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PWM control - rectangular pulse duty factor in LED lighting

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PWM control - rectangular pulse duty factor in LED lighting

PWM stands for Pulse Width Modulation which is another term for a kind of digital signal. PWM is employed in a wide range of applications including advanced control circuitry. Pulse Width Modulation control is commonly used in controlling the dimming of RGB LED lights.

In the application of PWM control in LED lighting, we can achieve a variety of results as pulse width modulation enables us to change the number of times the signal is high in an analogue way. Although the signal can be only low or high at any moment, we can regulate the amount of time the signal is high in contrast to when it is low over a specific interval.

PWM LED Lighting Control

The most common use of Pulse Width Modulation is to control LED lighting. They are used for dimming LED lights. It is a method that decreases the average quantity of supplied power of the provided electric signal. This process is done by dividing the signals effectively into small distinct parts.

PWM changes the current supplied to LED lights at high frequency from 0 to the output current rate to regulate the brightness of LED light. The Pulse Width Modulation dimming depends on the ability of the human eye to integrate the average light in the pulses. The observed brightness of LED light is almost proportional to the pulses’ duty cycle i.e. the ratio of on- and off-time of LED bulb.

This stream of pulses continues to proceed at a high frequency that makes it imperceptible or undetectable for the human eye or high-speed video cameras. PWM dimming can be employed in various LEDs that work with either constant current drivers or constant voltage drivers.

How does PWM dimming work?

PWM dimming operates by turning on and off the LED light at a very high speed. Even though this operation leads to flickering of light, it is fast enough to be imperceptible by human eyes. Pulse Width Modulation works by utilizing the same quantity of electric current required by an LED light. It is a continuous process that rapidly switches between zero and that specific amount of current. Therefore, the LED light is operating on its required power amount or is turned off.

An advantage of Pulse Width Modulation dimming is that it enables LED light to be turned off for less time. It will keep the internal temperature of the LED from increasing and potentially enhance the lifespan of the LED light. Another advantage is colour temperature. LED keeps on running at their required amount of electric current with PWM dimming. The projected colour temperature of the LED light doesn’t change and remains the same throughout dimming.

The disadvantage of Pulse Width Modulation dimming is the possibility of some noise production from the light bulb. If not using proper PWM frequency, there are chances of some flickering to occur. Moreover, PWM drivers sometimes can give rise to electromagnetic interference (EMI), which makes them inappropriate for some applications like medical.

Duty Cycle

When talking about PWM control in LED lighting, there are some terms that you will come across over and over again. One of them is the “duty-cycle”. The term belongs to the total number of times a pulse is turned on throughout the cycle. Thus, when the brightness is 50%, it means that the duty-cycle of the LED light is 50%. The duty cycle is usually measured in Hertz and gives us the value of cycles per second.
So, when the frequency is 50Hz, the cycle is 1 second/50 cycles that is equal to 0.02 seconds. As we are working with small-time measurements, it is good to use the smaller unit for seconds i.e. microseconds. In 1 second, there are 1,000,000 microseconds which result in 20,000 microseconds cycle duration which is 50Hz or 50 cycles per second.

In these 20,000 microseconds, we should either turn on or off the LED light based on the requirements of the duty cycle. For instance, a 75% duty cycle will need the pulse to be turned on for 15,000 microseconds and turned off for 5,000 microseconds later on.

The high signal is referred to as the “on-time”. To elaborate the quantity of “on-time”, we will make use of the duty cycle concept. The value of the duty cycle is given in percentage. The percentage duty cycle particularly shows the percentage of time the digital signal is turned on over a specific time or interval. This time is the opposite of the waveform frequency.

If a digital signal remains on for a half time and off for the other half time, it will indicate that the digital signal has a 50% duty cycle and is the same as a standard square wave. In case the percentage is more than 50%, it means that the digital signal stays in the high state for more time as compared to the low state and the other way around if the percentage is less than 50%. Voltage set at 5 volts or high is the same as 100% duty cycle whereas the ground signal is just like 0% duty cycle.

The square wave frequency needs to be high enough while regulating LED lights to gain an appropriate dimming effect. At 1 Hz frequency, a 20% duty cycle wave will be evident that it is turning off and on to human eyes while at 100 Hz frequency, a 20% or above duty cycle will appear dimmer than full light. Principally, the time can’t be too large if you want the dimming effect with LED light.

Examples

By regulating the duty cycle, it is possible to adjust the brightness of LED light.

It allows you to mix the proportion of three colours (red, green, and blue) in RGB LED lights to create the required colour. You can do this by dimming the three colours in different amounts.

When mixed in equal quantities, it will generate white light with varying brightness. Equally mixed green and blue colours will create teal. You can also try a complex combination of colours. Try adding 100% of red with a 50% duty cycle for green and blue completely turned off to create an orange colour ambience.

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