colors to achieve the same perceived brightness. The reason for this is the dependence of the human eye's spectral luminous sensitivity, which is described by the so-called V(λ) curve.

[0017] According to an advantageous further development, the method comprises determining the first on-time ratio and the second on-time ratio as a function of the mean value of the first current amplitude and the second current amplitude, and / or as a function of a predefinable current setpoint, and / or as a function of the value of the first current amplitude. This results in a variety of possibilities for selecting the current setpoint. Starting from a common predefinite value for the first on-time ratio and the second on-time ratio—for example, 0.8, i.e., an on-time of 80 percent of a respective cycle period and an off-time of 20 percent of the cycle period—the first measured value and the second measured value can be determined according to a predefinable weighting function. Examples of such a weighting function include the determination of a mean value (I avg=current amplitude*switching time / cycle period), which in the present case is identical to the so-called rectified mean value, or the determination of an RMS value (I rms = Current amplitude * √{switch-on time / cycle period} or expressed differently: I 2 rms = Current amplitude 2 * Switch-on time / cycle period).

[0018] Instead of a mean-value-based or RMS-based evaluation function, a light-source-specific evaluation function can be used. For example, in the case of a light-emitting diode (LED), such an evaluation function can be designed such that, at least within a predefinable range around an operating point—for example, 80 percent plus / minus 15 percent as the adjustment range of the on-time ratio—a proportional relationship is established between the respective measured value and the corresponding brightness of the light source. This evaluation function can be determined, for example, for a specific selected group of LEDs of a particular type, especially from the same production batch, which have been selected into predefined brightness classes according to a process called "binning," depending on a defined grouping current.

[0019] Another variant involves calculating an average value from the first and second current amplitudes. Based on this average current amplitude and a predefined proportionality factor, a corresponding first and second current through the respective light source is controlled by adjusting the corresponding first and second on-time ratios, respectively. A further possibility is to specify a fixed current setpoint, independent of the first and / or second current amplitudes, thus setting the respective current through the light source independently of the current amplitudes.Another possibility is to use one of the two currents, that is, the first current or the second current (or, in the case of a higher number of light sources, the corresponding number of electric currents), as a reference and to adjust the other currents accordingly.

[0020] Depending on the application, the evaluation of the first or second electric current with regard to its effect on the brightness of the respective light source can be based on the RMS value, the mean value, or a combination of both calculation methods.

[0021] According to a further advantageous embodiment, the method comprises determining the first and second on-time ratios as a function of the respective type-specific relationship between the brightness of the first light source and the first current, as well as between the brightness of the second light source and the second current. Thus, the current through the first and second light sources can be used as the respective control variable to adjust the desired brightness. Alternatively, it can be provided to determine the type-specific relationship between the brightness of the first and second light sources and the electrical power supplied to the respective light source.

[0022] An advantageous embodiment of the method comprises adjusting the first switch-on time ratio and the second switch-on time ratio during a production process of the household appliance or lighting device using a camera to adjust identical brightness when operating the first light source with the adjusted first switch-on time ratio at the first current amplitude and the second light source with the adjusted second switch-on time ratio at the second current amplitude, and non-volatile storage of a first adjustment value correlated with the adjusted first switch-on time ratio and a second adjustment value correlated with the adjusted second switch-on time ratio in the lighting device.

[0023] According to an advantageous further development, the lighting device can be mounted on the household appliance after the first and second on-time ratios have been adjusted. In this way, a corresponding lighting device can be pre-configured as a component with smaller dimensions than the fully assembled household appliance, thus eliminating the need for subsequent adjustments on the fully assembled appliance. This allows the first and second on-time ratios to be continuously adjusted based on stored calibration values, i.e., the calibrated first and second on-time ratios, depending on dynamic measured values.Such dynamic measurements can be based, for example, on the first and / or second current amplitude, thereby compensating for drift in a current source. Similarly, the aging of the light source can be taken into account based on its total operating time. To determine age-related degradation of the luminous flux or luminous intensity of the respective light source, the operating temperature of the light source, and in particular the operating time weighted by that temperature, can also be considered.

[0024] According to an advantageous embodiment of the method, the first driver circuit with the first on-time ratio and / or the second driver circuit with the second on-time ratio are controlled in a clocked manner using a pulse-width modulation (PWM) method. This means that the clocked control is performed with a constant, predefinable pulse period, namely the reciprocal of a PWM frequency, whereby a variable pulse width is set. The ratio of the pulse width to the pulse period is determined by the so-called duty cycle. Alternatively, a pulse-following control can be provided in which a constant, predefinable on-time is set, and the first on-time ratio or the second on-time ratio is realized by controlling the respective off-time.Similarly, a constant switch-off time can be provided and the switch-on time varied accordingly.

[0025] According to a further advantageous embodiment, the first and / or second on-time ratio is recalculated for each clock cycle of the pulsed control. This enables so-called cycle-by-cycle control. Accordingly, the first and / or second current is measured during the active pulse of each clock cycle, and as a result, the first and / or second on-time ratio is shortened or lengthened to compensate for current deviations. In this way, improved lighting results can be achieved. Even significant parameter fluctuations, including abrupt changes, can be compensated for with short delay times, which can be defined by the duration of a clocked control period.Alternatively, it may be possible to determine an average value of the first and / or the second current over a first group of consecutive clock cycles and to use this value to adjust the first on-time ratio and / or the second on-time ratio in a second group immediately following the first. Preferably, the number of clock cycles within the first and / or second group is a power of two. As a variation, it may be possible to evaluate only certain clock cycles within the first group, for example, the first and / or the last and / or one from the middle range, and to use these values ​​to adjust the first on-time ratio and / or the second on-time ratio of the clock cycles contained in the second group.

[0026] According to a further advantageous embodiment, the method comprises determining the first switch-on time ratio as a function of the total operating time of the first electric light source and the second switch-on time ratio as a function of the total operating time of the second electric light source. This allows for the correction of age-related degradation of the luminous flux or luminous intensity as a function of the electric current through the respective light source or the electric power supplied to the respective light source.

[0027] According to a further advantageous embodiment, the method comprises adjusting the first current amplitude to a value used for a manufacturer's classification of the first light source and / or the second current amplitude to a value used for a manufacturer's classification of the second light source. In this way, separate optical calibration of the first and / or second electric light source after installation in the lighting device can be dispensed with, since the manufacturer's selection process uses groups with a predefinable relationship between, for example, a current through the respective light source and a luminous intensity of the respective light source (the "binning" known particularly in connection with LEDs) to achieve a uniform brightness distribution.In principle, deviations in the curve shapes of individual light sources, i.e., the functional relationship between the current through the light source and the luminous intensity, do not appear during operation at the corresponding group current.

[0028] Furthermore, the invention relates to a lighting device in a household appliance comprising a first electric light source with a first driver circuit for operating the first light source with a first electric current which has a first current amplitude, a second electric light source with a second driver circuit for operating the second light source with a second electric current which has a second current amplitude, a control unit for controlling the first driver circuit and the second driver circuit, and a first measuring element for detecting a first measured value correlated with the first current as well as a second measuring element for detecting a second measured value correlated with the second current.

[0029] According to the invention, the lighting device is further developed by the fact that the control unit is designed to control the first driver circuit clocked with a first switch-on time ratio and the second driver circuit clocked with a second switch-on time ratio as a function of the first measured value and the second measured value in such a way that a brightness difference between the first light source and the second light source is reduced compared to operation with an identical first and second switch-on time ratio.

[0030] Preferably, the first and / or second electrical light source is a light-emitting diode (LED) or an organic light-emitting diode (OLED). The control unit preferably comprises a microprocessor and / or a microcontroller. Alternatively, the control unit can comprise a programmable logic circuit, for example, an FPGA (Field Programmable Gate Array), a CPLD (Complex Programmable Logic Device), or a PAL (Programmable Array Logic).

[0031] The first driver circuit, the second driver circuit and the control unit are preferably designed as components of the lighting device.

[0032] According to an advantageous embodiment, the first measuring element comprises a first measuring resistor connected in series with the first light source, and / or the second measuring element comprises a second measuring resistor connected in series with the second light source. A measuring voltage can be generated by means of such a measuring resistor (shunt), which can be supplied directly or via a filter and / or amplifier circuit to the control unit. This enables simple and cost-effective monitoring of the operating state of the first and / or second electrical light source.

[0033] The first and / or second measuring resistor can further be dimensioned such that, in addition to its function as a measuring resistor, it also performs a current-limiting function, allowing the first current amplitude and / or the second current amplitude to be adjusted as desired by appropriately selecting the resistance value of the respective measuring resistor. Such a configuration is advantageous when operating with a constant voltage source. In accordance with the specified order of the lower voltage limits listed below, a supply voltage greater than or equal to the following from the constant voltage source is increasingly preferred: 4 volts; 4.5 volts; 5.5 volts.Regardless of the lower voltage limit, the supply voltage provided by the constant voltage source is increasingly preferred in the specified order of the upper voltage limits listed below: 15 volts; 12 volts; 10 volts; 7.5 volts; 6 volts; 5.5 volts.

[0034] According to a further advantageous embodiment, the first driver circuit comprises a first switching element which is connected in series to the first light source, and / or the second driver circuit comprises a second switching element which is connected in series to the second light source. The first switching element and / or the second switching element can be clocked by the control unit, in particular directly, for example via a digital input / output unit of the control unit, which is preferably coupled to a clock generation unit. Such a configuration is provided as an internal functional block by a large number of currently available microprocessors / microcontrollers, which enables a simple implementation of clocked control. The respective switching elements are preferably designed as bipolar transistors; alternatively, they can be designed as field-effect transistors, in particular MOSFETs.

[0035] According to a further advantageous embodiment, the first driver circuit comprises a first current source connected in series with the first light source, and / or the second driver circuit comprises a second current source connected in series with the second light source. This can, for example, be a constant current source operated as a dynamic series resistor with a DC voltage source. In the simplest case, this can be an ohmic resistor that, when operated with a constant voltage source, acts as a current-limiting resistor. As previously described, individual components can perform multiple functions and thus simultaneously be part of several functional assemblies, so that, for example, a measuring resistor of the respective measuring element can simultaneously act as a current limiter as part of the respective driver circuit.Thus, it can be provided that the respective switching elements are designed as transistors with an integrated current limiting function.

[0036] According to a further advantageous embodiment, the first light source and the second light source are optically coupled to a common and / or to individual light guide elements arranged in a functional group. The light guide elements can, in particular, be strips and / or rings and / or ring segments, which are preferably arranged directly adjacent to one another, so that a closed light guide element contour is formed.

[0037] Preferably, a household appliance, particularly for the care of laundry, can have a lighting device according to the invention and a door that can be illuminated by the lighting device to visually indicate the operating states of the household appliance, resulting in a household appliance according to the invention. Such a household appliance can, in particular, have a ring around the door illuminated by the first and second light sources, the ring surrounding a loading opening of the household appliance. The illuminated ring allows for the indication of the operating states of the household appliance. Thus, a front-facing and centrally arranged optical visualization of operating states can be provided, which can be reliably understood and clearly perceived by a user even at a greater distance.

[0038] Further features of the invention are evident from the claims, the figures, and the description of the figures. The features and combinations of features mentioned above in the description, as well as those subsequently mentioned in the description of the figures and / or shown in the figures alone, are not only usable in the combinations specified, but also in other combinations without departing from the scope of the invention. Thus, embodiments that are not explicitly shown and explained in the figures, but which can be derived and generated from the explained embodiments by separate combinations of features, are also to be considered as encompassed and disclosed by the invention. Embodiments and combinations of features that do not exhibit all the features of an originally formulated independent claim are also to be considered disclosed.Furthermore, embodiments and combinations of features, in particular those set out above, are to be considered disclosed which go beyond or deviate from the combinations of features set out in the cross-references of the claims.

[0039] Further advantages and features will become apparent from the following description of exemplary embodiments, taking into account the accompanying figures. In the figures, the same reference numerals denote the same features and functions.

[0040] They show: Fig. 1 In simplified schematic front view a household appliance according to the invention; Fig. 2 in simplified schematic representation, one for operating each of the LEDs as they are used in the household appliance according to Fig. 1 are arranged, circuit arrangement used; Fig. 3. In simplified schematic representation, a modification of the one in the Fig. 2 Circuit arrangement shown for use in a lighting device according to the invention; Fig. 4 in simplified schematic representation a preferred embodiment of a lighting device according to the invention; Fig. 5. In a simplified schematic representation, a functional relationship between luminous intensity and current is shown using the example of two LEDs with significantly different characteristic curves and an ideal LED; and Fig. Figure 6 shows, in a simplified schematic representation, the aging behavior of LEDs depending on different operating temperatures.

[0041] A household appliance 1 for the care of laundry items is in the Fig. Figure 1 shows the appliance. This can be a washing machine, a tumble dryer, or a combination washer-dryer. In the illustrated embodiment, it is a so-called front-loading appliance, which has a front-opening door 2 that serves to close a loading opening 3, for example, a circular one. The appliance 1 comprises a housing 4 in which a drum 5 for holding laundry is arranged. The loading opening 3 is a front opening of the drum 5. The appliance 1 also includes an operating device 6, the design and position of which are to be understood as exemplary and not limiting, which has at least one control element 7 and a display unit 8. A drawer-like detergent dispenser 9 is also shown.

[0042] The pivotally arranged and, in particular, porthole-shaped door 2 comprises a ring 10 at its edge. Furthermore, the household appliance 1 includes a lighting device 11. In the exemplary embodiment, this lighting device 11 is arranged such that the ring 10 emits light from its outer surface 12.

[0043] In the exemplary embodiment, the lighting device 11 comprises a first light source 14a, a second light source 14b, a third light source 14c, and a fourth light source 14d. Each of these four light sources 14a, 14b, 14c, 14d can be formed from one or more light-emitting diodes (LEDs). Both the number and the arrangement of the light sources 14a to 14d, which in this example are offset from each other by a substantial, and in particular by exactly, 90°, is to be understood as an example only and not as an exhaustive list.

[0044] The first light source 14a, the second light source 14b, the third light source 14c and the fourth light source 14d are optically coupled via a light guide element 13 of the lighting device 11, in particular in the form of a light guide ring.

[0045] The lighting device 11 is designed such that the first light source 14a, the second light source 14b, the third light source 14c, and the fourth light source 14d can be controlled independently in a first operating state, and in a second operating state, all four light sources 14a, 14b, 14c, 14d can be controlled together, resulting in a uniform brightness distribution along the light guide element 13. In particular, this prevents any one of the four light sources 14a, 14b, 14c, 14d from appearing perceptibly brighter or dimmer to the human eye, thus avoiding a homogeneous overall impression.

[0046] The number of separately controllable light sources used is not limited to the values ​​suggested in the exemplary embodiments; rather, any number of light sources greater than or equal to two can be used.

[0047] One LED is typically used to operate each of the LEDs, as used in the household appliance 1 according to Fig. The circuit arrangement for driving a light-emitting diode LED20 is arranged in the Fig. 2 is shown as an example. The light-emitting diode LED20 represents, as a concrete embodiment of a light source, any one of the light sources 14a, 14b, 14c, 14d of the lighting device 11 (cf. Fig. 1).

[0048] The LED20 is connected between a positive supply terminal VCC of a DC voltage supplied to power the LED20 and a reference terminal GND. The circuit arrangement 15 includes a measuring resistor R20, which is connected to the reference terminal GND via one terminal. A current source CS20 is electrically connected to a second terminal of the measuring resistor R20. An electronic switch in the form of an NPN transistor Q20 is connected between the current source CS20 and the cathode of the LED20. The cathode of the LED20 is electrically connected to the collector of transistor Q20. The current source CS20 is electrically connected to the emitter of the NPN transistor Q20. The anode of the LED20 is electrically connected to the supply terminal VCC. A clocked input signal In20 is applied to the base terminal of transistor Q20.

[0049] For an NPN transistor typically used in such a circuit arrangement, the available gain factors, i.e., the ratio of collector current to base current, are so high (>> 100) that the collector current is almost identical to the emitter current, and the base current, as the difference between these two currents, is negligibly small. Therefore, in the following, the term "coupling a specific component in series with a light-emitting diode arranged in the collector path of the respective (bipolar) transistor" also refers to arrangements in which the specific component is arranged in the emitter path of the respective transistor. For a field-effect transistor, the terms drain instead of collector and source instead of emitter apply accordingly, whereby in this case, under steady-state conditions, the current through the drain path and the current through the source path are identical.

[0050] In the simplest case, the current source CS20 can be implemented as a resistive load. This results in a current limit through the LED20 that is essentially independent of the supply voltage between the supply terminal VCC and the reference terminal GND. The current limit is calculated by dividing the base-emitter voltage (typically around 0.7 volts, temperature-dependent) by the sum of the resistive load (replacing the current source CS20) and the measuring resistor R20. In this case, combining the resistors is particularly effective, allowing R20 to simultaneously determine the current limit in addition to its function as a measuring resistor. Therefore, the arrangement shown, comprising transistor Q20 and measuring resistor R20, represents a very simple and cost-effective driver circuit 15.

[0051] Furthermore, resistor R21 is connected at one end to the supply terminal VCC and at the other end, via diode D20, to the collector of transistor Q20 and the cathode of LED20. Diode D20 is positioned such that its anodes and those of LED20 are electrically connected. An output signal OUT20 can be tapped at the junction of resistor R21 and diode D20.

[0052] However, evaluating the output signal OUT20 only allows a statement as to whether the light-emitting diode LED20 is switched on.

[0053] By easily implementing a modification of the circuit, in particular by providing alternative assembly options on a printed circuit board on which at least some of the components of the lighting device can be arranged, the circuit arrangement can be modified according to the illustration in the Fig. 3. Modified. This is done in the same order as in the Fig. 2. A light-emitting diode LED30 is arranged in series with a transistor Q30, a current source CS30, and a measuring resistor R30 between the supply terminal VCC and the reference terminal GND. In contrast to the representation according to the Fig. 2 is according to the Fig. 3. It is provided that an output signal OUT30 is tapped at the junction between the measuring resistor R30 and the current source CS30. The output signal OUT30 thus serves to measure the current flowing through the LED30. In this way, it can be checked whether the LED30 is switched on, and, depending on this output signal 30, a compensation of the pulse width of a clocked control input signal IN30 supplied to the transistor Q30 can be used for compensation. The LED30, as a concrete embodiment of a light source, represents any of the light sources 14a, 14b, 14c, 14d of the lighting device 11 (cf. Fig. 1).

[0054] In the Fig. Figure 4 shows a preferred embodiment of a lighting device 11 according to the invention with several electric light sources in the form of light-emitting diodes LED41, LED42, ..., LED4n. For n = 4, the light-emitting diodes LED41, LED42, ..., LED4n can thus be considered a specific embodiment of the light sources 14a, 14b, 14c, 14d of the one described in the Fig. The lighting device 11, which is not described in further detail, is considered. The lighting device 11 thus comprises identically constructed branches, the first branch comprising a first series resistor RL41, a first light-emitting diode LED41, a first transistor Q41, and a first measuring resistor R41. The first series resistor RL41 is connected between the first light-emitting diode LED41 and the supply terminal VCC. The first transistor Q41 and the first measuring resistor R41 are connected between the first light-emitting diode LED41 and the reference terminal GND. Thus, the anode of the first light-emitting diode LED41 is electrically connected to the first series resistor RL41, the cathode of the first light-emitting diode LED41 is electrically connected to the collector of the first transistor Q41, and the first measuring resistor R41 is connected between the emitter of the first transistor Q41 and the reference terminal GND.The base terminal of the first transistor Q41 thus provides a first control input IN41, and a first signal output OUT41 is provided at the junction between the first measuring resistor R41 and the emitter of the first transistor Q41. The first series resistor RL41 therefore assumes the function of the current source CS20 or CS30. This means that the current source functionality can be implemented in the collector path of the first transistor Q41, where the LED 41 is also located, instead of in the emitter path. Alternatively, the first measuring resistor R41, in conjunction with the first transistor Q41, can provide the current source functionality, and the first series resistor RL41 can simply reduce the voltage, thus absorbing some of the power dissipation that would otherwise occur in the first transistor Q41.

[0055] The same structure is represented by a second branch comprising a second series resistor RL42, a second LED42, a second transistor Q42, a second measuring resistor R42 with a second control input N42 and a second output OUT42, and an nth branch comprising elements RL4n, LED4n, Q4n, and R4n with input IN4n and output OUT4n. A first electric current I1 flowing through the first LED41 is nearly identical to the current flowing through the first measuring resistor R41, which is identical to the current through the emitter of the first transistor Q41. The emitter current of transistor Q41 is equal to the sum of the base current and the collector current of the first transistor Q41, with the base current being several orders of magnitude smaller than the collector current.

[0056] A control unit 40 provides a first control signal PWM1 with a first on-time ratio to the first control input IN41, a second control signal PWM2 with a second on-time ratio to the second control input IN42, and corresponding further control signals up to an nth control signal PWMn at an nth control input IN4n. Similarly, the control unit 40 receives a first measurement signal M1 from the first output OUT41, a second control signal M2 from a second control output OUT42, and further signals up to an nth measurement signal Mn at an nth output OUT4n.

[0057] The modification options and explanations presented for the first branch apply accordingly to the second branch and to the further branches up to the nth branch.

[0058] The individual control signals PWM1, PWM2, ..., PWMn are driven with individually determined turn-on times for each branch to achieve the same brightness for each of the individual LEDs LED41, LED42, ..., LED4n. For example, if the second LED42 has a higher forward voltage than the first LED41, then, with otherwise identical circuit parameters, a lower second current I2 than the first current I1 will flow. The same effect occurs if the second series resistor RL42 is larger than the first series resistor RL41. Such discrepancies can occur, in particular, when using active components, which are employed instead of fixed resistors with comparatively precisely defined resistance tolerances to implement current sources. By adjusting the respective control signals, uneven currents through the individual LEDs LED41, LED42, ... can be corrected.The LED4n is compensated for in such a way that each LED achieves the same brightness. Therefore, if one LED or LED branch has a slightly higher current, the on-time ratio of the PWM signal is reduced to compensate for this effect. Conversely, if another LED has a slightly lower current, the on-time ratio of the PWM signal is increased in this case.

[0059] The target current value can be selected in various ways. One option is to adjust the duty cycles so that the first current, the second current, and subsequent currents up to the nth current are chosen in a predefined ratio to the average of all currents flowing through the respective branches. The averaging is performed using identical duty cycles for the respective control signals PWM1, PWM2, ..., PWMn, for example, 50 percent or 100 percent.

[0060] According to another alternative, a fixed setpoint is specified and the respective on-time ratio of the individual control signals PWM1, PWM2, PWMn is adjusted so that the value of the respective current, the first current I1, the second current I2, up to the nth current In assumes the desired setpoint.

[0061] According to another variant, one light-emitting diode or light-emitting diode array can act as a reference and the other light-emitting diodes can be adjusted accordingly.

[0062] Fig. Figure 5 schematically shows the functional relationship between a current I through a light-emitting diode and the corresponding luminous intensity in millicandela (mcd) as a curve for the first light-emitting diode LED41 and the second light-emitting diode LED42, as well as the curve of an ideal light-emitting diode LED42. id The ideal light-emitting diode (LED). id exhibits a linear relationship between the electric current I and the luminous intensity Iv. The curves of all three LEDs, the first LED41, the second LED42, and the ideal LED42, show this relationship. id intersect in a grouping stream I G(20 milliamperes) and a luminous intensity Iv of 100 millicandela. The first LED41 starts with a lower slope than the ideal LED. id and exhibits a steeper slope towards the end of the representation than the ideal light-emitting diode (LED). id In contrast, the second LED42 starts with a steeper slope than the ideal LED. id and ends with a lower slope than the ideal light-emitting diode (LED). id .

[0063] This shows that it is advantageous to operate the light-emitting diodes at a current amplitude equal to the grouping current I. G to operate, since in this case a reliable conclusion about the light intensity can be drawn from the operating current.

[0064] Fig.Figure 6 illustrates the influence of aging on a light-emitting diode (LED) at different housing operating temperatures. The abscissa of the diagram represents time t, ranging from 0 hours to 10,000 hours. The ordinate represents a relative luminous intensity Iv / Iv0, ranging from 80 percent to 105 percent. By definition, at the beginning of the operating time (0 hours), the LED shown here starts with a luminous flux Iv equal to the initial luminous flux Iv0, i.e., a relative luminous flux of 100 percent. This luminous flux depends on the housing operating temperature T. C This results in varying degrees of degradation of the relative luminous flux over time. For example, after an operating time of 6,000 hours at a housing operating temperature T, the luminous flux will be significantly reduced. CAt approximately 25 degrees Celsius, about 99 percent of the initial luminous intensity is still available, while at a housing operating temperature of approximately 60 degrees Celsius, only 95 percent of the initial luminous intensity remains. At a housing operating temperature of 85 degrees Celsius, the luminous intensity Iv has decreased to 90 percent compared to the initial luminous intensity Iv0. Based on knowledge of this aging mechanism, conclusions about the respective aging state of the LED can be drawn from the measured operating times and the housing operating temperatures encountered during these periods.Therefore, it is possible to incorporate the availability of the corresponding values ​​for age-related reduction in luminous intensity Iv into the functional relationship between current I and luminous intensity Iv, based on a computational model, and to use this to compensate for the age-related decrease in luminous intensity by appropriately increasing the respective on-time ratio. Compensating for age-related luminous intensity reduction is particularly useful for a washing machine and a tumble dryer installed together, which are operated at least temporarily.

[0065] In addition, with networked household appliances, corresponding data can be exchanged via a network and / or a suitable data interface in order to carry out coordinated brightness control according to the invention during operation.

[0066] The means proposed for realizing the invention can, of course, also be used to achieve a specific graduated brightness of the respective light sources. For example, it may be desirable to operate some of the light sources at half the brightness of the other light sources. In this case, if the functional relationship between the current I and the luminous intensity Iv of the light source, for example the first LED41 or the second LED42, is known, the corresponding on-time ratio can be calculated.

[0067] The exemplary embodiments serve only to illustrate the invention and are not limiting to it. In particular, the number of electric light sources operated in the lighting device 11 and a weighting function of the respective electric currents I1, I2, ..., In can be varied arbitrarily without departing from the concept of the invention.

[0068] The preceding section demonstrated how brightness compensation for separately controllable LEDs can be achieved simultaneously using a quasi-virtual series connection of light-emitting diodes (LEDs). In addition to simple current matching, methods for considering variable dependencies between the respective luminous intensity and the current flowing through each LED were also presented. Reference symbol list 1 household appliance 2 doors 3 Loading opening 4 cases 5 drum 6 Operating device 7 Control element 8 Display unit 9 detergent dispenser 10 rings 11 Lighting device 12 Outside 13 Light guiding element 14a first light source 14b second light source 14c third light source 14d fourth light source 15a first driver circuit 15b second driver circuit 15 Driver circuit general VCC supply connection GND reference connection LED20, LED30 Light-emitting diode Q20, Q30 Transistor CS20, CS30 power source R20, R30 measuring resistor D20 diode R21 Pull-up resistor IN20, IN30 control input OUT20, OUT30 signal output RL41 first series resistor RL42 second series resistor RL4n nth series resistor I1 first current I2 second current In nth stream LED41 first light-emitting diode LED42 second light-emitting diode LED4n nth light-emitting diode Q41 first transistor Q42 second transistor Q4n nth transistor R41 first measuring resistor R42 second measuring resistor R4n nth measuring resistor IN41 first control input IN42 second control input IN4n nth control input OUT41 first signal output OUT42 second signal output OUT4n nth signal output PWM1 first control signal with first duty cycle PWM2 second control signal with second duty cycle PWMn nth control signal M1 first measurement signal M2 second measurement signal Mn nth measurement signal 40 Control unit I current IV Light intensity LED id ideal light-emitting diode I G Grouping stream t time Iv0 initial luminous intensity T C Housing operating temperature

Claims

Method for operating a lighting device (11) of a household appliance (1) by: - ​​operating a first electric light source (14a, LED41) by means of a first driver circuit (15a) with a first electric current (I1) which has a first current amplitude, - operating a second electric light source (14b, LED42) by means of a second driver circuit (15b) with a second electric current (I2) which has a second current amplitude, - controlling the first driver circuit (15a) and the second driver circuit (15b) by means of a control unit (40), and - detecting a first measured value (M1) correlated with the first current (I1) by means of a first measuring element and detecting a second measured value (M2) correlated with the second current (I2) by means of a second measuring element,characterized by: - ​​clocked control of the first driver circuit (15a) with a first switch-on time ratio (PWM1) and the second driver circuit (15b) with a second switch-on time ratio (PWM2) depending on the first measured value (M1) and the second measured value (M2) such that a brightness difference between the first light source (14a, LED41) and the second light source (14b, LED42) is reduced compared to operation with an identical first (PWM1) and second switch-on time ratio (PWM2), and determining the first switch-on time ratio (PWM1) and the second switch-on time ratio (PWM2) depending on a sensitivity characteristic of the human eye, whereby the same brightness effect can be achieved when using different colors. Method according to claim 1, characterized by: - ​​determining the first on-time ratio (PWM1) and the second on-time ratio (PWM2) as a function of: ● the mean value of the first current amplitude and the second current amplitude, and / or ● a predefinable current setpoint, and / or ● the value of the first current amplitude. Method according to one of the preceding claims, characterized by: - ​​determining the first on-time ratio (PWM1) and the second on-time ratio (PWM2) depending on the respective type-specific relationship between the brightness of the first light source (14a, LED41) and the first current (I1) as well as between the brightness of the second light source (14b, LED42) and the second current (I2). Method according to one of the preceding claims, characterized by: - ​​matching the first on-time ratio (PWM1) and the second on-time ratio (PWM2) during a production process of the household appliance (1) or the lighting device (11) using a camera to adjust identical brightness when operating the first light source (14a, LED41) with the matched first on-time ratio at the first current amplitude and the second light source (14b, LED42) with the matched second on-time ratio at the second current amplitude, and - non-volatile storage of a first adjustment value correlated with the matched first on-time ratio and a second adjustment value correlated with the matched second on-time ratio in the lighting device (11). Method according to one of the preceding claims, characterized in that the clocked control of the first driver circuit (15a) with the first on-time ratio (PWM1) and / or the second driver circuit (15b) with the second on-time ratio (PWM2) is carried out according to a pulse width modulation method. Method according to claim 5, characterized in that the first on-time ratio (PWM1) and / or the second on-time ratio (PWM2) is determined again for each clock cycle of the clocked control. Method according to one of the preceding claims, characterized by: - ​​determining the first on-time ratio (PWM1) as a function of a total operating time of the first electric light source (14a, LED41) and the second on-time ratio (PWM2) as a function of a total operating time of the second electric light source (14b, LED42). Method according to one of the preceding claims, characterized by: - ​​setting the first current amplitude to a value (IG) which was used for a manufacturer-specific class classification of the first light source (14a, LED41) and / or the second current amplitude to a value (IG) which was used for a manufacturer-specific class classification of the second light source (14b, LED42). Lighting device (11) in a household appliance (1) comprising: - a first electric light source (14a, LED41) with a first driver circuit (15a) for operating the first light source (14a, LED41) with a first electric current (I1) which has a first current amplitude, - a second electric light source (14b, LED42) with a second driver circuit (15b) for operating the second light source (14b, LED42) with a second electric current (I2) which has a second current amplitude, - a control unit (40) for controlling the first driver circuit (15a) and the second driver circuit (15b), and - a first measuring element for detecting a first measured value (M1) correlated with the first current (I1) and a second measuring element for detecting a second measured value (M2) correlated with the second current (I2), characterized in that the control unit (40) is designed toThe first driver circuit (15a) is clocked with a first on-time ratio (PWM1), and the second driver circuit (15b) is clocked with a second on-time ratio (PWM2), depending on the first measured value (M1) and the second measured value (M2) in such a way that the brightness difference between the first light source (14a, LED41) and the second light source (14b, LED42) is reduced compared to operation with identical first (PWM1) and second on-time ratios (PWM2); and the first on-time ratio (PWM1) and the second on-time ratio (PWM2) are determined depending on the sensitivity characteristics of the human eye, thereby achieving the same brightness effect when using different colors. Lighting device (11) according to claim 9, characterized in that the first measuring element comprises a first measuring resistor (R41) which is coupled in series to the first light source (14a, LED41), and / or the second measuring element comprises a second measuring resistor (R42) which is coupled in series to the second light source (14b, LED42). Lighting device (11) according to claim 9 or 10, characterized in that the first driver circuit (15a) comprises a first switching element (Q41) which is coupled in series to the first light source (14a, LED41), and / or the second driver circuit (15b) comprises a second switching element (Q42) which is coupled in series to the second light source (14b, LED42). Lighting device (11) according to one of claims 9 to 11, characterized in that the first driver circuit (15a) comprises a first current source (RL41) which is coupled in series to the first light source (14a, LED41), and / or the second driver circuit (15b) comprises a second current source (RL42) which is coupled in series to the second light source (14b, LED42). Lighting device (11) according to one of claims 9 to 12, characterized in that the first light source (14a, LED41) and the second light source (14b, LED42) are optically coupled to a common light guide element (13) and / or to individual light guide elements arranged to form a functional group. Household appliance (1), in particular for the care of laundry items, with a lighting device (11) according to one of claims 9 to 13, and with a door (2) which can be illuminated by the lighting device (11) for the optical indication of operating states of the household appliance (1).

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