Author Topic: LED's - Whats the most efficient current please?  (Read 2017 times)

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gotwind2

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LED's - Whats the most efficient current please?
« on: July 26, 2008, 05:50:28 PM »
I know bright white LEDs are efficient and constant current devices - I have read 'commandas' masterclass 1-5 here - very good.


I have also read they are at their most efficient at lower current levels, Ghurd kindly advised me not to push LED's too hard 15-18 mA being the max for longevity.


My question is:

When are 5mm white LEDs at their most efficient? with a forward voltage of 3.6v.

2mA, 5mA? for a reasonable illumination.


Hope this makes sense.


Thanks

Ben.

« Last Edit: July 26, 2008, 05:50:28 PM by (unknown) »

ghurd

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Re: LED's - Whats the most efficient current?
« Reply #1 on: July 26, 2008, 12:54:13 PM »
Like everything, most efficient goes with most expensive.

IIRC, most 5mm LEDs are most efficient at about 2.5~3.0ma.  But not very bright at those currents, so it takes a lot of LEDs.


About 5 to 8ma is a nice compromise now that white LEDs are a lot cheaper.


Expect the Vf to be well below 3.6V shown in most data sheets.

Closer to 3.15V for 18ma.

Maybe 3.0V for 5ma.


Cheap LEDs, and sometimes even good LEDs, can get a funny color below about 8ma.

Just have to try every one in the batch at a few different currents.

I use one of these for testing color variations...

http://www.allelectronics.com/make-a-store/item/LT-100/LED-TESTER-/1.html

Must be available in UK too.


G-

« Last Edit: July 26, 2008, 12:54:13 PM by ghurd »
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Tritium

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Re: LED's - Whats the most efficient current pleas
« Reply #2 on: July 26, 2008, 01:21:12 PM »
not all 5mm leds are created equal. I have some 4 die 5mm white that are rated for 80 to 100 mA.


Thurmond

« Last Edit: July 26, 2008, 01:21:12 PM by Tritium »

gotwind2

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Re: LED's - Whats the most efficient current?
« Reply #3 on: July 26, 2008, 01:39:05 PM »
Thanks Ghurd for the prompt response.


2.5~3.0 mA (5 to 8mA better) was the answer I was looking for.


Unfortunatly, your 'testing color variations' link is dead here.


Regards

Ben.

« Last Edit: July 26, 2008, 01:39:05 PM by gotwind2 »

ghurd

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Re: LED's - Whats the most efficient current?
« Reply #4 on: July 26, 2008, 01:47:22 PM »
Go to All Electronics, search for "LED Tester".

Semi common item here.

One of the UK places should have them with a different logo.

G-
« Last Edit: July 26, 2008, 01:47:22 PM by ghurd »
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scottsAI

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Re: LED's - Whats the most efficient current?
« Reply #5 on: July 26, 2008, 05:07:17 PM »
Ben,

Be careful when you use ask about efficiency!

Item cost, power cost, how much light do you need?


Small LEDs can be cheap 10 cents each:

Lets look at a LED data sheet: (just picked one from eBay Item: 230274324488)

http://catalog.osram-os.com/jsp/download.jsp?name=LW_A673_Pb_free.pdf&url=media/_en/Graphics/0
0042257_0.pdf


See page 8 Relative luminous intensity vs current.

Yep as the current goes up, the efficiency drops.

If You Need 100ma of brightness, using 10ma ea requires maybe 7 LEDs. For the cost in efficiency I would drive it at 100ma to get the light I need. If the power was the killer then using many LEDs would be better. IF you want lots of light, the 1 watt LED are costly, would not drive then with less current than they are rated for. My thoughts!


Have fun,

Scott.

« Last Edit: July 26, 2008, 05:07:17 PM by scottsAI »

wooferhound

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Re: LED's - Whats the most efficient
« Reply #6 on: July 26, 2008, 06:48:07 PM »
The most Efficient lighting is, Not to use LEDs at all. Fluorescent Lighting still uses half the power as LEDs do.
« Last Edit: July 26, 2008, 06:48:07 PM by wooferhound »

independent

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Re: LED's - Whats the most efficient
« Reply #7 on: July 26, 2008, 07:34:20 PM »
Maybe, but I'm not seeing it. I got these 50-60lm Chinese no-name 1w star led's from ebay and they run on about 320mA. Five of them are powered by a ballast, I got a couple of 24v ones (24-28v or something like that), but you can get 12v ballasts too (10-15v). Ballasts are ~85% efficient, and the five leds draw 0.24A at 25v. 6w (guessing a watt for each led and another for the ballast) all up, they are really bright but very specular (need to be behind a diffusor). The colour is not that bad, I have a fenix torch with a Q5 100lm/w cree led in it and it's a bit more "blue" in comparison. The only downside is these cheap ballasts (I know I should say "constant current regulator" for LEDs) aren't dimmable via PWM. I would hazard a guess these are on a par with a good quality electronic ballasted fluoro for light output--seriously.

« Last Edit: July 26, 2008, 07:34:20 PM by independent »

commanda

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Re: LED's - Whats the most efficient current pleas
« Reply #8 on: July 26, 2008, 10:59:42 PM »
I agree with Scott; efficiency is a difficult beast to tie down precisely.


Whilst looking purely at power in vs lumens out, one may end up using far more individual leds, with the attendant increase in pcb real estate, and just generally more work, to save what may be, in the bigger picture, a tiny amount of actual power.


Like any design exercise, it's always a trade-off between conflicting compromises.


Amanda

« Last Edit: July 26, 2008, 10:59:42 PM by commanda »

frackers

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Re: LED's - Whats the most efficient current pleas
« Reply #9 on: July 27, 2008, 03:59:48 AM »
LEDs are funny things - and so is the brain<->eye connection.


For high brilliance, the normal method of operation is very high current pulses to the LED but at the low duty cycle. Try one of the 10mA LEDs already mentioned, pulse it at 50mA but with a 10:1 duty cycle so the average current is 5mA and hence the power dissipation is well within spec. This will produce a very bright light and at a high frequency (10KHz say) there will be no visible flashing.


The next bit is where the eye<>brain connection comes in. Try sitting in a dark room for 5 minutes and then fire either a photoflash or just turn a bright light on and off very quickly (say half a second). While sitting there in the dark you'll be able to 'see' the whole room due to the after image that persists in the photo receptors of the retina. Same with pulsed LEDs - you only see the bright bits, not the average or the dark bits in between.


My recommendation - never use a passive ballast on an LED if you want brilliance and low power.

« Last Edit: July 27, 2008, 03:59:48 AM by frackers »
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