The volt-ampere characteristic of the LED, that is, the current flowing through the LED PN junction varies with voltage, which can be visually demonstrated on an oscilloscope (see Figure 1). A complete volt-ampere curve includes forward characteristics. With reverse characteristics. In general, the reverse characteristic curve changes steeply. When the voltage exceeds a certain threshold, the current rises exponentially, which breaks down the LED PN junction. The forward voltage of the LED is also determined by its forward current. From the volt-ampere characteristics of the LED (Figure 1), the change in forward current causes a corresponding change in the forward voltage. Specifically, the decrease in the forward current also causes a decrease in the forward voltage. Therefore, when the current is turned down, the voltage of the LED is also reduced, which changes the relationship between the power supply voltage and the load voltage.
Therefore, from the volt-ampere characteristics of the LED, we can know that the dimming of the LED light source can not be achieved simply by reducing the input voltage or input current of the LED. In addition, the waveform of the sine wave of the LED is different from the waveform of the incandescent lamp, so it cannot Simply by changing its conduction angle, the purpose of changing its effective value (effective dimming) is achieved.
To make it easier for everyone to understand the above points, examples are as follows:
For example, in an LED lamp with an input of 24V, eight 1W high-power LEDs are connected in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V, then 8 series is 26.4V, so the load voltage is higher than the input voltage, so a constant current source of >24V should be used. However, in order to dim, the current is reduced to 100mA. At this time, the forward voltage is only 2.8V, and the 8 series is 22.4V. The load voltage becomes lower than the input voltage, so the constant current source of >24V cannot be used at all. Work, and finally the LED will flash.
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