Best LED (lumens/$) - The Planted Tank Forum
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post #1 of 29 (permalink) Old 03-13-2010, 01:59 AM Thread Starter
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Best LED (lumens/$)

I spent lunch today scouring some electronics suppliers for LEDs to see what is available:

LED Comparison

The best deal, at $27 (and 81 lumens/$):



Bridgelux BXRA-C2002-00000
Luminous Flux:2200lm (!!!)
LED Color:Cool White
CCT:5600K
Viewing Angle:120
Forward Voltage:16.2V
Forward Current:1.5A

A quick estimate with the Light Calculator says ONE of these will give you 123 umol/s/m^2 in a standard 10G (20"x10"X12")! I fiddled with the numbers a bit because the calculator assumes 60 lumens/W, and I do realize that getting one of these to light an entire 10G tank wouldn't work very well. Obviously, there are a few drawbacks. 5600k might not look the best and at 16.2V and 1.5A, this thing will be a pain to drive, but for $27, it may be worth giving a shot...


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post #2 of 29 (permalink) Old 03-13-2010, 03:35 AM
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Well as you've probably noticed, there is a lot more to the cost of the project then just the LEDs. Even if the LED is half the price per Lumen, if you have to spend twice as much for the components to drive it you aren't going to be saving any money on the project.

In terms of lighting too, it might be much more practical to use 4-6 LEDs just to get better coverage. If you have to get 4 LEDs then drive them at 1/2 power to keep the light levels where you need themthen the cost per lumen doesn't matter anyway.
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post #3 of 29 (permalink) Old 03-13-2010, 04:50 AM
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Nice find on the LED.

Erloas: I don't think the real point of DIY is to save money. It's to pique that human instinct to learn how things work and make something custom to the user.


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post #4 of 29 (permalink) Old 03-13-2010, 05:00 AM
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Quote:
Originally Posted by mistergreen View Post
Nice find on the LED.

Erloas: I don't think the real point of DIY is to save money. It's to pique that human instinct to learn how things work and make something custom to the user.
Yes, that is part of DIY, but this thread indicates that this particular LED is best because it gives the most lumens per dollar, indicating that saving money is a major concern. It isn't hard to make a LED light fixture that does well on a planted tank, if you aren't trying to keep the cost within some set amount. You just use lots of LEDs, with buckpuck type drivers, with a means of controlling the current to the LEDs, and using standard heatsink extrusions. That has been easy for over a year. The hard part is doing that at a cost that isn't outrageous.

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post #5 of 29 (permalink) Old 03-14-2010, 07:29 AM
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At the high amperage you get about 4 volts drop from the Cree's. So why not use 30 of them or so in series, and run them right off 120v with a rectifier, capacitor and a resistor. There's alot of variation in them so you can't get a precise voltage drop, but it should average out... If you get close enough to the 120v or whatever, you wouldn't need a ridiculous resistor (the more wattage you try to dump with a resistor, the more heat and larger it needs to be).

Would be about $150 in LEDs, if you use the XPG-s you could get about the same output as 4x65watt CF's, so enough for 55 - 100 gallon tank and be in the 'ridiculously high to high light' range.

You don't really need to use a buckpuck for these applications if you're using AC. We don't really care about efficiency, except perhaps to minimize heat from resistors, but proper design can minimize the inefficiencies without having to resort to a buck driver.

Alternatively, if you wanted to work with low voltage, you could plop $60 on a 12v power supply and do a similar thing, or plop $20-30 on a laptop power supply (~24 volt).

I'm still convinced you could do this all on 120v with $10 of components... For setups that are good for less than 50 gallon tanks, where you want to use less than 30 LEDs, I think you could just use a transformer to drop the voltage to whatever you wanted, then rectify it. Only need 1.5 amps from the transformer, so don't need anything terribly beefy.

XPG's can be had for about $5 or $6 individually. I'm sure you can get a volume discount. I don't think I'd use a lens on them since I believe the output is in a lambertian pattern, which is pretty much what we want anyways, but 30 lens's would only be a couple bucks total anyways.

This bridgelux is pretty sweet. 10% more lumens from the same amperage as 4 XP-G's, although it costs about the same as 4 XPGs. Realistically, you're right, I don't think the light output would be even enough for one of these to fill out. One of these really could light up a 10 gallon with a good amount of light. I have the same lumens on my high light 10 g. 6 or 7 of these on a 55 gallon would be kinda weird...
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post #6 of 29 (permalink) Old 03-14-2010, 09:46 PM
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You need to be able to set the current flowing through each LED. Putting them all in series means you only need to control the current flowing through the series group - same current goes through each one. But, if you supply those series LED's with a rectified 120 volt supply the current will be whatever the rectifier will allow to flow, and that may be far more than it takes to burn out the LED's. So, the suggestion to do it that simply, simply won't work. LED's are not resistors, so putting them in series doesn't result in a load that itself limits the current.

Ballast resistors work by reducing the voltage available to the LED as the current increases, and that stabilizes the circuit current to a fixed value. Buck pucks do the same but don't eat up power in voltage drop across ballast resistors. It isn't quite as simple and easy as it looks.

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post #7 of 29 (permalink) Old 03-15-2010, 01:17 AM
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I mentioned resistors four times. My point is that we don't normally take AC and use it for LED applications with simple circuits because the resistors used would be ridiculously large to bleed off all that extra voltage. If we bleed off the voltage with more LEDs from their voltage drop, we kill two birds with one stone. Smaller resistor is required and we get more light out of the deal.

I think I recall hearing that when you rectify the AC the voltage ends up being the source voltage times the square root of 2, so I think 120v AC turns into 160 volts. If that's true, then the easiest would be to use 40 cree's, not 30...

So lets say the rectified voltage is indeed 160V, and the average voltage drop from each cree @ 1.5 amps is 4 volts and we're using 40 of them, then we'd need a 2.2 watt capacity 1 ohm resistor. Radio Shack sells 10 watt 1 ohm resistors for $2. Easy. They sell a 4 amp rectifier for $2.59.

I'm not sure how much ripple current is acceptable, or if you'd even want to bother smoothing out the power coming from the AC. I'm not sure if the capacitor has to be rated for the maximum voltage or the voltage after all the drop, but if a high voltage capacitor is needed, then there might be a problem, as high capacity high voltage capacitors either don't exist, or are ridiculously big... I found somewhere else that suggested ~100,000 microfarad for 0.1 volts of ripple which pretty much does not exist in my Mouser catalog at 160 volts or above.
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post #8 of 29 (permalink) Old 03-15-2010, 01:32 AM Thread Starter
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Just as a note, one of the LEDs in the comparison is designed to be run directly from AC.

Manufacturer Seoul Semiconductor Inc
Manufacturer Part Number AW3200
Color White, Cool
Current - Test 40mA (3.3W)
Current - Max 40mA (3.3W)
Luminous Flux @ Current - Test 215 lm
Voltage - Forward (Vf) Typ 88 ~ 92 VAC
Wavelength 6300K
Lens Style/Size Round with Domed Top
Viewing Angle 130
Mounting Type Surface Mount
Package / Case 12.00mm Dia x 6.54mm H


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post #9 of 29 (permalink) Old 03-15-2010, 03:46 PM
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There isn't a big need to smooth out the ripple from the rectifier. You could easily run an LED on a sign wave if you wanted without a rectifier at all, you would just not be using power from half of your wave form.
The only issue you might have is that if the ripple is fairly big you might be able to see a noticeable pulse in the lights.

As for a cap to smooth it out, if I remember right the cap has to be parallel to the LEDs, between the rectifier and ground before the LED string starts. Which would mean a cap that can handle 160V. Probably the easiest place to find a cap that size is in a dead computer power supply, they almost always have 2-3 large caps (often what goes out in them too though). The exact rating doesn't really matter because you aren't really looking for great precision and don't need a really flat ripple anyway.

The main issue with using direct AC power is you loose a lot of control over the design. It pretty much gives you one set number of LEDs and you can't easily use more or less. If 40 or 80 LEDs are too big or too small for your project there isn't a whole lot you can do. It also makes it a lot harder to add any sort of programming or dimming to the circuit.
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post #10 of 29 (permalink) Old 03-15-2010, 10:06 PM
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Quote:
Originally Posted by Erloas View Post
There isn't a big need to smooth out the ripple from the rectifier. You could easily run an LED on a sign wave if you wanted without a rectifier at all, you would just not be using power from half of your wave form.
The only issue you might have is that if the ripple is fairly big you might be able to see a noticeable pulse in the lights.

As for a cap to smooth it out, if I remember right the cap has to be parallel to the LEDs, between the rectifier and ground before the LED string starts. Which would mean a cap that can handle 160V. Probably the easiest place to find a cap that size is in a dead computer power supply, they almost always have 2-3 large caps (often what goes out in them too though). The exact rating doesn't really matter because you aren't really looking for great precision and don't need a really flat ripple anyway.

The main issue with using direct AC power is you loose a lot of control over the design. It pretty much gives you one set number of LEDs and you can't easily use more or less. If 40 or 80 LEDs are too big or too small for your project there isn't a whole lot you can do. It also makes it a lot harder to add any sort of programming or dimming to the circuit.
As I recall, some LEDs have a limited capability to withstand a reverse voltage. If that is true, doesn't the reverse voltage half of AC damage the LED's when you use AC to drive them? If so, you could use twice as many LEDs and a couple of diodes to set it up so one half are driven with the + wave and the other half with the - wave. Right?

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post #11 of 29 (permalink) Old 03-15-2010, 10:53 PM
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Quote:
Originally Posted by Erloas View Post
The main issue with using direct AC power is you loose a lot of control over the design. It pretty much gives you one set number of LEDs and you can't easily use more or less. If 40 or 80 LEDs are too big or too small for your project there isn't a whole lot you can do. It also makes it a lot harder to add any sort of programming or dimming to the circuit.
Since we're only dealing with 1.5 amps, you can easily get/make a transformer to whatever voltage you want while it's still AC and use however many LEDs you want.

Quote:
Originally Posted by Hoppy
As I recall, some LEDs have a limited capability to withstand a reverse voltage. If that is true, doesn't the reverse voltage half of AC damage the LED's when you use AC to drive them? If so, you could use twice as many LEDs and a couple of diodes to set it up so one half are driven with the + wave and the other half with the - wave. Right?
The Cree XPGs only tolerate 5v reverse voltage. I'm not sure if the effect is cumulative if you hook them in series.
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post #12 of 29 (permalink) Old 03-16-2010, 04:49 AM
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You can make a simple rectifier with 4 diodes, at a very low cost, so why not do that? Then the flickering would be at 120 hertz and probably not even visible. And, I suspect you could buy a microcircuit package rectifier able to handle the voltage and current without spending much either. So, if you can cheaply buy a transformer to drop the 120VAC to whatever voltage is appropriate for the number of LEDs you want to use, could you drive a series string of them with the transformer/rectifier combination, plus a small ballast resistor?

One potential problem is that LEDs can fail short circuited. If a couple of them did so, you could burn out all of the remaining LEDs.

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post #13 of 29 (permalink) Old 03-16-2010, 07:05 AM
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Radioshack sells 4 amp rectifiers for $2.59. I talked to my friend and got some help with some more math. The conclusion is that controlling ripple to any reasonable level requires capacitors so ungodly huge and expensive that it ain't gunna happen with this setup. If you wanted to control ripple and get the most light output for the LED count you'd need to bump the frequency of the power up and basically make a switched mode power supply so that you can get away with a smaller capacitor.

On the positive, it looks like modern light dimmers would work for the simple setup..

Hoppy - I'm not sure if the LEDs would burn out or if they'd just pass the rest on to the resistor and burn that up..
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post #14 of 29 (permalink) Old 03-16-2010, 09:41 PM
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Quote:
Originally Posted by Hoppy View Post
As I recall, some LEDs have a limited capability to withstand a reverse voltage. If that is true, doesn't the reverse voltage half of AC damage the LED's when you use AC to drive them? If so, you could use twice as many LEDs and a couple of diodes to set it up so one half are driven with the + wave and the other half with the - wave. Right?
You probably couldn't do it actually just because LEDs weren't designed for that sort of thing. Though if you had long chains in a half or full bridge setup it might work. Probably not worth the effort of figuring it out though given how easy a normal bridge rectifier is to build/buy and the potential cost in lost LEDs.

Its been too long since I had those classes in college.

The main point I was trying to make though was simply that a bit of ripple from an unfiltered bridge rectifier isn't going to do anything to LEDs. They are designed to be turned on and off countless times, so even if the ripple was large enough to drop below the threshold voltage and turned the LEDs off it wouldn't harm them.



As for burning out an series of LEDs with one burning out, it kind of depends how you build the circuit. As far as I remember LEDs (and diodes in general) virutally always burn out as a short rather then an open. I don't remember seeing any series of LED strings go out because of a single one. Its still likely to drop some voltage across it, but no way of knowing how much. The danger to the rest of the LEDs depends on how the circuit is designed. If you use just a simple limiting resistor then dropping noticable more voltage across it is going to increase your current fairly drastically. If your resistor is close to its power limit it might blow first, otherwise it might be other LEDs. Then it depends on how hard you are driving the LEDs. If you design the circuit to run at 1.5A, and the LEDs are rated at 1.5A it isn't going to take much of a change to start burning them up. If you are driving the LEDs at 700mA and they can run at 1.5A, then you'll have to loose several before you run into high likelyhood of over-currenting them. Of course thats what fuses are for.
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post #15 of 29 (permalink) Old 03-17-2010, 01:04 AM
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Has anyone seen any LEDs driven by unfiltered AC, whether rectified or not? I'm really curious about whether the flicker is visible, and whether you get light consistent with what you would expect at that particular current. If the flicker isn't a problem this would make building a DIY LED fixture way easier and cheaper. (And transformers are big enough to handle without 5 thumbs getting in the way.)

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