WO2000065880A1

WO2000065880A1 - Light source consisting of several successively connected leds

Info

Publication number: WO2000065880A1
Application number: PCT/EP2000/003624
Authority: WO
Inventors: Roland Reiss
Original assignee: Sickinger, Monika
Priority date: 1999-04-22
Filing date: 2000-04-20
Publication date: 2000-11-02
Also published as: AU4913400A; DE19918336A1

Abstract

The invention relates to a light source, consisting of a number of LEDs (2) which are successively connected one behind the other and which are connected in series to a series resistance (4). The invention is characterised in that (a) the electric circuit of the LEDs (2) has, in addition to the series resistance (4), a rapid electronic switch (6) and a rectifier circuit (8) as the power source which, when in operation, is connected to the voltage of the public mains supply; (b) the rapid electronic switch (6) is connected to a pulse generator (10) which provides pulses with a repetition frequency of at least 70 Hz; (c) the series resistance (4) is smaller than a protective resistor which operates with a constant current (d) the arrangement is configured in such a way that the amplitude of the current to the LEDs (2) is greater than the maximum current which is permitted with a constant current.

Description

Light source from several cascaded

LEDs

The invention relates to a light source, comprising a plurality of LEDs connected in series, which are connected in series with a series resistor.

LEDs - light-emitting diodes - have a very high degree of efficiency when it comes to converting electricity into light. This efficiency is particularly high for LEDs for green light. But also so-called "white light LEDs" have one in particular in relation to

Incandescent lamps very good efficiency. The high level of efficiency and their long service life and thus high reliability make them particularly suitable for certain applications. Such applications typically include lighting of display boards, night lighting, for example in hotels or hospitals, emergency lighting and similar lighting installations in which a bundled, powerful light beam is not essential in the first place.

A circuit of this type consisting of a plurality of LEDs connected in series with a series resistor is common, especially if a higher light intensity is intended. Another advantage of connecting the LEDs in series is that this results in a higher voltage drop across the light source. Typically, an LED has an "operating voltage" of approximately 2V. If, for example, 20 LEDs are operated in the series connection, a sufficient light intensity is achieved for typical applications, so this arrangement requires an operating voltage in the order of 40V. This value differs significantly from the normally available mains voltage of 230V or 110V, so that the voltage for LED operation must either be transformed down using a transformer or via a

Series resistor is brought to this value.

Even in applications in which the mains voltage is transformed down to the required operating voltage for the arrangement of LEDs, there is a series connection with the LEDs

Series resistor required. If LEDs were operated without such a series resistor, the current through the LED would increase, which would lead to the LED heating up and ultimately being destroyed. The interposition of the series resistor leads to a current increase in the LED due to a higher voltage drop on

Series resistor is compensated.

The favorable efficiency of the LED for the conversion from current to light is understandably affected by the operational losses in the series resistor and possibly during the transformation, which affects the

Possible uses of LEDs severely hampered. In addition, the required operating voltage of typical LED light sources is not suitable for operation on the voltage of the public network, especially after rectification. The use of transformers for voltage adjustment is on the one hand with

Costs involved and, on the other hand, not always easy to implement due to space constraints, whereas a larger series resistor leads to significantly higher losses. The invention is therefore based on the object of providing a light source comprising a plurality of LEDs connected in series which can be operated at the voltage of the public network and in which excessive power losses in the series resistor are avoided.

According to the invention, this is achieved in the light source described above by

(a) that the circuit of the LEDs in addition to the series resistor has a fast electronic switch and as a current source a rectifier circuit, which in operation is connected to the voltage of the public

Power network is connected, has;

(b) that the fast electronic switch is connected to a pulse generator which delivers pulses with a repetition frequency of at least 70 Hz; (c) that the series resistor is smaller than it would be for operation at constant current, and that the arrangement is so constructed that the current amplitude at the LEDs is greater than the maximum current permitted during constant operation.

It is known to operate a plurality of LEDs with pulsed current. For example, DE-A-35 35 240 describes a traffic signal device with a light source comprising a plurality of LEDs which are fed in the frequency range from approximately 15 to 35 Hz. This low repetition rate is used to catch the eye's attention by flashing or flickering visibly

Increase road users. In the same document, reference is made to the prior art, in which LEDs are operated with current pulses with up to twice the mains frequency, ie 100 or 120 Hz. The reason for this is said to be an increase in the electrical power currently being supplied in order to produce an increase in the light intensity.

However, this prior art does not teach to select the series resistor smaller than it would be for constant current operation, nor does it teach to provide a current amplitude on the LEDs which is greater than the maximum current through the LEDs permitted during constant operation .

The possible energy saving effect will be illustrated in the following using a simple example. The case of constant operation is compared with operation with current pulses: The example is based on the mains voltage of 110V, which in the rectified state supplies an almost constant current with a voltage of the order of 150V. 20 LEDs should be considered in series at this voltage. In both cases, a total power of 1.5 W is assumed.

- Constant operation: With a constant current of 10 mA and a typical operating voltage of an LED of 2V, the following results:

Required series resistor: (150V - 20 x 2V) / 10mA = 11 kOhm Power loss: P x R = 10 mA x 10 mA x 11 kOhm = 1, 1W Converted power in the LEDs: 40V x 10 mA = 0.4W total power 1, 5W, "efficiency": 0.4W / 1.5W = 26.6%.

- Operation with current pulses: With current pulses of 100 mA, the operating voltage per LED increases from 2V to approx. 4V. A duty cycle (pulse duration / period) of 1/10 is assumed. Overall, the following comparison values result: Required resistance: (150V - 20 x 4V) / 100 mA = 700 Ohm power loss PxR = 100mA x 100mA x 700Ohm x 1/10 = 0.7W Converted power in the LEDs: 80V x 100 mA x 1/10 = 0.8W Total power 1.5W "efficiency": 0.8W / 1.5W = 53%.

As you can easily see, about 3/4 of the electrical energy in the resistor is converted into heat during constant operation, while in the second example it is significantly less than half. It is particularly favorable that in pulsed operation within the typical operating limits of the LEDs there is essentially no loss in their service life.

The pulse generator of the light source is preferably designed such that the pulse duty factor (pulse duration / period) of the current applied to the LEDs is less than 1/3, preferably less than 10 ^'1 , more preferably less than 10 ² and even more preferably less than 10 ^"3 Overall, it can be said that the smallest possible duty cycle is particularly preferred. However, in order to achieve sufficient brightness, a very high current must be brought into the LEDs in a short time with a very short duty cycle. In practice, problems can arise with a very small duty cycle occur.

The pulse generator is preferably designed such that the repetition frequency is greater than 130 Hz, preferably greater than 400 Hz, more preferably greater than 1 kHz and particularly preferably greater than

Is 10 kHz. Furthermore, the pulse generator is preferably designed so that the pulse duration is less than or equal to 10 ^"3 seconds, preferably less than 10 ^{~ 5} seconds and particularly preferably less than 10 ^{~ 6} seconds. The length of the pulse pauses can preferably be varied with the pulse length kept constant and the current constant. This enables the light source to be “dimmed”. The longer the pause between the individual current pulses, the lower the power introduced into the LEDs and consequently the output from the LEDs

Light output, the lower. It is easy to imagine that an infinitely long pause corresponds to the switched-off state. It is particularly advantageous to go into a high frequency range for the operation of the light source if a large dimmable range is to be covered, in particular in order to avoid flickering of the light from the light source.

The arrangement is preferably constructed in such a way that the current applied is greater than that permissible in constant operation by more than a factor of 2, preferably by more than a factor of 5, more preferably by more than a factor of 10 and very particularly preferably by a factor of 20 Is maximum current. The current-voltage characteristic of the LED specifies a certain "operating voltage" for each current value. A typical current-voltage characteristic for LEDs follows an exponential behavior. In principle, the greater the current, the greater the operating voltage. With constant operation, the current strength in the LED cannot be increased arbitrarily. This would ultimately lead to the destruction of the LED. With pulsed operation, the current intensity in the LED can be increased considerably compared to constant operation. The dimensioning rule is to choose the pulses so short that the LED does not heat up significantly in the pulse duration and in particular that the heat dissipation away from the LED is so high that there is no temperature increase in the LED from period to period. Typically, the Cables to the LEDs are the factor that limits the maximum pulse current. The LED chips are typically bonded with extremely thin gold or silver wires or electrically connected. If there is a current overload, a failure often occurs in the area of this connection. The type-specific performance data provide information about this type-dependent limit. It can generally be assumed that the "operating voltage" of an LED can be increased by a factor of 2 to 3 up to this value, ie from typically 2V to up to 5-6V.

The pulse generator and the fast electronic are preferred

Switch designed so that they cause a current limitation through the LEDs together with the series resistor. In particular, you can control the fast electronic switch so that it works in the manner of an adjustable resistor. This leads to very safe and constant operation, which prevents the LEDs from being destroyed by the occurrence of current peaks even in the area of their operating limit. As a result, the series resistor in particular can be chosen to be particularly small, which significantly reduces the power dissipated at the series resistor.

The pulse generator is preferably constructed with an electronic circuit which has:

(a) a capacitor chargeable from a constant current source;

(b) a voltage divider formed with two resistors for generating a reference voltage;

(c) a uni-function transistor connected to the reference voltage and the capacitor voltage, which fires as soon as the capacitor voltage becomes somewhat less than the reference voltage; and (d) a switch connected to the uni-function transistor and the capacitor, which is switched on by the ignition of the uni-function transistor for the duration of a pulse.

Such a structure can essentially be realized from standard components. With the preferred additional use of a MOSFET component as a fast electronic switch, such a ballast for the LEDs can be implemented relatively cheaply. In addition, there is the possibility of further integration of the corresponding circuit into an integrated circuit. The corresponding ballast can be manufactured with the appropriate number of items at extremely favorable conditions. The electronic circuit of the pulse generator is preferably connected to the rectifier circuit in parallel with the circuit of the LEDs. The

Rectifier circuit can be, for example, a one-way rectifier circuit or also a bridge rectifier circuit, wherein it is preferably equipped with a smoothing capacitor in order to provide an essentially constant current. The parallel connection of the electronic circuit of the pulse generator to the circuit of the LED is therefore particularly advantageous in that no additional network components are required for the circuit and the structure is significantly simplified. However, other connection options can also be provided. In particular, it is pointed out that the electronic circuit described above together with a

A plurality of LEDs, a fast electronic switch and a series resistor, and in particular without the rectifier circuit for connection to the voltage of the utility grid and without the series resistor being substantially smaller than it is for operation constant current would be required for independent inventive is considered.

The series resistor is preferably provided between the fast electronic switch and the base potential of the circuit, while the LEDs are provided between the fast electronic switch and the high potential, and even more preferably a voltage limiting diode is provided to limit the drive voltage of the fast electronic switch, thereby limiting the current is caused by the LEDs. With this

Circuit you can control the fast electronic switch in the manner of a variable resistor described above and ensure the constant current flow through the LEDs.

Preferably at least some of the plurality are LEDs

"White light LEDs". White light is particularly preferred for lighting purposes, in particular as night lighting. The white light LEDs can either be LEDs in which a plurality of LEDs with different colors are arranged on one chip in one plane. There are typically four

LEDs - one red, one green and two blue - are spatially very closely arranged. These LEDs are also typically operated in series. The eye perceives the light of these LEDs in an additive mixture as white light. White light LEDs, which are blue LEDs coated with phosphorus, have also recently become available. The blue radiation of the LEDs excites the electrons in the phosphor to higher energy levels and generates the white light when it returns to the basic state. In the case of the arrangement of a plurality of additive white light LEDs, ie white light LEDs which are made up of differently colored individual LEDs, it can be advantageous to in each case have the LEDs of the same color in series with a separate series resistor, a separate fast electronic switch and to provide a separate pulse generator, so that a total of light of a certain color can be generated by coordinating the brightness of the individual colors.

It is particularly preferred to use the described light source, i. H. the

Arrangement of LEDs, fast electrical switch, series resistor, pulse generator and rectifier circuit to be provided with a standard mounting base that can be used with a standard mounting socket. This makes it possible to change the lighting structure used up to now

To replace lamps, for example incandescent lamps, by the light source according to the invention.

The invention further relates to a traffic signal system having at least one light source of the type described above. Especially in traffic signal systems, ie in typical traffic light systems, the use of LED light sources is particularly favorable. An important cost factor in traffic signal systems is the relatively frequent replacement of the usual lamps. By using a light source of the type described above, the replacement intervals can be extended by a factor of 2 or more, which leads to a more than halving of the usual maintenance costs. On the other hand, the lower energy consumption of the LEDs is clearly noticeable in the running operating costs. In addition, the use of Color-matching light sources, for example red, yellow and green light sources in traffic lights, the energy efficiency can be further improved. The loss of light in the corresponding color filters (filter for green light, filter for red light, filter for yellow light) is extremely low. It is beneficial for two reasons

Color filters still to be retained. On the one hand, the already mentioned aspect of the simple replacement of the conventional lamps by the light source according to the invention plays a role here. On the other hand, the filters can serve to drastically reduce stray light, for example due to solar radiation. This applies in particular if the filters in their passband are adapted to the very narrow frequency band of the LEDs.

Especially in connection with the traffic signal systems, the use of the light source with conventional mounting bases is particularly preferred.

The possibility of varying the pulse pause in this context is also particularly preferred. It is easy to imagine that in traffic signal systems the lighting intensity during the day or

Night operation can and must be significantly different. A relatively high intensity must prevail during the day in order to ensure reliable detection of the signal state even in the most adverse sunlight. At night, however, care must be taken to ensure that the lighting intensity is not excessive in order to avoid being dazzled

To prevent road users safely. The described dimming effect can be used to advantage here. It is even conceivable to record the intensity at the moment depending on the actual lighting situation, for example with a photocell, and that at the moment Adjust the required light intensity of the signal system based on this information, for example automatically. Even during the day, such a control rarely requires the signal system to operate at the highest intensity of the light source. This enables even greater energy savings to be achieved.

The invention and refinements of the invention are explained in more detail below with reference to an exemplary embodiment shown in the drawing. Show it:

Fig. 1 shows schematically the structure of a light source according to the invention;

2 schematically shows the structure of another embodiment of the light source according to the invention;

3 shows the current pulses over time; and

Fig. 4 is a schematic circuit diagram of a possible embodiment of the light source according to the invention.

1 shows a light source according to the invention having a

A plurality of light emitting diodes (LEDs) 2 connected in series, which are connected in series with a series resistor 4. There is also a fast electronic switch 6 in the circuit of the LEDs. A rectifier circuit 8 is provided as the current source, which is designed for direct connection to the voltage of the public power grid, ie typically 230V or 110V AC voltage. The rectifier circuit can be, for example, a one-way rectifier circuit or a bridge circuit in which, for example, a capacitor is provided for smoothing the rectified AC voltage. For example, the rectifier circuit provides a voltage of the order of 150V for the circuit of the LEDs when connected to an IOOV power supply network, while when operating on the 230V power supply system the circuit has an essentially constant voltage in the Magnitude of 320V is provided. The fast electronic switch 6 is connected to a pulse generator 10. The pulse generator 10 supplies the fast electronic switch 6 with control signals for opening and closing the circuit. The control signals determine the length of the pulse duration, determine the

Repetition frequency of the pulses and thus the duty cycle, d. H. the ratio of pulse duration to period. The amount of current through the LEDs 2 is primarily dependent on the total resistance in the circuit of the LEDs, i. H. determined in particular by the size of the series resistor 4.

The fast electronic switch 6 itself can be, for example, a transistor and in particular a MOSFET transistor. N-channel MOSFETs have proven to be particularly suitable because, on the one hand, they are cheaper and, on the other hand, they switch faster than a p-

Channel MOSFET with the same electrical load capacity. To avoid losses, the fast electronic switch 6 should have only a very low resistance in the conductive state. It should also have particularly short switch-on and switch-off times in order to enable a particularly steep rise and fall of the current pulses.

The special type and number of LEDs 2 is selected depending on the intended use. LEDs are supplied in a wide variety of colors and with a wide variety of operating data. There are so-called "white light LEDs", which are white

Providing light as is typically particularly desirable for lighting purposes. So far, commercially available white light LEDs were made up of a plurality of LEDs of different colors, for example one green, one red and two blue LEDs, which are in one plane were spatially very close to each other. The additive color mixing gave the viewer the impression of blue light. Recently there has also been a type of white light LED that does not work on the principle of additive color mixing. These white light LEDs are based on a blue LED on their

Light emission side has a phosphor coating. The light from the blue LED stimulates the electrons in the phosphorus atoms to higher energy levels. On returning to the underlying energy levels, white light is emitted by the phosphorus atoms. This is done with such a high efficiency that an improvement in efficiency by up to a factor of 2 compared to conventional fluorescent lamps can be achieved with a light source according to the invention which works with such white light LEDs. Even with the previously used ad d it i v e n W e i ß l i ch t- L E D s l a s s e n s I achieve efficiency improvements of this magnitude.

With the additive white light LEDs known to date, which consist of diodes of different colors, the diodes of the same color 2r, 2g, 2b (for red, green and blue) of the individual LEDs are connected in series in the manner shown in FIG. 2 and one is formed essentially separate circuit for each color in these LEDs, i.e. H. With a separate fast electronic switch 6r, 6g, 6b and a separate pulse generator lOr, 10g, 10b, in principle any desired color can be generated by varying the brightness of the individual colors in the LEDs through the additive color mixing.

Conventionally, in the circuit of the LEDs, the upper rail 12 is at the high potential, while the lower rail 14 is at the reference potential, for example ground. 1 and 2 is the Series resistor 4 between the fast electronic switch 6 and the base potential 14 shown. However, it can equally well be provided between the fast electronic switch 6 and the high potential 12.

FIG. 3 shows the current pulses as they are applied to the LEDs 2 by the needle pulse generator 10 in connection with the fast electronic switch 6. One recognizes individual relatively short impulses that follow one another at regular intervals. 3 is the time t on the transverse axis and the current on the vertical axis

I specified. The constant level of the pulses, which corresponds to a constant pulse current I, can be seen. The pulse duration is indicated with ^. The period, i.e. H. for example, the duration from the beginning of a pulse to the beginning of the following pulse is indicated by T. The repetition frequency is thus determined as 1 / T. The

Time between two pulses, ie the pulse pause is given as T _P.

In Fig. 3, the current I _κ is also specified as the maximum current with which the LEDs can be operated at most in constant operation.

The size relationships shown in FIG. 3 correspond approximately to the size relationships on which the comparative calculation is based in the introduction to the description.

Fig. 4 shows schematically a circuit diagram of a possible

Embodiment of an LED light source according to the invention. In general, FIG. 4 essentially corresponds to FIG. 1 in its construction. Thus, on the far left in FIG. 4, the circuit of the LEDs 2 with the fast electronic switch 6 and the series resistor 4 can be seen. The pulse generator 10 is shown with a broken line for limitation in the circuit. It can be seen in particular that the pulse generator 10 is connected to the high potential 12 or the reference potential 14 of the circuit of the LEDs 2.

The pulse generator 10 is discussed in more detail below. The essential components of this circuit are the capacitor Cl, which can be charged by a constant current source, a voltage divider formed with the two resistors R13 and R14 for generating a reference voltage U _R , a uni-function transistor UJT connected to the reference voltage U _R and the capacitor voltage U _c , which ignites as soon as the capacitor voltage U _c becomes somewhat lower than the reference voltage U _R , and a switch S1 connected to the unijunction transistor UJT and the capacitor Cl, which is switched on by the ignition of the unijunction transistor UJT for the duration of a pulse.

In the present circuit, the constant current source for charging the capacitor C1 consists of the transistor T1, the resistors R1 and R2 and the diodes D20 and D21. The

Unijunction transistor UJT is formed from two transistors T3 and T5. The switch S1 is formed by the transistor T4 and the resistors R9 and RIO. It should be particularly pointed out that in particular the constant current source and the switch S1 can also be constructed completely differently. For example, there are IC circuits that can serve as a replacement for the structure of the switch S1 shown. Similar circuits can also be used as a constant current source for the capacitor C1. Other implementation options can also be imagined for the uni-function transistor UJT. Other components of the circuit shown are also not absolutely necessary. The transistor T6 essentially serves as an amplification transistor for the control pulses to be delivered to the fast electronic switch 6. This can depend on the special choice of the fast electronic switch 6 or on the

Selection of the switch Sl is omitted. Furthermore, a buffer capacitor C2 is shown in the circuit, for example, which is provided for stabilizing the circuit but is not absolutely necessary. Resistor R11 is also not absolutely necessary, it essentially facilitates the selection and tuning of resistors R13 and R14.

A voltage limiter diode D30 is also provided in the circuit, the purpose of which is to limit the drive voltage at the fast electronic switch 6. This component too, although advantageous, is not absolutely necessary.

During operation of the circuit, the capacitor Cl is charged by the constant current source, the capacitor voltage U _c at the lower end of the capacitor Cl gradually falling within a predetermined time interval until the capacitor voltage U _{c is} slightly less than that of the resistors R13 and R14 formed voltage divider generated reference voltage U _R. At this moment, the uni-function transistor UJT begins to conduct. The capacitor C1 is discharged via the uni-function transistor UJT and the resistors R9 and RIO. As long as current flows between the resistors R9 and RIO, the transistor T4 and the

Resistors R7, R8 of transistor T2 are turned on. By switching transistor T2 on, transistor T1 is switched off, ie the constant current source is switched off and the charging current to capacitor C1 is interrupted. At the same time it is also about a Switching on the capacitor T6, the control pulse is emitted by the pulse generator 10, ie applied to the fast electronic switch. As soon as the capacitor C1 is discharged, the transistors T4, T2 and T6 turn off and by turning off the transistor T2 the transistor T1 is turned on again, ie the constant current source starts to charge the capacitor C1 again and the process begins again.

It follows from what has been said that the discharge speed of the capacitor C1 and thus the pulse length can be varied over the total resistance of R9 and RIO. On the other hand, the size of the charging current of the constant current source for the capacitor Cl and thus the pulse pause can be regulated by changing the size of the resistor R1. A change in the capacitance of the capacitor C1, on the other hand, primarily results in a change in only the frequency with the same ratio of pulse length to period, i. H. with the same

Duty cycle.

In practical use, the possibility of varying the length of the pulse pause with the same pulse length is a very favorable option for the amount of current supplied to the diodes and thus the

Regulate the intensity of the light from the light source. Such a type of control is particularly desirable for lighting purposes, since in this way the possibility of dimming the light source is opened in a particularly simple manner.

As already stated above, it is necessary to limit the current through the LEDs 2. This is caused to a certain extent by the series resistor 4 in the present circuit. This current limitation can be overcome by modifying the circuit slightly improve the pulse generator 10 and the fast electronic switch 6, ie regulate very precisely. For this purpose, a voltage limiter diode D30, for example a zener diode, is provided. This diode D30 limits the drive voltage to the fast electronic switch 6. The fast electronic switch 6 is then not necessarily switched to completely conductive but is operated in the manner of a controllable resistor. However, the prerequisite is that the series resistor 4 is provided between the fast electronic switch 6 and the reference potential 14. The mechanism is as follows: When the current through the LEDs 2 increases, the voltage that drops across the series resistor 4 increases, which reduces the effective voltage between the gate G and the source S of the fast electronic switch 6 provided as a MOSFET. This voltage reduction reduces the conductivity of the MOSFET, ie increases its resistance and thus limits the current through the

MOSFET. Thus, by limiting the control voltage of the MOSFET on the one hand and by arranging the series resistor 4 between the source S of the MOSFET and the reference potential 14, a control circuit for the current flow through the MOSFET is predetermined. This control loop is essentially independent of what is happening between the high potential 12 and the drain D of the MOSFET 6, i. H. the distance in which the LEDs 2 are provided. In principle, any number of LEDs can be operated in series, provided that the sum of their operating voltages is in a reasonable ratio to the available voltage.

Claims

claims

1. Light source, comprising a plurality of series-connected LEDs (2), which are connected in series with a series resistor (4), characterized in (a) that the circuit of the LEDs (2) in addition to the

Series resistor (4) has a fast electronic switch (6) and as a current source a rectifier circuit (8), which is connected to the voltage of the public power grid during operation; (b) that the fast electronic switch (6) to a pulse generator

(10) is connected, which delivers pulses with a repetition frequency of at least 70Hz; (c) that the series resistor (4) is smaller than it would be for operation at constant current, (d) and that the arrangement is so constructed that the current amplitude at the LEDs (2) is greater than the maximum current permissible during constant operation .

2. Light source according to claim 1, characterized in that the pulse generator (10) is designed so that the pulse duty factor

(Pulse duration / period) of the current applied to the LEDs (2) is less than 1/3, preferably less than 10 ^"1 , more preferably less than 10 ^'2 and even more preferably less than 10 ³ .

3. Light source according to claim 1 or 2, characterized in that the pulse generator (10) is designed so that the repetition frequency is greater than 130 Hz, preferably greater than 400 Hz, more preferably greater than 1 kHz and particularly preferably greater than 10 kHz.

4. Light source according to one of claims 1 to 3, characterized in that the pulse generator (10) is designed so that the pulse duration is less than or equal to 10 ³ s, preferably less than 10 ^"5 s and particularly preferably less than 10 ^'5 s .

5. Light source according to one of claims 1 to 4, characterized in that the length of the pulse pause (T _P ) with a constant pulse length (T,) and constant current (I _P ) is variable.

6. Light source according to one of claims 1 to 5, characterized in that the arrangement is constructed such that the applied current (T _P ) by more than a factor of 2, preferably by more than a factor of 5, more preferably by more than that Factor 10 and very particularly preferably greater than the maximum current (I _k ) permissible in constant operation by more than a factor of 20.

7. Light source according to one of claims 1 to 6, characterized in that the pulse generator (10) and the fast electronic switch (6) are designed so that they cause a current limitation by the LEDs together with the series resistor (4).

8. Light source according to one of claims 1 to 7, characterized in that the pulse generator (10) is constructed with an electronic circuit which comprises: (a) a capacitor (Cl) which can be charged by a constant current source;

(b) a voltage divider formed with two resistors (R13, R14) for generating a reference voltage (U _R ); (c) one to the reference voltage (U _R ) and the

Ko nd ens ato r voltage (U _c ) on connected uni-function transistor (UJT), which ignites as soon as the capacitor voltage (U _c ) becomes slightly less than the reference voltage (U _R ); (d) and one to the uni-function transistor (UJT) and the

Capacitor (Cl) connected switch (Sl), which is turned on by the ignition of the uni-function transistor (UJT) for the duration of a pulse (T,).

9. Light source according to claim 8, characterized in that the electronic circuit of the pulse generator (10) is connected in parallel to the circuit of the LEDs (2) to the rectifier circuit (8).

10. Light source according to claim 9, characterized in that the

Series resistor (4) is provided between the fast electronic switch (6) and the base potential (14) of the circuit and the LEDs (2) are provided between the fast electronic switch (6) and the high potential (12) and that a voltage limiting diode ( D30) to limit the

Control voltage of the fast electronic switch (6) is provided, whereby a current limitation is effected by the LEDs (2).

11. Light source according to one of claims 1 to 10, characterized in that the plurality of LEDs (2) are "white light LEDs".

12. Light source according to claim 11, characterized in that the LEDs (2r, 2g, 2b) each have the same color of additive "white light LEDs" each in series with a separate series resistor (4r, 4g, 4b), a separate fast electronic switch (6r, 6g, 6b) and a separate pulse generator (lOr, 10g, 10b) are provided and, by coordinating the brightnesses of the individual colors, light of a particular color can be generated in total.

13. Light source according to one of claims 1 to 12, characterized in that the light source has a conventional mounting base for use with a conventional mounting socket.

14. Traffic signal system, comprising at least one light source according to one of claims 1 to 13.