Interesting Experiment: LED Lights Have A Very Special Start Feature

- Jul 16, 2018-

    There is a very interesting and strange phenomenon in a recent experiment. I hope to discuss with you and communicate with you to jointly analyze the causes of this phenomenon.


    Test in the laboratory, the indoor environment, the power supply of 220 v, 50 hz alternating current, voltage stability, lamps and lanterns is 400 wled lamp, measuring instrument for common fluke, test purpose is to measure startup properties of leds.


    Test is very simple, when the power is connected to the lamp and, access to the measuring instrument, check the wiring is correct, after turn on the switch, after electrify sensory moments of lamps and lanterns lit, instrument records the start process, waveform is shown in figure 1.



    Figure 1. Starting process waveform diagram


    As can be seen from figure 1, the start-up process has the following characteristics:


    1. High starting current


    The starting process has a very large peak current, with a maximum of 29A. For LED lamps with a rated current of only about 2A, the peak current is nearly 15 times higher than expected.


    2. Short start-up time


    The start-up time is relatively short. It only takes about 1 second from the start to the power supply to the current stability, and the maximum time is about 800 nanoseconds, which is very advanced.


    3. Negative peak


    As can be seen from the figure, in addition to the positive peak value, there is also a negative peak value during the start-up process, sometimes the negative peak value is larger than the positive peak value.



     Figure 2 shows a large negative peak


    4. Uncertain peak value


    Multiple experiments have found that the size of the peak is uncertain.

    This is related to ac power supply. The timing of the lamp's power connection is random, resulting in different peak value.



    The peak in figure 3 is uncertain



    The test shows that there is a large starting current, up to 15 times.

    Start-up time is short, only about 1 second, peak around 800ns;

    There is reverse peak;

    The peak value is uncertain and the peak current of multiple tests is different.

    After the publication of the article, the attention and messages of the vast number of Internet users have been aroused.

    The author tries to explain and discuss with you.



    FIG. 4 LED lamp drive circuit diagram


    Figure 4 shows the driving circuit diagram of an LED lamp. The power supply is 220V ac. Capacitors C1 and R1 play the role of reducing voltage and limiting current.

    VD1 ~ VD4 constitute the rectifier circuit, which changes the ac to dc.

    Then the capacitor C2 is filtered, and the dc voltage provides the working voltage for led VD5 ~ VD8.

    It can be seen that when the LED lamp is powered on, the capacitor C2 needs to be charged first, which is the fundamental reason for the peak current when starting.

    After the completion of C2 charging, the current gradually stabilizes, reaching steady state, and the current becomes rated current.

    The charging time of C2 determines the occurrence time of peak current.


    Therefore, in the design of LED lighting distribution system, the protection appliances should be matched with it.


    Therefore, the filter capacitor in the LED drive circuit is the reason for the large starting current.