LED driver on a thyristor CCR: flicker and stroboscopic effects
The CCR delivers a chopped current. A halogen filament smooths it, but an LED follows it unless the driver does the smoothing, and the lowest brightness steps are where it shows.
A thyristor CCR holds the loop current steady by switching on part-way through each half-cycle, so its output waveform is chopped. At the full step it is close to a sine wave. At the low steps the thyristors fire later, and the current flows in narrower pulses with longer gaps.
A halogen filament stays hot and averages this out. An LED responds almost at once, so what the driver does with the pulsed current decides whether the light is steady. A driver that cannot carry the light through the gaps lets it flicker, and a moving eye or camera may also see the stroboscopic effect.
A visible fluctuation in the light with time. It can be seen by a fixed observer, and is usually clearest at the lowest brightness steps.
Seen when the eye or a camera moves across a light that is rapidly changing in brightness. A steady light then looks like separate points or a broken trail.
| Aspect | Halogen | LED, driver not smoothing | LED, driver smoothing |
|---|---|---|---|
| Response to the pulses | Filament averages them out | Light follows the pulses | Energy store carries the light through the gaps |
| At the full step | Steady | Usually acceptable | Steady |
| At the lowest step | Steady, but dim and slow to respond | Most likely to flicker | Steady |
| Moving eye or camera | No strobing from the filament | Strobing or banding may show | Much reduced |
| Set by | The lamp | The driver design | The driver design and its match to the CCR |
| What to do | Nothing special | Choose another driver or a better matched CCR | Still test at every step |
What to check for flicker
The real combination
Test the actual fixture on the actual CCR type, not each on its own. A good driver on a poorly matched CCR can still flicker.
Every step
Check all the brightness steps, and look closely at the lowest, where the pulses are narrowest.
In motion
Look at the lights while moving, as a pilot does, not only from a fixed position.
Cameras and vision systems
Check how the lights look to the cameras or vision systems used on the airfield, because banding can appear on screen.
Compatibility data
Ask the driver and CCR manufacturers which combinations are approved, and keep the answer with the records.
After a change
Repeat the check when the CCR, its settings or the fixture type changes.
Frequently Asked Questions
Q1Why can an LED driver flicker on a thyristor CCR?
A thyristor CCR controls the current by switching on part-way through each half-cycle, so its output waveform is chopped. A halogen filament smooths that out because it stays hot. An LED responds almost at once, so unless the driver smooths the waveform, the light follows the gaps and pulses in the current.
Q2Why is flicker worse at the low brightness steps?
At the low steps the thyristors fire late in each half-cycle, so the current flows in narrower pulses with longer gaps between them. A driver has less to work with in the gaps, so any lack of smoothing shows most at the lowest steps.
Q3What is the stroboscopic effect?
It is the effect seen when the eye, or a camera, moves relative to a light that is rapidly switching in brightness. A steady light then appears as a row of separate points or a broken trail. It can be visible even when a fixed observer sees no flicker.
Q4Why does a halogen lamp not show the same problem?
Its filament has thermal inertia. It warms and cools slowly, so it averages out the fast pulses in the current and the light stays almost steady. This is the same slow response that makes a halogen light change more gradually at a step change.
Q5How is the problem reduced?
Mainly by the design of the driver, with enough energy storage or smoothing to carry the light through the gaps in the current, and by choosing a driver and a CCR that are made to work together. Some CCRs filter or shape their output. The right combination depends on the products, so check the manufacturers' data.
Q6How is it checked?
Test the real fixture with the real CCR, at every brightness step and with the lowest step looked at closely. Look at the light while moving and with the cameras or vision systems used on the airfield, and keep a record of the combination tested. Repeat the check if the CCR or the fixture type changes.
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