calendarNov 24, 2020
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What causes flickering in LEDs?

What causes flickering in LEDs? What should be considered when buying an LED workplace light? Is every LED light flicker-free?

What causes flickering in LEDs?

LEDs differ significantly in terms of how they work compared to conventional light sources such as e.g. B. compact fluorescent lamps. An LED is a semiconductor that only emits light when current flows in the forward direction. With negligible exceptions, an LED cannot be operated with pure mains voltage (alternating current) because the electrodes can no longer flow back - a rectified current is necessary. Special LED control gear/converters are used to supply the LED light with stabilized direct current/constant current. They ensure that the current no longer flows in two directions, but only in one direction.

The subject is different than with conventional light sources flicker with LED lights from other points of view. The extent to which flickering in LEDs can be eliminated depends largely on the quality of the LED converters used in the workplace lights and also the dimming process. As a Flickering lights in general arises, we have already shown in a separate article.

More on flicker in general

The quality of LED converters and the so-called ripple current

High-quality converters (LED drivers) use input and output filters to flatten the frequency waves so much that the frequency waveform is no longer perceived as flickering. The so-called ripple current is used to assess the residual ripple on the DC side. An alternating current superimposed with direct current is referred to as ripple current. Basically, the lower the residual ripple of the converted alternating current, the less "flicker" the LED light. From a technical and above all from an economic point of view, however, it is impossible to convert the residual ripple of the converted alternating current into a completely smoothed direct current.

The rule of thumb is that with LED converters with a ripple value of max. 10%, the residual ripple is so low that flickering of the LEDs is hardly visible to the naked eye. Our RMD workplace lights achieve ripple values ​​of less than 5% and are therefore "flicker-free".

The dimming process of the LED light

The second criterion when assessing the quality of dimmable converters is the dimming method used. In order to dim a workplace light, the operating current must be reduced. There are two options here:

Dimming via pulse width modulation (PWM for short):

With this method, the LEDs are switched on and off periodically. This gives the human eye the illusion of reduced brightness, since it is too sluggish to recognize this flickering. The average brightness is perceived by the user, i.e. the ratio between bright (LEDs switched on) and dark (LEDs switched off). The lower the dimming, the longer the "dark phase" of the LEDs, i.e. the time in which the LEDs are switched off.

Dimming an LED light using pulse width modulation, or PWM for short

Technically easy to implement, however, the PWM dimming process harbors a number of risks. The switching frequency of the LEDs is particularly important. Since the luminous flux of the LEDs is abruptly interrupted again and again, the lamp flickers without damping at the PWM frequency used. Very high frequencies are required to ensure that this flickering of LEDs is not actually perceived by the human eye. Especially in cheap lights or older models, problems occur again and again due to PWM frequencies that are too low, which are often noticed much too late. The consequences are negative effects on the human organism - the concentration drops rapidly and health problems such as headaches occur. It is particularly dangerous with moving objects. With low-frequency lights, strong stroboscopic effects can occur here, which can lead to dangerous situations.

In addition to digital dimming via PWM, there is also the option of analog dimming of the LED light. This method is referred to as so-called amplitude dimming.

Analog dimming (amplitude dimming):

With analog dimming, the luminous flux is maintained continuously. In this case, the dimming takes place by reducing the amplitude until the desired brightness is reached.

Analogue dimming of an LED workplace light (amplitude dimming)

A major advantage of this method is that flickering due to the dimming process used and a stroboscopic effect associated with it can be almost completely ruled out. This method reaches its limits with very small dimming settings. The brightness cannot be set as precisely here as with the PWM. Possible consequences can be minor color locus shifts or an uneven distribution of brightness. However, these restrictions are limited to very low dimming values, which are rarely used in practice.

Flicker-free light from RMD

In order to enable our customers to work trouble-free and, above all, safely, we rely on high-quality and state-of-the-art LED converters from the brands in our RMD workplace lights OSRAM  and tridonic. High-frequency PWM dimmer circuits and analog amplitude dimming are used as the dimming process, with the latter mainly being used in our LED workplace lights.

For example, we have been able to help our watchmaking customers see clockwork clearly which they previously had difficulty seeing due to low frequency lights dimmed via PWM.