11
APR
2019

Have you recently changed over some of your beautiful old Halogen lamps for new LEDs and wondered why they flicker, don’t dim as low as the old ones and in so many cases look much worse?

LED technology has virtually replaced traditional incandescent light sources and has been with us for the past 10 – 15 years.  In most cases LEDs are considerably more energy efficient than the light source they replaced and now provide huge benefits in energy saving, light output, colour temperature and effect. However, simply changing a lamp for a LED one is fraught with technical challenges especially if you want to dim them properly.

Until a few years ago about 90% of LEDs on the market were quite frankly not very good. Many of the mass market solutions were cheap and consequently of low quality, providing low output and undesirable colour temperature, which made quality finishes look anaemic. The remaining 10% were actually pretty good, but you needed to find specialist manufacturers or distributors as most electrical wholesalers did not stock these more specialist products.

If you have regular light switches without dimming control, the problem of poor-quality LEDs is reduced significantly, but as soon as you add a dimmer your problems can really start. So, you will either need a good understanding of the technical issues or work with someone (like Grahams) that does.

Almost any LED light can be dimmed by using the correct driver and dimming technology, but in order to do this with good results, you need to understand which components make up a complete fixture. Every LED requires a power supply which converts AC mains voltage to a suitable low voltage supply for the LED. Matching this to the LED itself is critical and the better solutions will have an external driver (a driver is conceptually the same as the old transformer you may have had for running 12v halogen lamps). It is called a “driver” to help differentiate from the old transformers, but the components required to deliver what the LED requires are more complex.   “Mains” LED lamps have this “driver” built in to the lamp base, and with low cost lamps the quality of these components are built-down to a price (costing a matter of pennies) and are consequently of much lower performance than an external driver.  A decent external driver carefully matched to the LED lamp source can cost anywhere from £15 - £50.  The best products that we use now actually do the dimming in the driver and are controlled using either low voltage 0-10v or better still, digital control signals such as DALI. 

The crux of the problems with LED flicker is that most traditional dimmers use a Traic which is an electronic high frequency switch; it typically turns on and off 50 times a second and by varying the amount of time each cycle stays on for, you can reduce the effect of the brightness of the lamp. This type of dimming is known as “Phase Dimming”. The first electronic dimmer was invented by Joel Spira, the founder of Lutron back in 1962.  At the time, this technology was perfectly suited to dimming incandescent filament lamps and these served us for many years.  The overall circuit was relatively simple by comparison to today’s LED solutions.  Phase dimming technology is not as well suited to dimming the more complex LED drivers which have to be designed to handle a varying voltage input signal to feed their electronics, and also work out how this variable signal can be used to vary the output to the lamp.

We won’t go into the different constant current (CC) or constant voltage (CV) types of LEDs, but generally most linear LED strips (or tape) work on 12v or 24v constant voltage drivers, and most other LED lamps work with a constant current driver.

This main cause of flicker is because the LED driver (either built into the lamp or the external one) can’t cope happily with the varying dimmed signal that comes from an electronic dimmer. One of the ways around this is to limit how dim the LED can go and many only down to 10-15%, but that’s hardly dim by comparison to your old halogen lamps which could dim to a tiny glow.  The very best LED drivers can dim to 0.1% which is pretty good, but most go to 1% which sounds low but is still quite a bit brighter than your old halogens. These numbers are also complicated by the “perceived light” you get; as the light entering your eye reduces, your pupils enlarge to compensate for this making the perceived light harder to measure and quantify.   So, for rooms like bedrooms you should make sure this is specified properly if low light levels are important to you.  For best dimming we recommend using DALI or 0-10v for control. DALI is our preferred solution as it is much simpler to wire, considerably more flexible, and with the correct programming, careful choice of lamps and drivers, can give you smooth flicker-free dimming.

This is just a taster of quite a complex subject matter. The important thing is to work with a specialist who understands the variables, can show you what the effects are and find out who will take responsibility for the result.  At Grahams we have been doing this for several years, but the only way to guarantee the results is to be in charge of the process from start to finish.  Please feel free to contact us to discuss your project or needs specifically.

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