Fullscreen HTML5Med Res (??MB)HTML4

By LEO SIMPSON LED downlights and dimmers In the past two years there has been a quiet revolution in the domestic lighting market. Incandescent and compact fluorescent lamps are rapidly disappearing, being supplanted by LED lamps of all sorts. But while LED lamps are great for low power consumption, they cannot be used with conventional leading-edge dimmers, which are ideally suited to incandescent lamps. This article surveys the domestic LED lamp scene and discusses the leading-edge, trailing-edge and universal dimmers. S ome years ago, incandescent lamps were largely choice for domestic lighting in new dwellings. This will banned from the Australian and New Zealand mar- be confirmed if you take a walk through any newly conkets, with a variety of incentive schemes sponsored structed home or home unit. Or have a look at any new proby governments to promote the use of more efficient com- ject home – downlights are used in just about every room. Apart from the whims of fashion, there are two main reapact fluorescent lamps (CFLs). sons for this trend. First, all new dwellWhile this may have been well-intentioned, CFLs ofings in Australia must meet the Building ten failed to live up to their promise of long life and for a Code of Australia for energy efficiency, number of reasons, they proved to be and in New South Wales, the BASIX ena less than satisfactory replacement ergy standard. for incandescent lamps in many apBoth standards set out how much plications. lighting, based on power consumption, Now CFLs are rapidly being discan used per unit area of the dwelling. placed from the market and LED This effectively rules out the use of inlamps are taking their place. Nocandescent, 240VAC halogen and parwhere is this more apparent than in ticularly 12V halogen lamps, because newly constructed homes and home they are power gobblers. units where the overwhelming lightThe second reason is that virtually all ing choice is flush-mounted ceiling new homes and home units in Australia LED downlights. are built with a minimum ceiling height But there is also an ever-expanding of 2.4 metres. This means that headroom range of direct substitutes for incanbelow hanging light fittings can be indescent lamps. sufficient for tall people. Who likes to In fact, apart from the occasional be banged on the head by a low-flying LED accent light or a rail light, or in pendant light fitting? situations where their use is preclud- These “Clipsal” trailing edge dimmers Conventional light fittings also often ed (eg, bathroom heat lamps) LED are specifically intended for use with downlights are now the universal mains-powered LED lamps (up to 400W). have poor light distribution and apart 24  Silicon Chip siliconchip.com.au We made this jig to show the differences in light colour from various types of lamps (unfortunately printing processes tend to mask the differences somewhat). On the left is a “cool white” (4000K) 15W CFL; second is a “warm white” (2500K) 15W CFL; third is a “warm white” (2700K) 60W incandescent (shh!) while on the right is a “cool white” (labelled 2700K!) 13W LED. from that, they are dust-catchers which can make small rooms seem even smaller. So flush-mounted ceiling downlights are a neat solution and they provide a bonus in giving lots of light for relatively little power; typically 10 to 14W. A further benefit is that when they are off, they virtually disappear into a white-painted ceiling. We should also note that there are LED equivalents to the familiar MR16 12V halogen downlights but these are less widely used and they are generally not as bright. The LED downlights we are referring to require a 92mm cut-out in the ceiling, vs 75mm for the MR16s. consequence, they are considerably less bright. But the high brightness of Cool White or Natural White LEDs can be too bright and harsh for many rooms, particularly in bedrooms but also in lounge and dining rooms where full brightness may not be required for most of the time. In those situations, dimmers become highly desirable. But most LED downlights, in fact most LED lamps, are not dimmable. Furthermore, all LED lamps are marked on their packaging as to whether they are dimmable or non-dimmable. More usefully, some LED downlights are labelled If a LED lamp is dimmable, it should as “trailing edge” dimmable. If you clearly say so on the body. If it doesn’t LED disadvantages cannot find any information on the say so, it isn’t! And then it’s only with a So what are the drawbacks? There trailing edge dimmer. packaging, you can assume that any are several but the main one is that LED lamp is not dimmable. most LED downlights are not dimmable – and those that The reason that most LED lamps are not dimmable is are will not work with conventional “leading-edge” dim- that they employ a switchmode power supply which will mers which are so effective with incandescent lighting. So work over a very wide range of AC mains voltage. Most why dim them at all? will work at voltages between 250VAC and around 80VAC, The over-riding advantage of LED downlights is that with very little change in brilliance. they are bright, particularly those that are rated Cool White (typically 5000K) or Natural White (4000K); Warm White SWITCH ON (typically 2700K). Warm white LED lamps are intended to imitate the lighting given by incandescent lamps and as a A S1 Ls Rt TRIAC DIAC N LAMP LOAD siliconchip.com.au G A2 Cs A1 Rs Ct Fig.1 (left) shows the circuit of a typical “leading edge”, triacbased light dimmer. Fig.2 (right) shows the waveform across the lamp load – when triggered early, the lamp is at its brightest, while it gets progressively dimmed as the trigger point (set by variable resistance Rt) is later in the half-cycle. SWITCH ON A EARLY TRIGGERING: HIGHER OUTPUT SWITCH ON SWITCH ON B LATER TRIGGERING: LOWER OUTPUT July 2017  25 S1 A Ls – Cs N SWITCH OFF + SWITCH OFF D ZERO CROSSING DETECTOR AND PULSE GENERATOR LAMP LOAD G HIGH S VOLTAGE MOSFET A LATER TRIGGERING: HIGHER OUTPUT SWITCH OFF Ls Fig.3 (above): a “universal” dimmer can be set to leading edge or trailing edge. A high voltage Mosfet is used to switch the load. Fig.4 (right): the waveform across the lamp is essentially the opposite of Fig.2 – in this case it’s for a “trailing edge” dimmer which turns the power off at a certain point in the cycle; the later it does so, the brighter the lamp. SWITCH OFF B EARLIER TRIGGERING: LOWER OUTPUT If you do try them out with a conventional light dimmer, they will not dim but will inevitably flicker uncontrollably. Dimmable LED lamps have a slightly more complicated switchmode power supply which allows their brilliance to vary in proportion to the mains supply voltage, although typically, their brilliance cannot be varied over the same wide range as that for incandescent lamps. Sadly, while a LED lamp may be labelled as being dimmable, they will not work if you try them with a standard dimmer that may have been installed in your home for a number of years. That is because it will be a Triac-based “leading edge” dimmer. What you need for LED lamps is a “trailing edge” or “universal” dimmer. But even if you have a trailing edge dimmer installed, it may not work satisfactorily with your particular “dimmable” LED lamp. Before we discuss why a dimmer may or may not work with a LED lamp, let us define and describe “leading edge” and “trailing edge” dimmers. As already noted, leading edge dimmers are based on Triacs and the general layout of a Triac dimmer is shown in Fig.1. A Triac is a a four-layer bi-directional semiconductor device which is non-conducting until a small trigger pulse is fed to its gate electrode. It then switches to a low resistance state, allowing current to be fed to the load until the voltage across it (the Triac) drops to zero or reverses in polarity. In more detail, as well as the Triac as the power switching element, there is an RC network (RT and CT) and a Diac (another four-layer semiconductor device) which provides the phase-delayed triggering pulse to the gate of the Triac (see Fig.1). The triggering network is effectively a pulse generator synchronised to the mains voltage waveform. So at the start of each half-cycle, the Triac will be off and the capacitor connected to one side of the Diac will charge, at a rate determined by RT, to between 30 and 40V where it reaches the “breakover” voltage of the Diac. The Diac then dumps the capacitor’s charge into the gate of the Triac, turning it on so it can pass current to the load. The inductor LS, capacitor CS and resistor RS are for interference suppression and voltage “snubbing” to reduce transient voltages when the Triac turns off at the end of each half-cycle. The method by which a Triac controls the power level in an AC circuit is referred to as “phase control”. Consider that the mains voltage is a 230VAC sinewave; the voltage varies sinusoidally between +325V and -325V at 50Hz (or 60Hz in the Americas and some other countries). Here’s an example of a “leading edge” dimmer triggering an incandescent lamp very late in the 230V mains cycle. Only a small amount of power is delivered to the lamp. Triggered much earlier in the cycle, the lamp would be almost as bright as it could be with a large amount of power delivered. 26  Silicon Chip siliconchip.com.au Fig.5: taken from the Fairchild data sheet, Similarly (to the earlier photo), at left is a 2.5W LED “COB” lamp (bordering on useless!); next is a “cool white” 16W LED; a decorative “warm white” 75W incandescent and finally the same 13W “warm white” (2700K[?]) LED as before. Even though that 2700K is on the label, we don’t believe it. The incandescent lamp to its left would be much closer to 2700K. The power fed to the lamp load by the Triac dimmer circuit is varied by the timing of the gate trigger pulse with respect to the sinusoidal voltage waveform. If the trigger pulse is early in each half-cycle of the waveform, the power level will be high and the lamp will be bright. Conversely, if the trigger pulse is late in each half-cycle of the waveform, the power level will be low and the lamp will be dimmed. The corresponding circuit waveforms are shown in Fig.2 and the scope screens below. These Triac circuits are also referred to as “phase-controlled” dimmers (sometimes also referred to as “phasecut” dimmers). So far so good but what is the basis for the term “leading edge”? In fact, leading edge refers to the “leading edge” of a pulse where the voltage rises from zero to maximum (either positive or negative). By extension, the end of the pulse is referred to as “falling” or “trailing edge”. So the voltage waveform fed to the lamp load by the Triac rises from zero to the instantaneous value of the sinusoidal voltage at the time of the trigger pulse to the gate – hence, by definition, that is a “leading edge”. Here’s what happens when you try to use a LED lamp with a leading-edge dimmer. It might work some of the time but is more likely to not work. This is an even worse example of the non-dimmable LED being dimmed. The LED was noticeably flickering, as you can see by this very confused waveform. siliconchip.com.au Trailing-edge dimmers OK. So now let’s look at a typical trailing edge dimmer. Instead of being based on a Triac, these are based on one or two Mosfets or IGBTs (Insulated Gate Bipolar Transistor) depending on the particular circuit. A typical single Mosfet trailing edge dimmer is fed by a July 2017  27 The typical driving circuitry of a mains-powered LED lamp, showing both sides of the power supply PCB. These flush-mounting 240V AC LED downlights from Altronics are available in 6 & 10W dimmable and 14 & 25W non-dimmable (the 25W are very bright!) in both cool white and natural white. Their rated life is 25,000 hours – much better than halogen bulbs! bridge rectifier from the 230VAC mains supply, arranged so that the Mosfet switching element is turned on at the beginning of each mains half-cycle and then turned off later in the half-cycle. A general form of this circuit is shown in Fig.3. The waveforms for the trailing-edge circuit are shown in Fig.4 and they are effectively the reverse of those for the leading-edge waveforms shown in Fig.2. In this case, for high power to be fed to the load, the Mosfet is turned on at the start of each mains half-cycle and is turned off late. Note the trailing-edge cut-off as the Mosfet switches off. For low power operation, the Mosfet turns off much earlier in each half-cycle. So why is the trailing-edge dimmer preferred for LED lamps? There are two main reasons. The first is that typical mains voltage-rated LED lamps use a small switchmode current driver, comprising a bridge rectifier feeding a capacitor, which provides a supply voltage of around 325V DC to feed the switchmode current driver; typically using a small high voltage Mosfet. These bridge rectifier capacitor input power supplies draw a very high current at the switching point on each half-cycle when feed by a leading-edge dimmer. This can play havoc with the operation of the dimmer as well as the LED current driver itself. The waveform of a trailing edge dimmer and incandescent lamp triggered very early in the 230V AC mains cycle. There would be very little light produced by the lamp. 28  Silicon Chip However, when the same LED lamp circuit is fed with the more benign voltage waveform from a trailing-edge dimmer, those nasty current pulses into the capacitor input power supply are much more subdued. However, there is another reason why leading-edge dimmers and typical LED lamps are not compatible and this has to do with the “holding current” specification for a Triac. For typical Triacs, this current is around 50mA. This is the current that needs to pass through a Triac for it to remaining a conducting state. If the current falls below the holding value, the Triac will switch off even though the total voltage across it and the accompanying lamp load might still be quite high. This is why a typical Triac dimmer has a minimum lamp load of 40W; any lower and the lamp will tend to flicker, regardless of the brightness setting. However, a LED lamp may only be rated at 10 to 15W (or less) – far below the minimum load for a Triac (leading-edge) dimmer. Universal dimmers Just to complicate the scene, there are “universal” dimmers which will provide a choice of leading-edge and trailing-edge operation. Hence, these can used in leading-edge mode to control incandescent and halogen lamps, or in The same dimmer/lamp combination triggered significantly later (about 1/3) of each cycle. In this case, the lamp would be glowing but not particularly brightly. siliconchip.com.au ing-edge dimmers and produced a series of waveforms, shown below. RZC Both the leading-edge and trailing-edge MONITOR K 1 ZC OC 10 Q1 dimmers were made by Deta, a low cost MONITOR SENSE1 G RGATE brand sold in Bunnings hardware stores. 2 DIM DRV 9 C GATE E CONTROL GATE The Deta 6031 trailing-edge dimmer VR IC1 3 8 adj E worked satisfactorily with the dimm-able VDD FL5150 OC SENSE2 G L-E LED downlights discussed in this article R2 4 DIM LOW 7 C3 Q2 POWER but the only way to be sure is to do a bench MODE C1 T-E R1 5 6 C test set up. VR VS GND offset Rather than comment on the waveforms Q3 K RSENSE2 in detail, we’ll let the waveforms and the C2 D1 captions tell the story. N A However, there are significant differences in performance between leading-edge, Fig.5: taken from the Fairchild data sheet, the FL5150 dimmer which can be trailing-edge and universal dimmers. Apart set for leading edge or trailing edge. from the fact that leading-edge edge dimtrailing-edge edge mode, to control dimmable LED lamps mers simply won’t work with LED lamps, they are better at driving incandescent and halogen lamps than the trailingand dimmable compact fluorescent lamps (CFLs). These typically use two high voltage Mosfets or IGBTs edge or universal types. With an incandescent lamp, the maximum brightness is controlled by a special driver such as the Fairchild FL5150 slightly higher (due to less conduction losses) and the minor the ST STEVAL-1LD005V1. Fig.5 shows the Fairchild FL5150 in a 230VAC circuit imum brightness is considerably lower, due to the fact that using two high voltage IGBTs; in this case set up for lead- the minimum conduction angle in each half-cycle is much smaller than can be achieved with a trailing-edge or uniing-edge operation (selected by the DIM mode pin). Note that regardless of whether you elect to use a trail- versal dimmer circuit (even when in leading-edge mode). On the other hand, a trailing-edge (or universal) dimmer ing-edge or universal dimmer with a dimmable LED lamp, there is no guarantee that they will work happily together. does have the advantage that it gives a soft turn-on rather Or you may get a situation where a dimmer will work than the instant snap-on effect with a Triac dimmer. A trailhappily with just a few LED lamps connected in parallel ing-edge dimmer always gives a slight delay between switchin a small room but may misbehave with a larger array of ing on and the lamp actually lighting up. So if your Triac leading-edge dimmer had failed and you LED downlights. Some licensed electricians will only use a particular have yet to replace it, it is worthwhile to replace it with a brand and model of universal dimmer because they may trailing-edge dimmer. This will provide the advantage of soft start which may also have found it to be reliable in the past. However, there is nothing to stop you from bench-testing avoid the sudden failures of incandescent lamps at switcha particular light dimmer and some LED lights to see if they on, together with the accompanying failure of the dimmer are OK, before they are installed by a licensed electrician. itself. This failure scenario is most common with lamp fittings where the lamp does not hang down but is upright. Waveforms and performance The installation instructions that come with some leadWe have run a series of tests with leading-edge and trail- ing-edge dimmers warn about this hazard. A RSENSE1 D2 Here a dimmable LED is being powered by a trailing-edge dimmer. The waveform is not as clean as the incandescent lamp but you’d be hard-pressed to notice the “step”. siliconchip.com.au A C And finally, a dimmable LED is being driven by the dimmer triggered quite late in the cycle. You’ll never get 100% of the LED’s brightness from a dimmer even at maximum. July 2017  29 K K ZD1 BZV55B15 A 47 F D7 S1M 25V A F F D1 S1M 100nF K A 2.2k P1B INT 6 x 12k F 100k 2.2nF D 1k 14 1M 1 560k 2 P1A 1M 8 3 150k 4 5 11 120k 12 13 Y1 F 9 K A3 F B1 B2 IC1 40 25 B Y2 G D3 S1M A1 Q1 6 A 2 VR1 VARISTOR 10k 1k 22k G S D6 1N4007 Q2 C1 C2 Y3 D STF17H62K3 100k 10 2.2nF C3 F J1 LINE 1 NEUTRAL CON3 A K 6 x 12k 2.2k D2 S1M K A 10k 0 D4 S1M F1 FUSE K S STF17H62K3 K B3 Vss 7 22nF A Vdd A2 22k D5 1N4007 A Fig.6: by contrast, this trailing edge dimmer uses two STF17N62K3 620V Mosfets and two diodes across the AC supply and in series with the lamp load. Interestingly, no microcontroller is used, with the switching being controlled by a CD4025BE CMOS logic gate device. It can handle lamp loads up to 300W on a 230VAC supply. But if you like the ability of a leading-edge dimmer to give very low brightness setting with incandescent lamps, or if you need to drive a bigger lamp load, then stick with those. Typically, trailing-edge dimmers are limited to a maximum of around 300-400W. On the other hand, if you want to provide for the day when you eventually change to LED lighting as incandescent lamps become too costly or hard to obtain, go for the trailing-edge dimmer. Having discussed cool white and warm white LED lamps, we should note that there is at least one interesting variant: a downlight which can vary its colour temperature from cool white to warm white in response to varying input voltage. Made by Opal Lighting (www.opallighting.com.au), they employ a special COB (chip on board) LED assembly from Sharp Corporation, called the Tiger Zenigata tuneable white COB. This COB has alternating LED stripes with cool and warm phosphor coating that ranges from 3000k down to 2000K, which most closely replicates the colour range of an incandescent lamp when dimmed by a leading-edge (Triac) dimmer. Indeed, it must be used with a leading-edge dimmer or a universal dimmer that is set to leading-edge mode. We have a couple of pictures at the start of this article (page 24) to demonstrate its range. We should also mention that colour sequenced LED pool The all-in-one, mains-operated Phillips SmartBright LED batten photographed with the end cap removed (not an easy job!) There is no separate LED tube in this fixture; you can see the row of SMD LEDs disappearing into the distance. The bottom of the fixture is polycarbonate, the top (semi-circular) section is an integral diffuser. This LED downlight is the popular 12W/220-240V AC cool white model and, as is marked, is trailing-edge dimmable. Light output is an impressive 900 lumens. They are designed to replacee the MR-16 12V halogen downlights used in millions of homes and offices – but no 12V transformer is needed. They also need a larger (92mm) ceiling cutout. Variable colour LED lights 30  Silicon Chip siliconchip.com.au for use in standard 36W T8 fluorescent battens. (See www. siliconchip.com.au/Article/277). These are now available much more cheaply but you can now also purchase LED battens such as the Philips SmartBright LED Batten (see https://reductionrevolution.com.au/ products/philips-smartbright-led-batten). These do not have a separate LED tube but use a 1.2mlong PCB with a row of SMD white LEDs under a white diffuser. A switchmode mains power supply is incorporated in the polycarbonate housing of the batten which will operate down to about 80VAC with very little change in brightness. They are rated cool white (4000K), consume 21W and are not dimmable. At left is a 15W 240V decorative “candle” MES globe, with its 4W warm white LED globe equivalent alongside. Below, the same two lamps are fitted in the one “wall sconce”. Their brightness and colour are not all that different. Conclusion lights are available which can typically provide a choice of three colours, white, blue and green, cycled each time the lamp is turned on. LED fluorescent battens In the September 2010 issue, we featured an article on then relatively new (and expensive) LED fluorescent tubes As this article shows, there is now an enormous range of LED lamps to replace virtually every incandescent and fluorescent lamp used in homes and offices. About the only lamp application where a LED replacement could not be used is in conventional electric ovens, microwave ovens and lamps incorporated into kitchen exhaust hoods. It also may not be advisable to replace the incandescent lamp used in combination bathroom exhaust fan/heat lamps as the housing can become quite hot. One final point: you may have noticed that we refer to both LED lamps and incandescent lamps as “240V” in this feature when the mains supply in Australia is (nominally) 230V AC. The reason is that most lamps are labelled 240V AC, (some are labelled “250V AC”). Indeed, many LED lamps are labelled “220-240” or even “220-250” V AC. SC The SILICON CHIP Inductance - Reactance - Capacitance - Frequency READY RECKONER For ANYONE in ELECTRONICS: HU 420x59G4Em on heavy photo pa m per You’ll find this wall chart as handy as your multimeter – and just as ESSENTIAL! Whether you’re a raw beginner or a PhD rocket scientist . . . if you’re building, repairing, checking or designing electronics circuits, this is what you’ve been waiting for! Why try to remember formulas when this chart will give you the answers you seek in seconds . . . easily! Read the feature in Jan16 SILICON CHIP (you can view it online) to see just how much simpler it will make your life! All you do is follow the lines for the known values . . . and read the unknown value off the intersecting axis. It really is that easy – and quick (much quicker than reaching for your calculator! Printed on heavy (200gsm) photo paper Mailed flat (rolled in tube) or folded Limited quantity available Mailed Folded: Mailed Rolled: ORDER NOW AT $10.00 $20.00 inc P&P & GST www.siliconchip.com.au/shop siliconchip.com.au inc P&P & GST JJuly uly 2017  31 2017  31