On 1st September 2016 the EC’s ban on inefficient directional halogen lamps came into effect. That means that there will be a lot of GU10 sockets, for example, into which LED replacements will be fitted. For many people this will not be a smooth process: LED lighting is very prone to flicker and instability when dimmed, because the LED chips themselves react so quickly to changes in the electricity supplied to them. Whereas with more traditional light sources, such as incandescent lamps, it was normal to swap lamps from different manufacturers and to expect similar behaviour, that’s no longer the case with LEDs. Instead it is now crucial to consider compatibility between each part of the whole LED system; the lamps, the dimmers and the drivers which supply power to the LEDs themselves. However help is at hand! Here are some handy hints to help you achieve success with LED dimming:

  • All LED systems have a driver: the electronics that converts the incoming AC mains voltage into the low-voltage DC which the actual LED chips require. For mains-dimmable lamps the components to achieve this are usually hidden away inside the lamp, often located in the stem, or as a small internal PCB. For LED fixtures this is often a remote-mounting box, joined by a short cable to the “light engine”. The compatibility of the driver and the dimmer is the most crucial element in determining smooth-dimming, flicker-free success in an LED project.
  • Remember that not all LED lamps are created equal – different models will perform very differently, even when connected to the same type of dimmer. Even different lamp SKUs from the same manufacturer may give very different results, despite looking similar from the outside. Of course, some lamps are not dimmable at all! That information is often buried in the small print on the packaging and is therefore easy to miss. Make sure that you only use lamps which have been tested to show that they are compatible with the dimmer you’re using.
  • “Minimum load” is an issue on many older dimmers, but not all! Some dimmers require at least 40W of load in order to work, whereas newer dimmers, designed specifically with LED in mind, may not have this restriction. Depending on the internal design of the LED lamp you may achieve success with older dimmers, but the only way to know this for sure is to properly test the combination of lamps and dimmer.
  • A well-designed dimmer will have a setting known as “low-end trim”. This is the minimum power level at which the dimmer will first switch on the lamp. For those lamps which suffer from flicker problems at the lower end of the dimming range this setting can ensure that they are never dimmed into this “unstable range”, thereby avoiding flicker issues.
  • The lowest stable brightness level will vary considerably between different lamp models, i.e. some lamps are better at being dimmed lower than others. The lighting industry usually uses “measured power” to describe dimming levels, e.g. 1%, 5% or 10% etc. However “measured brightness” and “perceived brightness” are not the same thing: the pupils of our eyes open up at low light levels to let more light in, and therefore we see small brightness changes at low levels more clearly. Be aware that a lamp+dimmer combination which can dim down to 10% power will still look as if it’s at 30% brightness. To achieve a 10% perceived brightness you’ll need to find a combination that can dim as low as just 1% measured light, which narrows your choice of lamp vendor considerably. Lutron lists both the minimum power and the perceived brightness achievable with each lamp tested on our LED report-cards.
  • Some LED lamps are compatible with leading edge dimmers, and some are compatible with trailing-edge dimmers. Some more advanced dimmers, known as “phase-adaptive”, can perform either way round, meaning that a wider selection of lamps will be compatible. Proper testing, with the right equipment, is the only way of knowing which option to use!
  • For lamps which are dimmed using the leading-edge method there will be an effect known as “repetitive inrush current” on each half-cycle of the chopped-up mains input to the lamp. The quantity of lamps which can be connected to any one dimmer will be determined by the total size of this inrush, along with the total Wattage of the load. The only way to determine the safe maximum permitted quantity of lamps is by measuring the repetitive inrush current, during testing (you may be spotting a theme around testing here… see item 10 for good news!). The quantity of lamps that can be connected to one dimmer can vary very widely indeed: you can often halve or double the permitted quantity simply by choosing a different lamp.
  • Mains dimming isn’t the only option for dimming lamps, although in a retro-fit scenario it is likely to be the first choice. There are other types of LED driver, which do not take a chopped-up incoming mains supply in order to dim them, but instead use a fixed mains supply with a low-voltage signal to indicate the required output brightness. These low-voltage signals use a protocol such as 0-10V, DALI or DMX. Generally you’ll experience more success in using this type of system because the mains supply to the driver is not chopped up, but be aware of the cabling requirements.
  • Why dim lamps? There are two answers: Firstly, dimming LEDs, i.e. reducing the amount of power which is going through them, will lower the LED chips’ “junction temperature” which in turn will increase the lifetime of the LED, thereby reducing maintenance cycles. The other answer is far more aesthetic: dimmed light creates an atmosphere, reduces energy usage, so if you’re planning a cozy evening-in then find a compatible lamp-and-dimmer combination, and dial in the romance!
  • Since 2009 Lutron has had a team of engineers dedicated to testing compatibility between LED lamps and dimmers. Over 2000 combinations have been tested, with the results published, free, at www.lutron.com/LED The resulting LED report cards, of all of those lamps which “pass”, give details of the minimum and maximum number of lamps which can be connected to each dimmer zone, along with the minimum and maximum output power, and the resulting perceived low-end level which can be achieved. Customers are increasingly turning to these LED Report Cards to give them the confidence to choose a combination of items in their LED systems that will give the flicker-free dimming performance that they require.