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LED Dimming with Pulse Width Modulation (PWM)

The term modulation in essence refers to a form of control. Pulse width modulation in lighting is a specific method of control, used to effectively alter LED brightness. The PWM technique minimises the typical amount of deliverable power to single or multiple LEDs. This is a popular dimming method as with PWM, users can expect a wider range of brightness levels with high accuracy.

So how does it work?Pulse width modulation (PWM) applies the method of rapidly switching between load and source, i.e., turning the LED on and off at a very quick rate. It controls, connects, and disconnects the current source to the LED. By essentially turning the light on and off very rapidly, the duty cycle is altered. As the average current is controlled in this way, the light emitted from the LED/s appears to steadily dim or increase in brightness. The PWM signal can be prompted by a timer or digital device and can be controlled by an analogue or digital input such as a potentiometer, sensor, or communication protocol. PWM dimming allows for dynamic control of LED brightness and colour, as it can be synchronised with other devices, including sensors, cameras and displays.

The duty cycle refers to the percentage of the time period that the light is actively on. When at full brightness, the duty cycle will be 100%. When the light is dimmed, the duty cycle and power consumption will be decreased. By decreasing the average current and thus the power dissipation, LED power consumption and heat dissipation are also minimised. For example, if the duty cycle is 50%, this means that the current is off for 50% of the time and on for 50% of the time.

Pulse width modulation diagram showing visually what duty cycle refers to at different percentages

This method of dimming is ideally suited to lighting circuits that operate at a constant voltage. In such applications, the LEDs cannot be dimmed by simply reducing the current passing through the circuit. So, by using PWM dimming to pulse the LEDs, the brightness can successfully be altered by adjusting the percentage of time the LEDs are on (100%), versus the time the LEDs are off (0%). Utilising PWM dimming in this way is great for enabling a flexible and broad range of dimming levels between 0-100%, without compromising LED performance or lifespan. The colour quality of the light emitted is preserved and the LED efficiency is optimised, without altering forward voltage or current level.

In any application, the frequency of the LED pulses should always be high enough to avoid any visible flickering and subsequent noise. Because the duty cycles are so rapid and the pulsing of the LED so constant, typically the human eye cannot perceive the light source flickering on and off. Typically, the PWM frequency needs to be above 80 Hertz to ensure this. In lighting the unit Hertz refers to periods per second.

Although human sight may not notice any light flickers, cameras are more sensitive to this effect and can register PWM flickers at much higher frequencies than the human eye. Photography images can show light distortions if there is PWM frequency of up to several hundred hertz. Plus, when using digital viewfinders, light flickers may also be visible, making it hard for a photographer to focus a shot. Therefore, in any applications where cameras are used, the suitability of this technology must be considered. Particularly in professional photography and video applications. As these much higher-quality cameras can detect light flickers at much higher frequencies than normal cameras can.

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