Abstract:
A method for controlling an output of an electrical AC voltage U comprising the following steps: switching on a current flow I induced by the AC voltage as soon as an absolute value of the AC voltage U exceeds a switching-off target voltage, and switching on the current flow I as soon as the absolute value for the AC voltage U falls below a switching on target voltage. The switching-off target voltage and the switching-on target voltage are defined as positive and the switching-on target voltage is lower than or equal to the switching-off target voltage. The method according to the invention serves to dim an LED lamp in a brightness range of 0% to 100% of a maximum brightness of the LED lamp.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for controlling an output of an electrical AC voltage. Furthermore, the invention relates to a dimmer for an electrical AC voltage as well as an assembly of such a dimmer having a lamp comprising at least one LED element. 
       BACKGROUND OF RELATED ART 
       [0002]    Methods for controlling an output of an AC voltage are used in particular in dimmers for controlling the brightness of lamps. Known methods for controlling the output are the phase trailing-edge control and phase leading-edge control, for example. With the phase leading-edge control, a current flow delayed after the zero crossing of the AC voltage is switched on. With the phase trailing-edge control, the current flow is already switched off prior to the zero crossing of the AC voltage. 
         [0003]    In case of capacitively coupled loads, methods of this type have the disadvantage, that the switching-on procedures of the current flows result in large voltage differences, in particular to a change in the sign of the voltage at the capacitors. The large voltage differences lead to undesired current pulses, which cause disruptions, and can damage components. In particular, these methods are not suitable for dimming LED lamps when the LED lamps are connected to an AC voltage network via a rectifier and a coupling capacitor. So far, such LED lamps have been regarded as not dimmable. 
       SUMMARY OF THE INVENTION 
       [0004]    The object of the present invention is thus to create an improved method for controlling an output of an AC voltage with which current pulses of this type can be avoided, such that LED lamps can be dimmed with this method. 
         [0005]    This object is achieved according to the invention by a method having the features in Claim  1 . 
         [0006]    The method according to the invention is distinguished in that a current flow induced by the applied AC voltage is always then interrupted when the absolute value of the AC voltage exceeds a switching-off target voltage. The switching-on of the current flow then occurs when the absolute value of the AC voltage again falls below a switching-on target voltage, wherein the switching-on target voltage is lower than or equal to the switching-off target voltage. The switching-on target voltage and the switching-off target voltage are defined hereby as positive. With a switching regulation of this type, it is ensured that the AC voltage does not pass through a zero crossing between the switching off and the switching on. The AC voltage thus does not change signs between the switching off and the switching on. Voltage differences at the capacitor connected thereto are reduced between the switching procedures. Undesired current pulses resulting from the voltage differences are reduced and in particular, damages are prevented. 
         [0007]    The applied AC voltage is a mains AC voltage in particular. For a periodic AC voltage with a defined frequency, the temporal course of the AC voltage is clearly determined. There is a fixed relationship between the absolute value of the AC voltage and the time axis. The switching-off and switching-on target voltages thus can be assigned to periodically repeating points in time at which the current flow is switched on or off. The method can thus also be defined through the specification of switching-on and switching-off points in time. 
         [0008]    The AC voltage that is switched on and off by means of the method is applied as an input voltage to at least one LED lamp. The current is thus switched on and off in the lead to the LED lamp. As a result, a dimming of the LED lamp is enabled, without it having to contain an integrated switch-mode power supply. 
         [0009]    A brightness of the LED lamp can be controlled in a brightness range of 0% to 100% of a maximum brightness of the LED lamp. Currently known dimmable LED lamps can be dimmed in a limited brightness range. By means of the method according to the invention, the full brightness range can be exploited, including that below 10% of the LED lamp. In particular, it is possible to control the brightness of the LED lamp in a brightness range of 0% to 20%, in particular from 0% to 15%, and in particular from 0% to 10%. 
         [0010]    If the output serves to operate numerous LED lamps, the latter can be grouped in particular in a parallel circuit. Furthermore, the individual LED lamps can each comprise a capacitive mains adapter having an input-side coupling capacitor. 
         [0011]    The switching-on target voltage is preferably substantially equal to the switching-off target voltage. This is to be understood to mean that the difference between the switching-off target voltage and the switching-on target voltage is less than twice the voltage that drops at a smoothing capacitor for the LED lamp. By way of example, it may be provided that the switching-on target voltage is at least 80%, in particular at least 90%, in particular at least 99% of the switching-off target voltage. As a result, voltage differences during the switching on and off of the current flow are further reduced. Disruptive current pulses are consequently prevented. It is ensured in particular that the difference between the switching-off target voltage and the switching-on target voltage is smaller than the voltage drop at a connected load. 
         [0012]    A substantial equivalence of the switching-on target voltage and the switching-off target voltage has the further advantage that the current flow is symmetrically interrupted on both sides of the maximum absolute value of the applied AC voltage. This ensures that the average current flow is approximately proportional to the switching-on target voltage and/or to the switching-off target voltage. The output can thus be quantified via the switching-on target voltage and/or the switching-off target voltage. 
         [0013]    Advantageously, the switching-on target voltage and/or the switching-off target voltage can be set in an adjustable manner. This has the advantage that the output can be adjusted. The output can also be defined indirectly via a switching-on target voltage and/or switching-off target voltage that can be set in an adjustable manner. 
         [0014]    Moreover, the invention relates to a dimmer for connecting to an electrical AC voltage having the features of the independent claim  4 . The dimmer according to the invention is also suitable for loads that cannot be operated with a voltage that is controlled by means of a phase leading-edge control or phase trailing-edge control. 
         [0015]    The dimmer according to the invention comprises a switch element for switching a current flow induced by the AC voltage on and off. Moreover, a control element is provided, which provides for the switching on and off of the current flow in accordance with the method outlined above by means of a control voltage. The advantages of this dimmer correspond to those of the method according to the invention. 
         [0016]    The dimmer preferably has a rectifier for the applied AC voltage. With rectified voltage, the switch element can be designed as a transistor. This provides for an economical and simple construction of the dimmer. 
         [0017]    The control element can comprise a potentiometer. The switching-off target voltage and the switching-on target voltage can be set in an adjustable manner with the potentiometer. 
         [0018]    Furthermore, the invention relates to an assembly having a dimmer and at least one lamp, which has at least one LED element. The advantages of this assembly correspond to those of the method and/or the dimmer of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    An exemplary embodiment of the invention shall be explained below based on the drawings. Therein: 
           [0020]      FIG. 1  shows a schematic circuit diagram for an assembly having a dimmer and an LED lamp, and 
           [0021]      FIG. 2  shows a temporal course of a period of an applied AC voltage as well as a current flow that is switched on and off according to the method according to the invention, when an ohmic load is connected thereto. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      FIG. 1  shows circuit diagram of an assembly comprising a dimmer  2  and an LED lamp  4 . The dimmer  2  and the LED lamp  4  are connected to a voltage source  6 . A periodic AC voltage U is provided by the voltage source  6 , in particular a mains AC voltage. 
         [0023]    The LED lamp  4  comprises an input-side coupling capacitor  8  for limiting the flowing current. A rectifier  10  composed of four diodes  12  is connected to the coupling capacitor  8 . The rectifier  10  serves to convert an input AC voltage to a DC voltage. In order to smooth the DC current, a smoothing capacitor  14  is connected downstream of the rectifier  10 . Two LED elements  16  are connected in series for converting the DC current to light. Alternatively, the LED lamp  4  can also comprise one or more than two LED elements  16 . 
         [0024]    LED lamps  4  of this type are known. They have the disadvantage that a brightness emitted from the LED elements  16  cannot be dimmed by means of conventional dimmers. Furthermore, an undisrupted operation is not ensured with conventional dimmers. The reason for this is that the voltage jumps caused by conventional dimmers induce disruptive current pulses at the capacitors  8  and  14 . These current pulses cause disruptive noise and can damage components of the LED lamp or the conventional dimmer. 
         [0025]    In the assembly shown here, the LED lamp  4  is connected via a connecting lead  18  to the dimmer  2  and the voltage source  6 . The dimmer  2  has an input-side rectifier  20 , which corresponds to the rectifier  10  of the LED lamp  4  in terms of its construction and function. The rectifier  20  is connected to a switch element  22 , which serves to switch the current on and off. 
         [0026]    In the present exemplary embodiment, the switch element  22  is a transistor, in particular a normal blocking metal-oxide-semiconductor field-effect transistor (MOSFET). This means that without control voltage, the MOSFET  22  blocks the current flow through the dimmer  2 . In order to enable current flow, a gate  24  of the MOSFET  22  is connected to a control output  26  of a control element  28 . The control element  28  can use voltage dropping via the MOSFET  22  for its own current supply. The voltage dropping via the MOSFET  22  is proportional to the value for the AC voltage U when the transistor is switched off and no voltage is applied to the load. The control element  28  can induce a control voltage at the gate  24  via the control output  26 . With an induced control voltage the MOSFET  22  becomes conducting. Applying the control voltage at the gate  24 , a current flow through the LED lamp  4  can be switched on. The switching off or interrupting of the current flow is obtained through the control element  28  by switching the control voltage at the gate  24  off. The current flow through the dimmer  2  is thus switched on and off by the control element  28 . In particular, a current flow is only possible when a control voltage is applied to the gate  24  of the MOSFET  22 . By switching the current flow on and off by means of the dimmer  2 , the voltage and output tapped into by the connecting lead  18  is varied. 
         [0027]    The method for switching the dimmer shall be explained below, based on  FIG. 2 . For this, the temporal course of a period of the applied AC voltage U is depicted in  FIG. 2 . For this, the curve of the AC voltage U is plotted along the time axis t. Moreover, a current I flowing through the dimmer  2  is plotted over the time axis t in  FIG. 2 , when an ohmic load is connected thereto. When an LED lamp  4  is connected, the current flow is approximately proportional to the derivation of the voltage over time. 
         [0028]    The AC voltage U runs in a sine wave over time t. This means that it passes through a zero crossing at the start of the depicted period, at the middle of the depicted period, and at the end of the depicted period in each case. After passing through the first zero crossing at the start of the period, the AC voltage U increases, and thus its absolute value increases. Because the control element  28  applies a control voltage at the gate  24  of the MOSFET  22  at this point in time, the current I also flows, which is approximately proportional to the derivation of the AC voltage as a function of time. When the absolute value for the AC voltage U exceeds a switching-off target voltage  30 , the control element  28  switches the control voltage at the gate  24  off, and the MOSFET  22  blocks the current flow. The current I decreases abruptly to 0. After passing through its maximum, the AC voltage U decreases. As soon as the absolute value of the AC voltage falls below a switching-on target voltage  32 , the control element  28  applies a control voltage to the gate  24 , and activates the MOSFET  22 . The current I is again applied abruptly. 
         [0029]    When the MOSFET  22  is blocked, the current flow I is interrupted. 
         [0030]    In order to obtain the advantages of the method described above, it is necessary that the switching-off target voltage  30  and the switching-on target voltage  32  are defined as positive. The switching-on target voltage  32  must not be greater than the switching-off target voltage  30 . In the example shown here, the switching-on target voltage  32  is substantially equal to the switching-off target voltage  30 . This means that the switching-on target voltage  32  is at least 80%, in particular at least 90%, in particular at least 99% of the switching-off target voltage  30 . The difference between the switching-off target voltage  30  and the switching-on target voltage  32  is less than twice the drop in voltage at the smoothing capacitor  14 . As a result, voltage jumps at the capacitors  8 ,  14  of the LED lamp  4  are prevented. Disruptive current pulses at the capacitors  8 ,  14  are avoided. 
         [0031]    By switching on and off, the current flow I is temporally interrupted. It is interrupted in particular in the ranges of the maximum absolute value of the AC voltage U. As a result, the average current flow, and thus the output tapped at the connecting lead  18 , is reduced. If the switching-off target voltage  30  and the switching-on target voltage  32  are substantially equal, the average current flow as well as the output applied to the LED lamp  4  are approximately proportional to the switching-off target voltage  30  or the switching-on target voltage  32 . The power applied to the LED lamp  4  can thus be controlled continuously by defining the switching-off target voltage  30  and the switching-on target voltage  32 . As a result, it is possible to control the brightness of the LED lamp  4  in a brightness range of 0% to 100% of the maximum brightness of the LED lamp  4 . 
         [0032]    Because the temporal course of the AC voltage U is predefined, the exceeding of the switching-off target voltage  30  can be assigned to a switching-off point in time  34 . The switching on when the voltage has fallen below the switching-on target voltage  32  can be assigned to a switching-on point in time  36 . In the time interval from the switching-off point in time  34  to the switching-on point in time  36 , the current flow I is zero. 
         [0033]    After the current flow I has been switched back on at the switching-on point in time  36 , the absolute value of the AC voltage U decreases until the AC voltage U passes through a zero crossing. Because the absolute value of the AC voltage U is independent of the sign of the AC voltage U, the switching procedures are also carried out when the poles are reversed. The switching off of the current flow I occurs here at a switching-off point in time  34 ′, the switching back on of the current flow I occurs at a switching-on point in time  36 ′. 
         [0034]    Due to the periodic course of the AC voltage U, the switching-off points in time  36 ,  36 ′ and the switching-on points in time  34 ,  34 ′ correlate with twice the frequency of the AC voltage U. The periodic temporal course of the AC voltage makes it possible thereby to define the switching procedures by means of the switching points in time  34 ,  34 ′,  36 ,  36 ′, instead of the target voltages  30 ,  32 . The control element  28  can thus also be designed as a timer correlated with the periodicity of the AC voltage U, which defines the switching points in time  34 ,  36 ,  34 ′,  36 ′ through the temporal course of the control voltage. 
         [0035]    Other circuits can also be used for the dimmer. The exemplary embodiment shown here has the advantage of a simple and robust construction. 
         [0036]    The important aspect of the dimmer is that a switch element  22  is provided with which the current flow can be switched on and off, and that a control element  28  is provided, which controls the switch element  22 . The control element  28  equalizes the absolute value of the applied AC voltage U to a switching-off target voltage  30  and/or a switching-on target voltage  32 . In order to set these target voltages, it is advantageous when the control element  28  includes a potentiometer  38 , with which the target voltages  30 ,  32  can be set. 
         [0037]    Alternatively to the normal blocking MOSFET  22 , a normal conducting MOSFET may be provided. This means that the current I flows in the absence of a control voltage at the control output  26  of the control element  28 . The current I is then switched off by applying the control voltage. 
         [0038]    In an exemplary embodiment that is not shown, numerous LED lamps connected in parallel are connected to the dimmer, each of which comprises an input-side coupling capacitor.