JPH0338841Y2 - - Google Patents

Description

[Detailed description of the invention] [Technical field] The present invention relates to a circuit that controls the brightness of a large number of light emitting diodes connected in series and parallel to be used as a light source for vehicle lamps, etc., to a constant level regardless of fluctuations in power supply voltage. It is.

[Technical Background] It has been considered to connect a large number of light emitting diodes in series and parallel for use as a light source for a vehicle lamp.
In that case, the relative luminous intensity of the light-emitting diode changes depending on the ambient temperature, and the forward current may differ greatly from the set current due to variations in the power supply voltage and the forward voltage V _F of the light-emitting diode. Therefore, it is desirable to control the current flowing through the circuit in order to keep the brightness constant.

[Objective of the invention] An object of the invention is to provide a brightness control circuit for a light emitting diode that can maintain constant brightness regardless of fluctuations in power supply voltage.

[Summary of the invention] The invention consists of a circuit that detects changes in the combined current of a parallel circuit of light emitting diodes caused by power supply voltage fluctuations, a comparator that compares the output of this detection circuit with a reference value, and a comparator that compares the output of this detection circuit with a reference value. A power source comprising an oscillator whose duty ratio of an output signal changes according to the output of the device, and a switching element inserted into each branch of the parallel circuit of the light emitting diodes and whose switching operation is controlled by the output signal of the oscillator. This device is characterized by controlling the circuit current according to voltage fluctuations to keep the brightness of the light emitting diode constant.

[Embodiment] Fig. 1 shows an embodiment of the present invention, in which a large number of light emitting diodes 1 are connected in series and parallel, and a current limiting resistor 2 and a switching element, such as a transistor 3, are inserted in each series circuit. . One end of the parallel circuit, for example, the transistor 3 side, is grounded, and the other end is connected to the power supply terminal (voltage 10~
16V) is connected to 5.

The voltage across the resistor 4 is input to an integrator 6, its output is compared with the voltage of a variable voltage power supply 8 for reference voltage by a comparator 7, and the duty ratio of the output signal of the oscillator 9 is changed by the comparator output. In this way, the on/off ratio of the transistor 3 is controlled, that is, the circuit current is controlled.

With such a configuration, when the power supply voltage is a predetermined value, the duty ratio (second
The ratio of the on time t to the period T in the figure becomes a predetermined value, and accordingly, the transistor 3 turns on.
The light emitting diode 1 is turned off and the set current flows through the light emitting diode 1.

When the power supply voltage fluctuates, the integrator input, that is, the voltage across the resistor 4 increases or decreases, and the output of the integrator 6 increases or decreases. As a result, the difference from the reference voltage changes, the output of the comparator 7 changes, and the oscillator 9 is controlled. For example, when the power supply voltage increases, the duty ratio of the output signal of the oscillator 9 becomes smaller (on-time t in FIG. 2 is small), and the on-time of the transistor 3 becomes shorter. In other words, an increase in circuit current due to an increase in power supply voltage is suppressed and the set current is maintained, and a constant brightness is maintained.

Conversely, when the power supply voltage decreases, the on-time of the transistor 3 increases, thereby compensating for the decrease in circuit current and keeping the brightness constant.

Note that the period of the output of the oscillator 9 is set to a time that cannot be followed by the human eye.

[Effects] As described above, according to the present invention, changes in circuit current due to fluctuations in power supply voltage are detected and compared with a reference value, and the set current is maintained by changing the on/off ratio of the switching element. Therefore, the brightness of the light emitting diode can be kept constant. Moreover, since the circuit current is controlled, excessive current can be prevented, and damage or deterioration of the light emitting diode can be prevented.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing one embodiment of the present invention;
The figure is an output waveform diagram of the oscillator in the same embodiment. 1... Light emitting diode, 2... Current limiting resistor,
3...Transistor, 4...Detection resistor, 6...
Integrator, 7... Comparator, 8... Reference voltage power supply,
9...Oscillator.

Claims (1)

[Scope of utility model registration request] a circuit that detects changes in the combined current of the parallel circuit of light emitting diodes caused by fluctuations in power supply voltage; a comparator that compares the output of this detection circuit with a reference value;
It is equipped with an oscillator whose duty ratio of an output signal changes according to the output of the comparator, and a switching element that is inserted into the energizing circuit of the light emitting diode and whose switching operation is controlled by the output signal of the oscillator. A light emitting diode brightness control circuit characterized by controlling the circuit current according to fluctuations and keeping the brightness of the light emitting diode constant.