Technique for adjusting the brightness of LED lamps

Abstract

A method is described for adjusting the brightness of at least one LED according to a changing supply voltage provided to the at least one LED includes acquiring a parameter which indicates the changing supply voltage; and providing, on the basis of the acquired parameter, a control voltage of the at least one LED for adjusting an LED current such that the at least one LED has a predefined relative brightness change over the predefined voltage range. A control unit and an LED lamp which implement the method described here are also discloses. Furthermore, an LED lighting apparatus and a medical diagnostic device having the LED lamp disclosed here are described.

Claims

1. A method for adjusting the brightness of at least one LED according to a changing supply voltage provided to the at least one LED, wherein the supply voltage is provided by means of an electrical supply apparatus, which is designed to provide supply voltages over a predefined voltage range to adjust the brightness of an incandescent lamp, wherein the method is carried out by means of a control unit, which is provided for electrically coupling the electrical supply apparatus (100) to the at least one LED, comprising the following steps: acquiring a parameter which indicates the changing supply voltage, wherein acquiring a parameter comprises selectively coupling and decoupling an electrical load resistor, which simulates an electrical resistance of the incandescent lamp, to and from the supply apparatus, and acquiring the parameter by measuring the voltage which drops at the load resistor; and providing, on the basis of the acquired parameter, a control voltage of the at least one LED for adjusting an LED current such that the at least one LED has a predefined relative brightness change over the predefined voltage range.

2. The method according to claim 1, wherein the step of providing a control voltage comprises: calculating the control voltage on the basis of the acquired parameter and at least one predefined dimming characteristic curve, which, for the predefined voltage range, describes a dependence of the control voltage on the acquired parameter according to a predefined functional relationship.

3. The method according to claim 1, wherein the control voltage is provided such that a normalized brightness-voltage characteristic curve, which describes the relative brightness change, is substantially identical or similar to a normalized brightness-voltage characteristic curve of an incandescent lamp over the predefined voltage range.

4. The method according to claim 1, wherein the steps of acquiring a parameter and providing a control voltage are repeated at chronologically successive intervals.

5. The method according to claim 4, wherein the steps of acquiring a parameter and providing a control voltage are repeated at a predefined frequency.

6. A control unit for adjusting the brightness of at least one LED according to a changing supply voltage provided to the at least one LED, wherein the control unit is designed for electrically coupling the at least one LED to an electrical supply apparatus, which is designed to provide supply voltages over a predefined voltage range, to adjust the brightness of an incandescent lamp, comprising: an electrical load resistor which is designed for simulating the electrical resistance of the incandescent lamp and provided for being selectively coupled to and decoupled from the electrical supply apparatus; an acquisition apparatus which is designed to acquire a parameter which relates to the changing supply voltage, wherein the acquisition apparatus is designed to acquire the voltage which drops at the load resistor upon coupling of the electrical load resistor to the electrical supply apparatus and to provide said voltage to a provision apparatus as a parameter; and the provision apparatus, which is designed to provide, on the basis of the acquired parameter, a control voltage of the at least one LED for adjusting an LED current such that the at least one LED has a predefined relative brightness change over the predefined voltage range.

7. The control unit according to claim 6, wherein the provision apparatus is designed to calculate the control voltage on the basis of the acquired parameter and at least one predefined dimming characteristic curve, which, for the predefined voltage range, describes a dependence of the control voltage on the acquired parameter according to a predefined functional relationship.

8. The control unit according to claim 6, further comprising: a switching apparatus, which is designed to implement at least two switching states, wherein the switching apparatus electrically couples the load resistor to the electrical supply apparatus in a first switching state.

9. The control unit according to claim 8, further comprising a clock generator apparatus, which is electrically coupled to the switching apparatus and is designed to generate an electrical pulse sequence, in order to switch the switching apparatus back-and-forth between the at least two switching states.

10. The control unit according to claim 9, wherein the clock generator apparatus is designed to generate a pulse sequence having a predefined frequency.

11. The control unit according to claim 9, wherein the clock generator apparatus is furthermore electrically coupled to the acquisition apparatus and is designed to provide the generated pulse sequence to the acquisition apparatus for sampling the voltage which drops at the load resistor.

12. An LED lamp, which is designed for electrical coupling to an electrical supply apparatus, wherein the electrical supply apparatus is designed to provide supply voltages over a predefined voltage range, in order to adjust the brightness of an incandescent lamp, the LED lamp comprising: A control unit, an IED driver which is electrically coupled to the control unit, and at least one LED, which is electrically coupled to the LED driver; wherein the control unit is designed to provide a control voltage on the basis of the supply voltage provided by the electrical supply apparatus, and comprises: an electrical load resistor which is designed for simulating the electrical resistance of the incandescent lamp and provided for being selectively coupled to and decoupled from the electrical supply apparatus; an acquisition apparatus which is designed to acquire a parameter which relates to the changing supply voltage, wherein the acquisition apparatus is designed to acquire the voltage which drops at the load resistor upon coupling of the electrical load resistor to the electrical supply apparatus and to provide said voltage to a provision apparatus as a parameter; and the provision apparatus, which is designed to provide, on the basis of the acquired parameter, a control voltage to the LED driver; wherein the LED driver is designed to generate an LED current for the at least one LED on the basis of the provided control voltage such that the at least one LED has a predefined relative brightness change over the predefined voltage range; and wherein the at least one LED is designed to generate a luminous flux on the basis of the provided LED current.

13. The LED lamp according to claim 12, wherein the LED lamp is provided for use in medical diagnostic devices.

14. An LED lighting apparatus, comprising: an LED lamp; and an electrical supply apparatus, which is coupled to the LED lamp and is designed to provide supply voltages over a predefined voltage range to continuously adjust the brightness of an incandescent lamp; wherein the LED lamp comprises a control unit, an LED driver which is electrically coupled to the control unit, and at least one LED, which is electrically coupled to the LED driver; wherein the control unit is designed to provide a control voltage on the basis of the supply voltage provided by the electrical supply apparatus, and comprises: an electrical load resistor which is designed for simulating the electrical resistance of the incandescent lamp and provided for being selectively coupled to and decoupled from the electrical supply apparatus; an acquisition apparatus which is designed to acquire a parameter which relates to the changing supply voltage, wherein the acquisition apparatus is designed to acquire the voltage which drops at the load resistor upon coupling of the electrical load resistor to the electrical supply apparatus and to provide said voltage to a provision apparatus as a parameter; and the provision apparatus, which is designed to provide, on the basis of the acquired parameter, a control voltage to the LED driver; wherein the LED driver is designed to generate an LED current for the at least one LED on the basis of the provided control voltage such that the at least one LED has a predefined relative brightness change over the predefined voltage range; and wherein the at least one LED is designed to generate a luminous flux on the basis of the provided LED current.

15. The LED lighting apparatus according to claim 14, wherein the supply apparatus comprises a fixed voltage source and a variable series resistor for adjusting the supply voltage in the predefined voltage range, wherein the load resistor can be coupled to the variable series resistor.

16. A medical diagnostic device, comprising an LED lighting apparatus according to claim 14.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an electric circuit of a lighting apparatus having an incandescent lamp according to the prior art;

(2) FIGS. 2A and 2B show an electric circuit of a lighting apparatus having an LED lamp with and without brightness adjustment according to the prior art;

(3) FIG. 3 shows a flow chart to illustrate a method according to the invention for adjusting the brightness of an LED lamp;

(4) FIG. 4 shows a circuit diagram of a control unit for adjusting the brightness of an LED lamp according to one exemplary embodiment;

(5) FIG. 5 shows a circuit diagram of a control unit for adjusting the brightness of an LED lamp according to a further exemplary embodiment;

(6) FIG. 6 shows a circuit diagram of a control unit for adjusting the brightness of an LED lamp according to a further exemplary embodiment; and

(7) FIGS. 7A and 7B show two diagrams, which illustrate the brightness change in an LED lamp and an incandescent lamp on the basis of a rheostat setting.

DETAILED DESCRIPTION

(8) FIG. 1 shows, on the basis of a circuit diagram, an electrical circuit of a lighting apparatus 10, as is used, for example, in medical diagnostic devices. The lighting apparatus 10 comprises an electrical supply apparatus 100 and an incandescent lamp 160, which is electrically connected to the supply apparatus 100. The supply apparatus 100 comprises a voltage source 110 (generally a battery, which provides a predefined fixed voltage) and a rheostat 120 arranged in the electrical circuit 140. The supply apparatus 100 can be designed as an independent subunit of the lighting apparatus 10 that is installed separately in a medical diagnostic device, for example (shown in FIG. 1 by the dashed line).

(9) The rheostat 120 represents an adjustable series resistor, which is used for the continuous adjustment of the brightness (dimming) of the incandescent lamp 160. By actuating the rheostat 120, the series resistance value R.sub.rheo changes and the following relation applies for the voltage (or partial voltage) U.sub.lamp applied to the incandescent lamp 160:
U.sub.lamp=U.sub.bat*R.sub.lamp/(R.sub.rheo+R.sub.lamp),  (1)
wherein U.sub.bat designates the voltage provided by the voltage source 110 (battery), R.sub.lamp designates the resistance of the incandescent lamp 160, and R.sub.rheo designates the variable resistance of the rheostat 120.

(10) It should be noted that the luminous flux generated by the incandescent lamp 160 (or equivalently the brightness of the incandescent lamp) is directly proportional to the consumed electrical power. Changes in the electrical power cause corresponding changes in the brightness of the incandescent lamp 160. Therefore, for the further considerations, the electrical power consumed by the incandescent lamp 160 will be used to describe the brightness.

(11) Since the incandescent lamp 160 represents an ohmic consumer, the electrical power consumed at the incandescent lamp 160 can be calculated according to the equation
P.sub.lamp=U.sub.lamp*I.sub.lamp=U.sub.lamp.sup.2/R.sub.lamp=(U.sub.bat*R.sub.lamp/(R.sub.rheo+R.sub.lamp)).sup.2/R.sub.lamp  (2)

(12) Equation (2) shows that the electrical power of the incandescent lamp 160 is a function of the square of the partial voltage U.sub.lamp which drops over the incandescent lamp 160. The partial voltage U.sub.lamp which drops over the incandescent lamp 160 is in turn a function of the ratio of internal resistance of the incandescent lamp R.sub.lamp to total resistance (R.sub.rheo+R.sub.lamp), wherein the total resistance is variably changeable based on the rheostat setting.

(13) For example, if the value R.sub.rheo=0, the maximum voltage provided by the voltage source 110 is thus applied to the incandescent lamp 160, whereby a maximum lamp power
P.sub.lamp max=U.sub.bat.sup.2/R.sub.lamp  (2a)
and therefore a maximum brightness of the incandescent lamp 160 results. In contrast, if the rheostat assumes the value R.sub.rheo=R.sub.lamp at full deflection (in the following example, it is assumed that this is reached at a 90° deflection), the lamp power would thus be reduced to
P.sub.lamp=U.sub.bat.sup.2/(4*R.sub.lamp)=P.sub.lamp max/4  (2b)

(14) In other words, the electrical power consumed at the incandescent lamp 160 would drop to 25% of the maximum lamp power P.sub.lamp max. Accordingly, the brightness of the incandescent lamp 160 can be dimmed by the rheostat 120 to 25% of the maximum incandescent lamp brightness. In the example discussed here, the electrical power and therefore the brightness can be adjusted variably between 100% and 25% using the rheostat 120. The dimming characteristic of the incandescent lamp 160 is illustrated by the brightness-voltage characteristic curves “P_lamp” and “P_lamp_norm” shown in FIGS. 7A and 7B. Specifically, in FIG. 7A, the electrical power (this is a measure of the brightness of the incandescent lamp 160) is plotted as a function of the rheostat deflection (this is a measure of the voltage applied to the incandescent lamp). The characteristic curve shows the quadratic dependence described in equation (2) of the power (brightness) of the incandescent lamp 160 on the supply voltage U.sub.lamp. In FIG. 7B the characteristic curve “P_lamp” shown in FIG. 7A is plotted in a manner normalized to the maximum power (brightness) for better comparability. It goes without saying that lower lamp powers can also be achieved with a higher resistance value of the rheostat 120 (R.sub.rheo>>R.sub.lamp). The example described here is used solely for illustration of how the electrical power changes according to the rheostat setting.

(15) A lighting apparatus 10a will now be described in conjunction with FIG. 2A, in which an LED lamp 300a is used instead of the incandescent lamp 160 shown in FIG. 1. The LED lamp 300a comprises an LED 340 and a conventional LED driver 320a, which is electrically connected to the LED 340. This driver is designed to output a constant current to the LED 340 for activating the LED 340. The input of the LED driver 320a is in turn electrically coupled to the supply apparatus 100 discussed in conjunction with FIG. 1.

(16) If the input voltage (U.sub.in) is changed via the rheostat 120, the LED current I.sub.LED does not change. In other words, the rheostat 120 has no effect in the LED lamp 300a having the LED driver 320a. The following equations apply here:
P.sub.LED=U.sub.LED*I.sub.LED=η*U.sub.in*I.sub.in,  (3)
wherein P.sub.LED is the electrical power of the LED 340 and η is the efficiency of the LED driver 320a. In equation (3), generally η<1. This means that the electrical power consumed by the LED driver 320a (P.sub.in=U.sub.in*I.sub.in) is greater than the power provided to the LED 340. U.sub.LED is nearly constant as a physical condition. I.sub.LED is kept constant by the LED driver 320a. That is to say, when U.sub.in becomes smaller, I.sub.in becomes greater and vice versa. The LED lamp 300a shown in FIG. 2A is not dimmable.

(17) A lighting apparatus 10b having a dimmable LED lamp 300b will be described in greater detail on the basis of FIG. 2B. FIG. 2B shows an LED lamp 300b, which is again electrically coupled to the electrical supply device 100 described in conjunction with FIG. 1. The difference from the LED lamp 300a shown in FIG. 2A is that a “dimmable” LED driver 320b is provided. This driver is provided with an additional control input. A control voltage U.sub.dim for dimming the LED 340 can be applied to the control input and can be tapped at a voltage divider 330b arranged in parallel with the LED driver 320b. Such LED drivers are generally available on the market and the following relations apply:
I.sub.LED=I.sub.LED max*(U.sub.dim/U.sub.dim max) and  (4a)
U.sub.dim/U.sub.dim max=U.sub.in/U.sub.in max,  (4b)
wherein I.sub.LED max designates a maximum LED current, U.sub.dim max describes a maximum control voltage, U.sub.in max designates a maximum input voltage, and U.sub.in designates the set input voltage.

(18) The maximum control voltage U.sub.dim max is reached when the rheostat 120 assumes the value zero, i.e. when the following equation applies
U.sub.in max=U.sub.bat (when R.sub.rheo=0).  (4c)

(19) In this case U.sub.dim=U.sub.dim max and I.sub.LED=I.sub.LED max apply, as can be derived immediately when equations (4a-4c) are considered together.

(20) Since the LED lamp 300b only consumes a fraction of the electrical energy of the incandescent lamp 160 to generate the same luminous flux (the same quantity of light), the following relations furthermore apply:
P.sub.LED max=v.sub.e*P.sub.lamp max, or  (5a)
P.sub.LED=v.sub.e*P.sub.lamp, with v.sub.e<1,  (5b)
wherein P.sub.lamp and P.sub.lamp max designate the (maximum) power of the incandescent lamp 160 and P.sub.LED and P.sub.LED max designate the (maximum) power of the LED 340. v.sub.e designates a proportionality factor, which assumes values between 0.16 and 0.2 in current LEDs having high-quality light.

(21) A so-called virtual resistance of the LED lamp 300b (LED 340 and LED driver 320b together) can now be estimated with the aid of the equations (4a-5b), and is defined by the relation
R.sub.LED virt=U.sub.in/I.sub.in  (6)

(22) Similarly to equation (2) above, the following equation applies for the circuit in FIG. 2B having the rheostat 120 arranged in the circuit 140
U.sub.in=U.sub.bat*(R.sub.LED virt/(R.sub.rheo+R.sub.LED virt)).  (7)

(23) By rearranging equations (3) and (5b), the following may also be written
I.sub.in=P.sub.LED/(η*U.sub.in) and I.sub.in=(v.sub.e*P.sub.lamp)(η*U.sub.in)  (8)

(24) Equation (8) inserted into equation (6) results in
R.sub.LED virt=U.sub.in/((v.sub.e*P.sub.lamp)/(η*U.sub.in))=η*U.sub.in.sup.2/(v.sub.e*P.sub.lamp)=(η/v.sub.e)*R.sub.lamp.  (9)

(25) By inserting common values for n=0.8 and v.sub.e=0.2, a value can be estimated for the virtual resistance R.sub.LED virt=4*R.sub.lamp. In other words, the resistance of the LED lamp 300b is higher by a factor of 4 than the resistance at the incandescent lamp 160, which generates a similar luminous flux to the LED lamp.

(26) As a result of the much higher resistance of the LED lamp 300b, a rheostat 120 provided for regulating the brightness of an incandescent lamp having similar luminous flux is not suitable for performing a similar brightness adjustment on the LED lamp 300b. This will be illustrated further on the basis of the example described here.

(27) By means of equation (9), equation (7) can be converted into the equation
U.sub.in=U.sub.bat*(((η/v.sub.e)*R.sub.lamp)/(R.sub.rheo+((η/v.sub.e)*R.sub.lamp)))  (10)

(28) If R.sub.rheo=0, then U.sub.in=U.sub.bat and the following equations apply:
I.sub.in=I.sub.in max=v.sub.e*P.sub.lamp max/(η*U.sub.bat)=(v.sub.e/η)*(P.sub.lamp max/U.sub.bat) and  (11a)
I.sub.in max=(v.sub.e/η)*I.sub.lamp max.  (11b)

(29) If the rheostat 120 is adjusted to the value R.sub.lamp at full deflection (90° deflection as in the circuit in FIG. 1), the following equation furthermore applies:
U.sub.in=U.sub.bat*(((η/v.sub.e)*R.sub.lamp)/(R.sub.lamp+((η/v.sub.e)*R.sub.lamp)))=U.sub.bat*1/((v.sub.e/η)+1).  (12)

(30) At values of n=0.8 and v.sub.e=0.2, an input voltage as follows results therefrom
U.sub.in=U.sub.bat*0.8(=U.sub.in max*0.8).  (13)

(31) The following equation results from the combination of equation (13) with equations (4a) and (4b):
I.sub.LED=I.sub.LED max*0.8.  (14)

(32) This means that the rheostat 120 in the present example can cause a maximum reduction of the LED current, and therefore also of the LED power (since U.sub.LED=constant), of only 20%. The brightness change thus achieved is hardly perceptible because of a brightness perception of the human eye which follows a power function (the perceived sensitivity of the brightness is proportional to the Stevens power function of the stimulation). The circuit is therefore not suitable in this form for replacing incandescent lamps with LED lamps.

(33) The linear dimming characteristic of the LED 340, which is derivable from equation (14), is illustrated by the dashed brightness-voltage characteristic curve shown in FIGS. 7A and 7B having the designation “P_LED” and “P_LED_norm”. The electrical power is again plotted as a measure of the brightness according to the rheostat deflection as a measure of the provided supply voltage (input voltage U.sub.in in FIG. 3). For better comparison of the LED characteristic curve to the incandescent lamp characteristic curve, reference is made to the normalized illustration in FIG. 7B. While the brightness of the incandescent lamp 160 has a quadratic dependence on the deflection of the rheostat 120, the LED 340 displays a linear dependence.

(34) It can be stated that the virtual resistance of the LED lamps 300a, 300b is (η/v.sub.e)-times higher (four times higher in the example described here) than the resistance of the incandescent lamp 160. The deflection of the rheostat 120 thus causes only a minor change in the input (and simultaneously dimming) voltage of the LED lamps. A further problem is that the reduction of the input (and dimming) voltage only reduces the LED current and not the LED voltage at the same time.

(35) A method according to the invention for adjusting the brightness of at least one LED 340 according to a changing supply voltage provided by the supply apparatus 100 of the at least one LED 340 will now be described on the basis of FIG. 3, which voltage further improves the LED dimming described in conjunction with FIGS. 2B and 7. The method can be implemented by a control unit shown in FIGS. 4 to 6.

(36) According to the method according to the invention, in a first step 510, a parameter is acquired, which indicates a changing supply voltage provided to the at least one LED 340. The acquired parameter can be a voltage value, which directly indicates the supply voltage provided by the electrical supply apparatus 100. Alternatively, the acquired parameter can also be a deflection value of the rheostat 120, which is proportional to the provided supply voltage.

(37) For the direct acquisition of the supply voltage provided by the electrical supply apparatus 100, step 510 can comprise the following substeps. Coupling an electrical load resistor 408, which simulates an electrical resistance of the incandescent lamp 160, to the electrical supply apparatus 100; and acquiring the parameter by measuring the voltage which drops at the load resistor 408. In this case, the load resistor 408 simulating the incandescent lamp 160 has a resistance value which corresponds to the resistance value of the incandescent lamp 160 during operation. Therefore, the resistance of the incandescent lamp 160 in the circuit is simulated by means of the load resistor 408, wherein the voltage (or partial voltage) which drops at the load resistor 408 corresponds to the supply voltage provided to the incandescent lamp 160.

(38) Depending on the acquired parameter, in a subsequent second step S20, a control voltage of the at least one LED 340 for adjusting an LED current is provided such that the at least one LED 340 has a predefined relative brightness change over a voltage range predefined by the supply apparatus 100. This can be similar or identical to a brightness change in an incandescent lamp 160. Alternatively, the control voltage can also be provided on the basis of the acquired parameter such that the at least one LED 340 has a brightness change (brightness characteristic curve) deviating from the incandescent lamp 160 over the predefined voltage range.

(39) The predefined brightness change can be achieved by providing at least one predefined dimming characteristic curve. The predefined dimming characteristic curve describes a functional dependence of the control voltage on the acquired parameter for the predefined voltage range. This functional dependence can follow a predefined power function or logarithmic function. According to one variant, a quadratic dimming characteristic curve can be provided, which describes a quadratic change in the control voltage on the basis of the acquired parameter. A quadratic dimming characteristic similar to the dimming characteristic of the incandescent lamp 160 can thus be achieved.

(40) According to a further variant, steps S10 and S20 can be repeated at short successive chronological intervals (for example, at intervals of a few milliseconds).

(41) Further implementations of the method described here will be described in conjunction with the LED lamps illustrated in FIGS. 4 to 6.

(42) FIG. 4 shows the circuit diagram of a lighting apparatus 40. It comprises the electrical supply apparatus 100 having a voltage source 110 and rheostat 120, which was described in conjunction with FIG. 1. Furthermore, the lighting apparatus 40 comprises an LED lamp 400, which is electrically coupled to the supply apparatus 100. The LED lamp 400 can be designed such that it can be removably electrically coupled (not shown in FIG. 4) to the supply apparatus 100 via (two) electrical contacts. That is to say, the LED lamp 400 can be electrically coupled and decoupled to and from the supply apparatus 100 at any time.

(43) The LED lamp 400 comprises at least one LED 340 (for generating a white light) and an LED driver 430 having a control input for dimming the LED light. The driver 430 can be a commercially available driver (as described above in conjunction with FIG. 2B) in this case. In contrast to the configuration shown in FIG. 2B, the LED lamp 400 comprises a further electrical control unit (shown by the dashed box 402 in FIG. 4), the output of which is electrically coupled to the LED driver 430. This control unit 402 comprises the load resistor 408, which is described above in conjunction with FIG. 3, an acquisition apparatus 410 (designated in FIG. 4 with “sample & hold”), a provision apparatus 420, a switching apparatus 409, and a clock generator apparatus 440, which is electrically coupled to the switching apparatus 409 and the acquisition apparatus 410. The acquisition apparatus 410, provision apparatus 420 and clock generator apparatus 440 are electrically coupled directly to the supply apparatus 100. The load resistor 408 and the LED driver 430 are electrically coupled to the supply apparatus 100 via the switching apparatus 409.

(44) The clock generator apparatus 440 is designed to switch the switching apparatus 409 back-and-forth between a first switching state and a second switching state. For this purpose, the clock generator apparatus 440 generates an electrical pulse sequence, which consists of current pulses or voltage pulses. The pulse sequence can be generated in this case such that it repeats the current pulses or voltage pulses at a predefined frequency. The current pulses or voltage pulses can be generated, for example, at a frequency between 10 Hz and 10 000 Hz, preferably at a frequency between 100 Hz and 10 000 Hz. The switching apparatus 409 is accordingly switched back-and-forth between the two switching states at the set frequency.

(45) In the first switching state, the switching apparatus 409 connects the electrical supply apparatus 100 to the load resistor 408, which has the resistance value R.sub.load. A short time span is typically sufficient to acquire (measure) the voltage (partial voltage) U′.sub.dim, which drops at the load resistor, according to a rheostat actuation. This can take a few microseconds. The dwell time of the switching apparatus 409 in the first switching state is accordingly substantially restricted to the required measurement time. The dwell time can be adjusted via the pulse duration of a pulse. After each measurement, the switching apparatus 409 is put back into the second switching state. In the second switching state, the switching apparatus 409 couples the LED driver 430 to the supply apparatus 100 for the electrical supply of the driver 430. It should be noted that the switching apparatus 409 predominantly remains in the second switching state and only decouples the LED driver 430 from the electrical supply apparatus 100 during the acquisition of the partial voltage U′.sub.dim which drops at the load resistor 408. As a result of the short dwell time in the first switching state, this has no significant effects on the driver supply and the driver control. This is because the LED driver 430 has at least one buffer capacitor for temporarily storing electrical power. This is sufficient to bridge the short interruptions.

(46) Whenever the switch is switched in the first switching state, the partial voltage U′.sub.dim applied at the load resistor (this is dependent on the current rheostat deflection) can be measured. As a result of the high switching frequency, the measurement of the partial voltage U′.sub.dim which drops at the load resistor 408 can be repeated many times per unit of time (100 to 10 000 measurements per second are possible at the above-mentioned repetition rate frequency). Each chronological change in the partial voltage U′.sub.dim applied at the load resistor as a result of a rheostat actuation can be acquired “practically continuously”. It is thus ensured that the brightness change to be adjusted is adapted to every resistance change at the rheostat 120 without noticeable delay.

(47) The acquisition apparatus 410, which is coupled to the electrical load resistor 408, is provided for acquiring the partial voltage U′.sub.dim which drops at the load resistor. It acquires the potential difference applied at the electrical load resistor 408. The acquisition apparatus 410 is designed in the control unit 402 formed in FIG. 4 as an analog sample and hold circuit (frequently also referred to as an S/H circuit). The S/H circuit is designed to sample and briefly hold the partial voltage U′.sub.dim which drops at the load resistor 408. The sample and hold phases of the S/H circuit are in this case synchronized with the switching states of the switching apparatus 409. Specifically, the switching apparatus 409 and the S/H circuit are synchronized with one another such that the S/H circuit performs the sampling when the switching apparatus 409 is in the first switching state. In contrast, the S/H circuit is in the hold phase when the switching apparatus 409 is in the second switching state. For synchronization, the pulse sequence provided by the clock generator apparatus 440 is applied to the S/H circuit. According to one implementation, the pulse sequence (Clk1) provided to the S/H circuit can have a slight time delay in relation to the pulse sequence (Clk2) applied to the switching apparatus 409. It is thus ensured that the S/H circuit switches into the sample phase with a slight time delay in relation to the switching in the first switching state. By way of the slight time delay, settling effects in the circuit upon switching into the first switching state can be blanked out from the measurement.

(48) In order that a similar dimming characteristic is achieved with the LED 340 as with the incandescent lamp 160 (see FIG. 7: quadratic dimming characteristic), the partial voltage U′.sub.dim acquired and provided by the acquisition apparatus 410 is processed further. This is performed by the provision apparatus 420, the input of which is electrically connected to the S/H circuit 410 and the output of which is connected to the control input of the LED driver 430.

(49) The provision apparatus 420 is designed to convert the partial voltages U′.sub.dim provided by the S/H circuit 410 into control voltages U.sub.dim. The provision apparatus 420 can comprise an electrical circuit for this purpose, which converts the provided partial voltages U′.sub.dim (as the input signal) into a control voltage U.sub.dim (as the output signal) according to the following equation:
U.sub.dim=a*U′.sub.dim.sup.2 with a=U.sub.dim max/(U.sub.bat).sup.2.  (15)

(50) By means of equation (15) and the relationship I.sub.LED=I.sub.LED max*(U.sub.dim/U.sub.dim max) (see equation (4a) above), the following observation results:
I.sub.LED=I.sub.LED max*((a*U′.sub.dim.sup.2)/U.sub.dim max)  (16a)
I.sub.LED=I.sub.LED max*(((U.sub.dim max/(U.sub.bat).sup.2)*U′.sub.dim.sup.2)/U.sub.dim max)  (16b)
I.sub.LED=I.sub.LED max*(U′.sub.dim/U.sub.bat).sup.2.  (16c)

(51) Since U′.sub.dim max=U.sub.bat, the following relationship follows:
I.sub.LED=I.sub.LED max*(U′.sub.dim/U′.sub.dim max).sup.2.  (17)

(52) The acquisition described here of the partial voltage U′.sub.dim which drops at the load resistor is repeated at an interval of, for example, several milliseconds. The brightness adjustment of the LED thus reacts very rapidly to the adjustment of the series resistance. In this case, the virtual resistance of the LED system does not play a role, since the voltage ratio which is predefined by the rheostat 120 is not ascertained at the input of the LED system, but rather briefly at the load resistor 408 (the resistance of which corresponds to the comparable incandescent lamp 160).

(53) The following then applies for the electrical power of the LED according to the rheostat setting by means of the general relationship illustrated above in equation (3):

(54) If the value R.sub.rheo=0 is set at the rheostat, U.sub.in=U.sub.bat and the following equations apply:
I.sub.LED=I.sub.LED max  (18a)
P.sub.LED=U.sub.LED*I.sub.LED=U.sub.LED*I.sub.LED max=P.sub.LED max(=v.sub.e*P.sub.lamp max)  (18b)

(55) For example, if the rheostat is set to the value R.sub.rheo=R.sub.lamp=R.sub.load, thus:
U′.sub.dim=U′.sub.dim max/2  (19a)
I.sub.LED=I.sub.LED max*((U′.sub.dim max/2)/U′.sub.dim max).sup.2=I.sub.LED max/4  (19b)
P.sub.LED=U.sub.LED*I.sub.LED=U.sub.LED*I.sub.LED max/4=P.sub.LED max/4  (19c)

(56) It can be seen from the equations (18a/b) and (19a-c) that a deflection of the rheostat 120 from the value 0 to the value R.sub.lamp causes the same relative power and therefore brightness change as in the incandescent lamp 120 (see equations (2a) and (2b)), namely from 100% to 25%. Of course, lower LED powers can also be set with a higher resistance value of the rheostat (R.sub.rheo>>R.sub.lamp).

(57) In general, the following equivalent relationships apply for the LED power with the control voltage provided by the provision apparatus 420 (see equation 17):
P.sub.LED=U.sub.LED*I.sub.LED=U.sub.LED*I.sub.LED max*(U′.sub.dim/U′.sub.dim max).sup.2  (20a)
P.sub.LED=U.sub.LED*I.sub.LED=U.sub.LED*I.sub.LED max*(U.sub.in/U.sub.in max).sup.2  (20b)
P.sub.LED=U.sub.LED*I.sub.LED=U.sub.LED*I.sub.LED max*(U.sub.in/U.sub.bat).sup.2  (20c)
P.sub.LED=U.sub.LED*I.sub.LED=v.sub.e*P.sub.lamp max*(U.sub.in/U.sub.bat).sup.2  (20d)
P.sub.LED=U.sub.LED*I.sub.LED=v.sub.e*(U.sub.in.sup.2/R.sub.lamp)  (20e)

(58) It can be seen from equations (20a-e) that the dimming characteristic U.sub.dim of the LED lamp 400 obtained by the provided control voltage U.sub.dim corresponds to the dimming characteristic of the incandescent lamp 160 with a v.sub.e-fold downscaling. The dimming characteristic obtained by the activation according to the invention of the LED lamp 400 is illustrated by the dotted curve “P_LED_new” in FIG. 7a. The normalized dimming characteristic is coincident with the normalized dimming characteristic of the incandescent lamp (P_Lamp_norm) in FIG. 7B and is therefore not visible.

(59) The specified values are examples. The principle can also be applied to other rheostat resistances, η- and v.sub.e.

(60) The quadratic dimming characteristic curve (V.sub.0=a*V.sub.in.sup.2) which is implemented in the provision apparatus 420 can alternatively also be formed as an interpolation, consisting of multiple linear characteristic curves.

(61) A further circuit diagram of a lighting apparatus 40a having an LED lamp 400a is shown on the basis of FIG. 5, which comprises a control unit 402a according to the invention, which implements the method illustrated in conjunction with FIG. 3.

(62) The circuit of the LED lamp 400a substantially corresponds to the circuit of the LED lamp 400 in FIG. 4. Reference is made in this regard to the description in conjunction with FIG. 4. The LED lamp 400a only differs on account of the embodiment of the provision apparatus 420a of the control unit 402a integrated in the LED lamp 400a.

(63) As shown in FIG. 4 and FIG. 5, in addition to the LED driver 430, the acquisition apparatus 410, the provision apparatus 420, 420a and the clock generator apparatus 440 are coupled to the electrical supply apparatus 100 and are powered thereby. Depending on the voltage source used and in particular in the case of rheostats having higher resistance values (R.sub.rheo), the input voltage U.sub.in at the LED lamp 400a can be adjusted to be sufficiently low that the electronic components of the control unit 402a (i.e. the acquisition apparatus 410, the provision apparatus 420, 420a and the clock generator apparatus 440) no longer function. To ensure the functionality of the electronic components, the input voltage U.sub.in cannot fall below a minimum voltage U.sub.in min. However, this would have the result that arbitrarily low LED brightnesses cannot be set using the control unit 400a described here.

(64) If the threshold value for the minimum input voltage is at U.sub.in min and a characteristic similar to an incandescent lamp is desired in the operating range between U.sub.bat and U.sub.in min, such that slightly above U.sub.in min the brightness of the LED can be reduced to 0%, the control signal U.sub.dim at the control input of the LED driver 430 has to be modified further, specifically according to the following considerations.

(65) If U′.sub.dim=U′.sub.dim max=U.sub.bat, U.sub.dim=U.sub.dim max should be reached (i.e. 100% of the LED brightness). In contrast, if U′.sub.dim=U.sub.in min, U.sub.dim=0 should be the case (i.e. 0% of the LED brightness).

(66) Accordingly, the provision apparatus 420 shown in FIG. 4 is modified further such that it generates a control signal U.sub.dim and provides it to the LED driver 430, which converts the partial voltages U.sub.dim provided by the S/H circuit according to the following equation:
U.sub.dim=a′*(U′.sub.dim−U.sub.in min).sup.2 with a′=U.sub.dim max/(U.sub.bat−U.sub.in min).sup.2.  (21)

(67) If the rheostat is adjusted to the value R.sub.rheo=0, U′.sub.dim=U.sub.bat and it follows with equation 21:
U.sub.dim=a′*(U′.sub.dim−U.sub.in min).sup.2=(U.sub.dim max/(U.sub.bat−U.sub.in min).sup.2)*(U.sub.bat−U.sub.in min).sup.2=U.sub.dim max.  (22)

(68) 100% of the LED brightness is therefore reached at R.sub.rheo=0.

(69) In contrast, if the rheostat is adjusted so that the minimum input voltage U.sub.in=U.sub.in min is reached (i.e. at maximum rheostat actuation), the following thus applies:
U.sub.dim=a′*(U′.sub.dim−U.sub.in min).sup.2=(U.sub.dim max/(U.sub.bat−U.sub.in min).sup.2)*(U.sub.in min−U.sub.in min).sup.2=0.  (23)

(70) 0% of the LED brightness is therefore reached at R.sub.rheo=R.sub.rheo max.

(71) The provision apparatus 420a therefore generates a control voltage which is offset-shifted by U.sub.in min.

(72) A further circuit diagram of a lighting apparatus 40b having an LED lamp 400b is shown on the basis of FIG. 6, which lamp comprises a control unit 402b according to the invention, which implements the method described in conjunction with FIG. 3.

(73) The LED lamp 400b differs from the LED lamps 400 and 400a shown in FIGS. 4 and 5 on account of the embodiment of the control unit 402. Instead of an analog S/H circuit 410 and provision apparatus 420, 420a, a microcontroller system 415 is now used, which performs both the clock generation and also the quadratic scaling (or scaling according to another power function or logarithmic function) of the acquired partial voltage U′.sub.dim which drops at the load resistor 408 and optionally the offset shift (by the value U.sub.in min) described in conjunction with FIG. 5.

(74) Furthermore, the control unit 402b comprises an A/D converter 416, which is designed to convert the partial voltage which drops at the load resistor 408 into a digital signal (bit sequence), and a D/A converter 418, which is designed to convert the digital control voltage signal provided by the microcontroller system 415 into an analog control voltage. The switching apparatus 409 and electrical load resistor 408 have the same arrangement and function within the control unit 402b as in the control units 400 and 400a of FIGS. 4 and 5. Reference is made in this regard to the corresponding description above.

(75) By means of the technology described here LED lamps can be provided which have a similar power-voltage characteristic curve and accordingly a brightness-voltage characteristic curve like an incandescent lamp, but with only a fraction of the power consumption with equal light yield. The LED lamps can therefore be used to replace conventional incandescent lamps, without the electrical supply apparatuses having to be adapted for this purpose. This is particularly advantageous if electrical supply apparatuses are permanently installed in a device. This is frequently the case in medical diagnostic devices.