Many sources now for these devices, but the few I have inspected all have relatively large electro caps on the input side, to make DC from which the current controller PWM draws its supply for the LED drive.
I strikes me that with the published figures for LED lifetimes of 50,000 hours, the electro caps in these are probably going to fail long before the LEDs and other associated circuitry.
Has anyone investigated sensible ways of running the PWM circuit direct off the ( full-wave ) rectified line, and accepting the fact that for a few msec, the input voltage will fall below the 10 volts or so needed to drive the LEDs ?
Seems to me that would extend the potential lifetime of the driver considerably.
Any thoughts ?
Adrian Jansen adrianjansen at internode dot on dot net
Hello, if the working temperature is low enough and the capacitors good enough (pricy?) they should last enough :) And also the optocouplers tend to age.
There are many design examples that lack a large electro cap on the input. AFAIK this is done mainly for high power factor, but as a side effect the large high voltage electro disappears. The single stage High PF flyback is simple to do for low power requirements, and moves the cap from rectified mains to led output side. For higher power one could use the usual PFC+flyback (or PFC+resonant) and use high voltage film capacitors as storage. Check appnotes for ST L6561/2, powerint linkswich-PH, if I remeber correctly also fairchild and ON should have appnotes on the subject.
Fabio Eboli nella vita reale...
I don't quite understand why they don't use good old inductive ballast (or current-limiting transformer) in those led things? Cheap shit ballast is way more reliable than any cheap shit switchmode psu.
We already have that, and they have much the same problems of CCL lamps
- the temps generated by the lamp itself impact on the electronics. Much better to separate the LED and its heat from the electronics.
With the long life ( quoted ) on the LEDs, they essentially become fixtures in the building, rather than replacable elements like the tungsten lamps we have all used. So it makes more sense to have the entire lamp installation permanent. That is, if the rest of the driver can also be made long life.
I rather doubt that inductive ballasts can supply the relatively stringent constant current requirements of LEDs, although it is tempting. Certainly the simplicity is attractive. And weight is not really an issue in permanent fittings. Copper and iron cost maybe.
Adrian Jansen adrianjansen at internode dot on dot net
We'll build a high-reliability circuit, conservatively rated transistors, big transformer/coils, lots of heatsinking, and leave the inevitable electrolytic hanging out the side, mounted in a screw base! Patent pending!
Ya know, the scary thing is the plausibility. They need to hurry up and make high voltage aluminum polymers work. Then this'll (mostly) go away.
Deep Friar: a very philosophical monk.
you never know, I just read of a guy making a fuss with safety agencies and the manufacturer because his PH lamp fell down when the plastic gizmo it was hanging in broke. It had been hanging there for 27 years!
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For domestic that may be considered fixture, but for business use 50000 hours is just 2.5 times fluorescent tube lifetime ie. about 5 years. Lack of replaceable
*standardized* bulbs is big drawback for led technology. Leds are damn expensive to begin with and the need to replace the whole luminaire every five years means much more installing work (expensive).
Also at 50000 h they typically quote something like 70% light output remaining. It means that to achieve some minimum lighting level at end of life, you must design with 30% extra power initially so most of lifetime you are making more light than necessary. Fluorescents drop only by 10% so not much a problem. Of course if you have feedback on lighting level, then it is not any problem.
Energy saving of leds compared to modern well designed fluorescent lighting is anyway questionable at best. Of course if you compare leds to some old non-designed system with 18W halophospate tubes with D-class ballast, plastic diffuser and no reflector you get impressive numbers to put in ads.
Even if you spend big bucks for led lighting you do not necessarily save any energy over fl. lighting. But using conventional light sources, natural lightning and good design does not give you any "Green PR" like leds do.
It seems that fluorescent life varies a bit over the range of 10,000 -
20,000 hours, LEDs range from 50,000 - 100,000 hours. I thus challenge your only 2.5 times the life.
Depending on usage LEDs can deliver more lumens per watt, near equivalent CRI, dimmability (something fluorescent does not handle well at all) and some other useful trade offs. For some applications LEDs rock, for others there are plenty of lamp technologies.
Induction lighting in particular is still giving all other technologies fits in several applications.
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Fluorescent lifetime varies very much with use pattern and ignition method. I have read that in the US they use these rapid/instant start systems that reduce lamp lifetime somewhat. Around here we use glow starters nearly allways if using ballast. I think most here haven't even heard of those rapid start things Also if you look at two first links you find that typical lamp lifetime is given with many on/off cycles. In typical business use lamps are on for long time (if not 24/7) so startup wear is not that much concern. If you look at graph in second link you can find that under these conditions it is reasonable to expect about 30k life for a tube. And 2.5 times 30k hours is 75k hours. Of course if there is much on/off cycles lifetime drops and that 10k figure might be realistic for home use with instant on starter.
Modern fluorescent lamp with electronic ballast and good reflector give system efficacy of about 90 lm/W (tube only is approx 110 lm/W) so they are about equally good as good led lamps. Also modern control gear is not that bad at dimming.
Yes, certainly there are many places where leds are very good choice. It just irritates me when green weenies are pushing led lighting as saving the world wonder thing. Every magazine and newspaper is nowadays full "leds save 60% energy" articles with no hint as to where on earth they are pulling those numbers from. And then city engineers have hard time telling politicians why changing every lamp in city to leds ain't that great idea.
The lifetime is defined when the light output is reduced to a specific amount (say to 70 %). The rate of reduction can be measured with relative short (1000 h) tests and extrapolated from there.
Those LEDs that dim in a few thousand hours are usually driven with Imax, i.e. a LED with Imax=350 mA ("1 W") at 350 mA. This causes severe thermal problems (high junction temperatures), heating the PSU. Running at such high current also reduces the efficiency [lm/W].
Running the LED at Imax/3 will slow down the degradation considerably, making 50000+ hours believable. The junction temperature is much cooler, even when running three Imax/3 LEDs on the same heatsink as one at Imax (Rth j-c is in parallel). It might even be possible to reach over 100 lm/W claimed efficiency. Of course, to get similar initial light output as the one LED at Imax, you need nearly three of those devices at Imax/3, which of course increases the cost significantly.
For even longer lifetime, reduce the current even further and add more LEDs, but of course this drives the costs up and is only justifiable, if the replacement costs are high.
Look carefully at the LED specs, optical characteristics are more or less useless at Imax, many manufacturers specify the optical characteristics at some more usable fraction of Imax.
Those over 100 lm/W efficiency claims are usually measured below Imax/10 and at 25 C junction temperature (just when turned on :-). In addition the highest outputs only apply to "cool white" i.e. those with a very strong blue spectral line.