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The solution to the low voltage at both ends of the LED lamp

elecfans | 11/05/2021

After the 5V LED lamp is connected to the 5V USB power supply through a MB10F patch rectifier bridge, the brightness of the LED lamp becomes dark. This is because the forward voltage drop of the rectifier bridge is too large, resulting in a low voltage across the LED lamp. . There are two solutions: one is to use a Schottky diode with a small forward voltage drop as the rectifier bridge, and the other is to reduce the current-limiting resistance of the 5V LED lamp to enable it to work at a lower voltage. Below we introduce these two methods separately.

The input end of the 5V LED lamp is connected to a rectifier bridge so that the lamp can be lit regardless of the power supply connection. This method is possible, but the MB10F rectifier bridge is selected. The forward voltage drop of each tube is about 0.7V. The 5V LED lamp is connected to the USB power supply through the rectifier bridge. The voltage applied to the two ends of the lamp will drop by about 1.4V, which is actually only 3.6V. The lights will dim. If you want to reduce the loss of the rectifier bridge, you can choose 4 1N5819 Schottky diodes to connect as a low-dropout rectifier bridge as shown in the following figure, so that the 5V LED lamp can be normally lit.

In the above figure, D1～D4 are 4 1N5819 Schottky diodes, and RL is a 5V LED lamp. After the input 5V voltage is converted through the polarity of D1～D4, only about 0.4V is reduced, so that the brightness of the 5V LED lamp is basically constant.

For 5V low voltage LED lamps, each lamp bead is connected in series with a current-limiting resistor, and then connected in parallel with a 5V power supply (see the figure above). If you want to use the MB10F rectifier bridge with a large forward voltage drop as the polarity conversion, At this time, the brightness of the lamp can be improved by reducing the lamp bead current-limiting resistance.

In the above figure, the current limiting resistance of each white LED lamp bead is 180Ω. When it is working at a voltage of 5V, the current flowing through each lamp bead is about 11mA. Assuming that after passing through the MB10F rectifier bridge, the voltage applied to the two ends of the lamp drops to 3.6V. If you want to make the lamp bead's working current still 11mA, you can change the current limiting resistance of each lamp bead to 51Ω. The brightness of the 5V LED lamp remains unchanged.

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