general efficiency of LED's - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: August 07, 2010, 08:52:37 am
content: I'm wondering what is the typical efficiency of a white LED ?I have an old 8mm film projector I just spent the best part of 20 quuid on for a bulb but am wondering if i can replace the bulb with a led(s) to make replacement a non issue and increase it's efficiency on power consumption (100 W lamp), the lamp is run off a 12V transformer so having the power available is not a problem

author: RayJones
author_rank: Frequent Contributor
timestamp: August 07, 2010, 09:09:38 am
content: If I recall correctly, a Halogen bulb runs at something like 20-25 lumens / wattSo you have something like 2500 Lumens to create.The typical best available LEDs at the moment are 80 or lumens/watt, so you'd need a fair stack of LEDs to get the same level of light, not considering heatsinking or even focusing all those individual light sources for projection.Unfortunately the halogen bulb is still most likely the best choice for now.

author: Simon
author_rank: Unknown
timestamp: August 07, 2010, 09:33:50 am
content: hm 25-30 W of led, can you get a single led to make that ?my consideration was that the led would emit a faily focused beam so should be optically more efficient too ?

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2010, 10:24:20 am
content: Yes you can buy 25W LEDs but it isn't that simple.The colour rendering index of LEDs is inferior to halogen so you might mess the colour up.Anyway are you sure it's halogen? Some of the better projectors use HID which is more efficient, although £20 seems too cheap for HID unless you got a cheap ebay deal.You're looking at more than £20 for an LED in that power range but of course it will probably never need replacing to you might save in the long run.Here's a link to such an LED on RS, £33.50 including VAT, then you need a heat sink and an SMPS.http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6491373What voltage does the lamp in the projector run at? If it's something like 12V or 24V it might make it easier to buy/build an SMPS rather than mains.

author: Simon
author_rank: Unknown
timestamp: August 07, 2010, 10:32:47 am
content: this is a 1960's 8 mm film projector and is one of the few that uses a 12v bulb so yes I have a perfect power source and available space for a SMPS, I can use the original bulb as a plug in so as to not damage the projector and ultimatel;y owing to the popularity of this projector might make a nice project that could become a saleable product

author: RayJones
author_rank: Frequent Contributor
timestamp: August 07, 2010, 11:30:34 am
content: The usual problem with the power LEDs is the emission area is a large rectangular area, and that causes problems to focus correctly, but on the upside, this square area may be a better fit to a rectangular projection window...But yeah, 20 quid would only be a small fraction of the cost of a LED, if you could get one, then you'd still need drivers and heatsinking.I would like to think you could get reasonable colour rendition. There's a hell of a lot of TV's now using LED backlights through TFT, so you'd expect there would be public outrage if these more expensive LED sets were a poorer cousin to the EL backlights...Does anybody actually make a single 25W LED with a small point source emitter? The referenced RS unit has 6 emitters, and I'm certain once you focused and projected that you'd have a nice Bill Gates backdrop (windows) on your movies!

author: Simon
author_rank: Unknown
timestamp: August 07, 2010, 12:09:48 pm
content: well the projector bulb itself is not a perfect light source and the projector handles it, I'm sure that the led when positioned correctly would provide optimum lighting, yes I'd need a driver etc, I think wasting less light i might get away with a slightly less powerful led, the original bulb has a square filament so as to fill the frame as best as possible but then it emits light at 360 degrees and mostly back and front yes there is a reflector in the back but I'm sure lots of light is wasted.I'm sure there are some cheaper leds on ebay, but then i need to test out the 1 and 3W leds I've already bought to see if they are trustworthy

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2010, 12:42:16 pm
content: I've found another LED, lower power, cheaper but it claims 90lm/W and appears to have a more concentrated beam, although I didn't read the data sheet.http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0537993#header

author: Simon
author_rank: Unknown
timestamp: August 07, 2010, 12:47:58 pm
content: hm thats 11ish watts might even do, I don't think I'd get away with 2 of them as I'd never be able to pisition them.It's not something I'll be doing any time soon as I'm busy with decorating and other projects but will sure give it some consideration as I'm interested in led lighting anyhow

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 07, 2010, 01:01:55 pm
content: I have actually modified a cheap video projector with a 1W Luxeon LED. I'm going to make it a projection weather clock, just need to put the parts into an enclosure and have some sort of microcontroller to actually generate the video signal.

author: EEVblog
author_rank: Unknown
timestamp: August 07, 2010, 01:05:53 pm
content: Quote from: RayJones on August 07, 2010, 09:09:38 amThat's way behind.Cree now have cracked the 200lm/W mark 6 months ago:http://www.cree.com/press/press_detail.asp?i=1265232091259The Cree XPG I used in my deck lighting some time back are measly 130lm/W or so.Dave.

author: Zero999
author_rank: Super Contributor
timestamp: August 07, 2010, 01:29:01 pm
content: Quote from: EEVblog on August 07, 2010, 01:05:53 pmNot available in shops so I wouldn't get too excited yet.QuoteQuote from: RayJones on August 07, 2010, 11:30:34 amBut TFT screens are designed with LED blacklights in mind, old celluloid film will be designed for halogen.My concern came from looking at the spectra emitted by the LED on the datasheet, there's a huge blue peak at 460nm so it may look slightly bluish, even if the colour temperature matches a high temperate halogen.Perhaps that's the problem: maybe the LED I linked to has too higher colour temperature?Quote from: Simon on August 07, 2010, 12:09:48 pmYes you're right, in that case the LED I suggested may even be slightly overkill, although you can always run it at a lower power and get a much longer life out of it.QuoteYes, you were very lucky with your UV LEDs last time but next time you might not be so lucky.I don't think think the price in RS is too bad, even if you manage to make the whole for £50 then it seems reasonable.Don't forget, if you want to make to sell then you can go for bulk discounts but you'll need to make sure there's a market first.How much does a lamp normally cost? I know you picked one up for £20, is that the retail price or ebay?

author: Simon
author_rank: Unknown
timestamp: August 07, 2010, 03:16:48 pm
content: well apart from my UV leds that are not very consistent but adequate i have some white ones I've not tried yet,I got my replacement bulb from a Bristol camera shop @ £19.09, the same shop is welling them in batches of 3 for £50 on ebay and I have seen listings and other sites run by shops selling them at as much as £30 each. it's probably something that would not sell massively, a very niche market but hey once I've designed and built my own if there is someone as mad as me to pay the parts plus assembly charge I'll not be calling them mad. The idea would work for other projectors too, at the end of the day you have to meet the bulb spec and not the projector spec. This one is a particularly popular projector, I spent twice what I paid for it with a screen and some old film on the bulb.

author: EEVblog
author_rank: Unknown
timestamp: August 07, 2010, 10:57:08 pm
content: Quote from: Hero999 on August 07, 2010, 01:29:01 pmThe Cree XM LED's are already sampling (maybe in full production by now?), and they do 160lm/WI'm excited.Dave.

author: RayJones
author_rank: Frequent Contributor
timestamp: August 07, 2010, 11:14:45 pm
content: The XM carrot has been getting dangled by CREE for a loooonnnggg time and no sign of release to the wild yet.But yeah 160 lm/W - bring it on - the LED headlight on my bike is the first candidate.

author: Simon
author_rank: Unknown
timestamp: August 08, 2010, 06:09:21 am
content: sound pretty good, so just as a comparison of light sources what is the typical outoput in Lm/W of incandescents and CFL's ? have LED's surpassed them yet ?

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2010, 08:12:36 am
content: Quote from: Simon on August 08, 2010, 06:09:21 amYes and no.Most LED light sources on the market (including power supply and casing) are less efficient than CFLs. Cree sells LEDs which are much more efficient than fluorescents but they're naked and don't include an SMPS so will be only a little bit more efficient than a fluorescent when fully assembled into a luminaire with a PSU.However, LEDs give a more direct light source than CFLs so might be more efficient for applications requiring a direct light source, which is why your UV exposure works so well: the efficiency of your LEDs is probably less than a UV fluorescent but the comparative performance was the same, if not better because they give a more direct light source, which is better for that application.

author: EEVblog
author_rank: Unknown
timestamp: August 08, 2010, 08:16:32 am
content: Quote from: Simon on August 08, 2010, 06:09:21 amYes, LEDs have surpassed them:http://en.wikipedia.org/wiki/Luminous_efficacyDave.

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2010, 08:50:09 am
content: Quote from: EEVblog on August 08, 2010, 08:16:32 amBe careful reading that table, it's very misleading!All right not CFLs but fluorescent tubes in general still seem to be better than LEDs, the most efficient LED is 13.6% but the most efficient fluorescent is 15.63%.The Cree LED which is 22% is without the PSU (all other ratings are given with the PSU) and it's naked so will need an insulated case to work. Once assembled into a usable product, you're probably talking about an efficiency between 15.8% and 18.8%, yes better than a fluorescent but not as good as the table suggests.Also no mention is mention is made of the CRI, colour temperature, ageing or the application so it's not that simple.I think it's advisable to wait a while before ripping out fluorescents and installing LEDs. Perhaps you could try replacing a few CFLS with LEDs but the LED lights I've seen in shops seem to be very unimpressive. Building your own from good LEDs, from reputable suppliers (not ebay) is probably your best bet.Maybe you can start replacing fluorescent when reasonable priced, high powered >16% efficient LEDs, with a good CRI start appearing.