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Polarity-Pulsed LED operation-LED PCB

Polarity-Pulsed LED operation-LED PCB


Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity. When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated on an alternating current voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.

Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage above this threshold. The cause of damage is overcurrent resulting from the diode breakdown, not the voltage itself. LEDs driven directly from an AC supply of more than the reverse breakdown voltage may be protected by placing a diode (or another LED) in inverse parallel.

The manufacturer will normally advise how to determine the polarity of the LED in the product datasheet.

Pulsed LED operation

Many systems pulse LEDs on and off, by applying power periodically or intermittently. So long as the flicker rate is greater than the human flicker fusion threshold, and the LED is stationary relative to the eye, the LED will appear to be continuously lit. Varying the on/off ratio of the pulses is known as pulse-width modulation. In some cases PWM-based drivers are more efficient than constant current or constant voltage drivers.[3][4] It is also done to allow digital intensity control without a more complex digital-to-analog converter. Several types of microprocessor boards use this technique to control the on-board LED, such as the Arduino and the Texas Instruments MSP430 LaunchPad.

Most LED data sheets specify a maximum DC current that is safe for continuous operation. Often they specify some higher maximum pulsed current that is safe for brief pulses, as long as the LED controller keeps the pulse short enough and then turns off the power to the LED long enough for the LED to cool off.

LED as light sensor

Mobile phone IrDA

In addition to emission, an LED can be used as a photodiode in light detection. This capability may be used in a variety of applications including ambient light detection and bidirectional communications.

As a photodiode, an LED is sensitive to wavelengths equal to or shorter than the predominant wavelength it emits. For example, a green LED is sensitive to blue light and to some green light, but not to yellow or red light.

This implementation of LEDs may be added to designs with only minor modifications in circuitry. An LED can be multiplexed in such a circuit, such that it can be used for both light emission and sensing at different times.

Professional Manufactur Custom Design Circuit Board Electronic Pcb

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