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aardvarcus
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Posts:169
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| 11/26/2008 10:56 AM |
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I am going to be constructing some DIY cabinets later on this year, and I was considering incorperating led's as task and accent lighting, instead of buying those ready made disks. I would only have one rectifier in the system, and would run dozens of led's off of this circuit. Has anyone done anything similar? Any tips?
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Brock
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Posts:431
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| 11/26/2008 3:04 PM |
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In our last house I ran under cabinet 12v halogen's off a 12v battery that was charged via solar. Whatever you do I would wire it to be able to relatively easily switch over at least the power line to 120vac so if you ever change what is up there it's not a big deal. Just run 14-2 or 12-2 to a location you could put a power supply (basement?).
Probably the easiest / cheapest thing to do would be a small 12 sealed battery on a 12v trickle charger, the advantage is you wouldn't need as larger of a power supply and they would work if the power was out. |
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Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1.8kw solar PV setup, 3400 sq ft |
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Dana1
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Posts:648
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| 11/26/2008 5:10 PM |
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The LEDs themselves are not voltage-operated devices, but current dependent. If you supply them a constant voltage the current (and light output) will vary considerably with temperature (possibly with component-destructive results.) Unless your "LEDS" are modules that include the necessary current-regulation your results will likely be unsatisfactory (and low efficiency as well.)
When you say, "I would only have one rectifier in the system", what do you actually MEAN by that? (What DC voltage/current are you designing for?) An individual LED has a forward drop of only a few volts (actual drop varies with temperature and current)- be sure you're using appropriate voltages & current controls for the LEDs you intend to use.
The efficiency specs (lumens per watt) for existing general-illumination LED fixures are quite poor relative to electronically-ballasted fluorescents (and the color-rendering specs are even worse, except in the most-expensive LED fixures.) If you're on-grid, high-efficiency T5 fluorescents will deliver better efficiency and light quality (depending on which tubes you buy- you can still buy the crummy ones it you go out of your way to ;-) ) at a fraction of the price of so-so LED fixures. About the only time an LED will beat fluorescent technology on raw efficiency is in a narrow-beam spot light fixture with a minimum of optics and NO diffuser. In under-cabinet lighting situations you end up with multiple shadows & uneven illumination with LEDs, and in overhead lighting naked high-output LEDs have severe glare. It will be awhile before they can compete head-on in efficiency numbers (let alone price!) with fluorescent technology as ambient-lighting sources. But for fiery li'l accents & decoration they can be pretty good- way more efficient than halogens, with a sparkle that fluorescent technology can never achieve.
(FWIW: I've designed LED control circuits for infra-red illuminators used in eye-surgery equipment, as well as high-precision current & temperature controls for LEDs in fiber-optic gyroscopes for aerospace applications, but never for general-purpose illumination.) |
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aardvarcus
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Posts:169
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| 11/27/2008 8:38 AM |
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| I have used led's in several custom flashlights I have made. I was planning on having one source of ac power, running it though a transformer to get it down to about 6 volts, and running it through a full bridge rectifier to a small capacitor bank. From here, I would run 22 ga. wire through a 100ohm resistor and through one led, and back to the other side of the bank. Each led would be run in parallel with it's own resistor. |
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Dana1
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Posts:648
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| 12/01/2008 11:05 AM |
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>Each led would be run in parallel with it's own resistor.
Using resistive ballast as your current control is the LEAST efficient method- you're burning literally half of the energy from your DC grid as heat in the resistor. (Not to mention you've also started out by burning ~12% of the power in the rectifier & step-down transformer used to create the DC grid, assuming you're using standard silicon rectifiers.) Small switching power-supply chips with current feedback are available for driving single or small arrays of LEDs that can deliver the current with better than 85% efficiency. To read up on it, you can start here: http://www.national.com/analog/led (many other vendors exist, but National has a good amount of design-info on their website.) Without something like that you'll be hard-pressed to beat halogen-incandescent efficiency, measured as lumens per AC watt:
If the LED per the spec sheet produces 40-lumens/watt, measured at the LED at a nominal 25C (you have to keep 'em cool to stay that efficient while on), and you're burning half of the DC in the ballast resistor, supplied by an 88% efficient rectifier/transformer you're well under 20 lumens/AC-watt, which is halogen territory. Add any optics or diffusers, and you'll drop under 15 lumens/AC-watt pretty quickly.
Higher efficiency LEDs exist, but read the spec-sheets carefully to figure out what the actually deliver under the current & temps you intend to run them (many come dangerously close to fraud on the title page!)
Bottom line, designing a truly efficient LED lighting fixture isn't a trivial task- even many commercial products fall woefully short of expectations/claims!
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