LED Array for a Pico

[AndyL]

http://www.canreef.com/photopost/showgalle...php?ppuser=1293 Tossed up a few pictures, i'm too lazy to resize to 600x for n-r.com

[mushroom head]

Sweet!I need LED lighting for a azoo plam filter because im having the azoo palm filter a refugium.Can one of you guys make me LED lighting?

[neanderthalman]

If you were to make the supplies and design available, I am able to build an array for you no problem. It wouldn't be much more effort for me to build it at the same time as I'm building my new array. It'll be a few weeks tho, there's been a delay with shipping my new LEDs. Canada's a big place, where are you?

[neanderthalman]

I just realized I never addressed Blind's comments on LEDs in parallel. Yes each branch will have slightly different currents flowing in each. That slight difference will create slightly different intensities from each LED. The amount of current flowing in one branch will not directly affect any other branch.

 

Within a branch, each LED will have a slightly different I-V curve. Since the current through each LED will be the same, they will each have slightly different voltages. The effects of this are minor, as the only effect may be a slight variation in light intensity, so long as the current and voltage remain within safe values. LEDs are not identical, but are close enough.

 

The same issue is a greater issue if you were designing a solar array. Solar cells are basically LADs (Light Absorbing Diodes) and operate on the same basic principle. When I was part of a team designing and building a solar powered car, it was an issue because mismatched cells reduce your efficiency. The key was to match the output current of each cell, keeping the light intensity constant.

 

Matching cells were placed in series, as the current flowing through the branch is limited by the weakest cell. Those branches are then placed in parallel to add the current together. When adding the current together, circuits called Peak Power Trackers (PPTs) were used to balance the slightly different output voltages.

 

Since that sort of efficiency is not needed, and light intensity need not be consistent from LED to LED, ensuring that the LEDs are closely matched isn't a high priority. Perhaps if I had done a lot more testing of each LED, I could have optimized the groupings and gotten a few more Lux out of it, but it just doesn't seem necessary.

 

Andy, your array looks good, I'd like to hear an update on that in a little while once it's been running, perhaps with a shot of it lighting your tank. With regards to your MH comment, part of the reason I'm trying LEDs is that they apparently give a lot of the MH "sparkle" to a tank. From what I've seen, I think it's true. Certainly looks better than PCs.

[taekwondodo]

Starting to put the 5.5G together (now that the 20 is up and running).

 

Is there a way to compare LEDs in intensity to MH?

 

i.e., how many LEDs at what MCD would one need to be comparable to a 70W DE?

 

TIA,

 

- Jeff

[mushroom head]

Im in Weyburn wich is in Saskatchewan.What would i need to get

?

[neanderthalman]

There is a way to compare the intensity of the MH to the intensity of the LEDs. All I need is a 70W DE MH to put my luxmeter under, and I can compare it to the array I built. I don't have a MH fixture handy, so that option is pretty much out. So far, the only comparison that I can make is that the LEDs had a light output 14 times as intense as my 50/50 PC at the same wattage. If you could find a comparison between the intensity of PC vs MH, then a rough comparison could be made.

 

My guess would be that you wouldn't need 70W of LED to equal 70W of MH. MH bulbs get very hot, so a large portion of that 70W must be wasted as heat. LEDs are not perfect, they do waste some energy as heat. They don't get very hot, not like MH, so they must be wasting less energy as heat, so more of the energy goes into light. If more energy is going into light, then you would need fewer LEDs to produce the same light intensity.

 

Ok. Just for fun, I'm going to make some assumptions to get some basic numbers. This could get ugly. Assuming that you needed 70W of LED to imitate 70W MH, how many LEDs do you need? Each of my 10,000 mcd LEDs is running at about 0.2W. You would need five LEDs per watt, so, 350 LEDs would be a 70W fixture.

 

As I discussed with regards to heat, you shouldn't need 70W of LEDs to duplicate 70W of MH. Lets say you needed 35W LED instead. At 0.2W per LED, you'd need 175 LEDs. I don't think LEDs produce twice as much Lux/Watt than MH, but they are more efficient than MH. My rough guess would be that you'd need 200-300 10,000 mcd LEDs to duplicate a 70W MH.

 

That's not so bad. My current array has fifty LEDs, and I only spent about five minutes with a soldering iron in my hand. There was a lot of prepwork mind you, so I probably spent about three or four hours total mounting and wiring the LEDs. Then came the fun part of putting the array in the hood....bleh! I came to the conclusion that stock hoods suck.

 

Mushroom head, in order to figure out what you need to get, you need to figure out what you need. This is for a fuge, right? How big is the fuge? You're into picos...so this is probably a HOB fuge. My best guess would be that 10 LEDs would be enough. Doncb had 25 white LEDs and a couple blues over his, but that's in the back chamber of a 24g NC, which is a LOT deeper than a HOB fuge. What are the dimensions of your proposed fuge? How do you intend to put it over your fuge? I can't do much more than mount and wire the LEDs.

[AndyL]

Andy, your array looks good, I'd like to hear an update on that in a little while once it's been running, perhaps with a shot of it lighting your tank.  

 

I'll have pics up eventually, yesterday the buisness end of the array was getting dipped in liquid electrical tape to seal it up nice. All thats left now is getting it's tank up and running.

 

Definitely prefer the eggcrate as a mounting surface, keeps things running much cooler than the other methods I've tried.

 

Andy

[neanderthalman]

I'm thinking of maybe using eggcrate as well, how did you attach the lights to the eggcrate? Did you just fold the leads around it or something? The other option to reduce heat is to mount them in sheet metal. I've tried to keep as much metal away from my tank as possible, and I'm still waiting for a suitable peice of aluminum to show up in the scrap bin at work.

[AndyL]

I glued them with superglue gel - the leads basically straddle the plastic.

 

I have a big hunk of 1/8" finned aluminum here, decided after testing the eggcrate that I prefer to use it. It'll be a lot less work to change out the arrays if one burns out - or if I upgrade sometime. The LEDs can be popped off with a utility knife and reglued, I figure the only way to get them out of metal (other than pressure fitted) is to drill them out - not my favorite thought.

 

Andy

[wannabeafarmer]

Sorry to poke my nose into this, but does anyone think that they could give me a diagram for the electronically challenged? I am interested in trying a small array for my AC500 HOB Fuge on my ten gallon. I would like to install it in the actual lid of the AC 500, to reduce light bleed into the tank.

 

I'm a little confused about how you wired the leds together, and then added the power.

 

Thanks in advance for any help!

 

John

[neanderthalman]

Sure thing John

 

Each individual triangle on the diagram is an LED. The electricity will only flow through the LED in one direction. The electricity will flow in in the direction that the triangle points. That means that you need to put the positive side of a battery on the flat side of the triangle.

 

This is a diagram of a single LED

 

 

You can then put the LEDs into groups, with the negative side of one LED attached to the positive side of the next LED, like this pic.

 

 

Once you've got three of them arranged in a group like that, which is called a "series" connection, you can take more of those groups and wire them side by side, called a "parallel" connection.

 

 

That diagram shows four groups of LEDS wired in parallel, and each group is made up of three LEDs in series. It also shows how the 12V power supply is wired to it. This is now a 12 LED array.

 

The full schematic is essentially four separate 12 LED arrays. The positive side of the power supply is wired to the positive side of each 12 LED array, and the negative side is wired the same way.

 

Here's the schematic again so you don't have to keep scrolling up the page.

 

 

The positive side of an LED can be identified by the length of the leads. The longer lead is the positive side. On many LEDs, there is a flat spot on the casing next to the negative lead. This is handy if you've cut the leads short and forget which end is which. The longer lead corresponds to the flat side of the triangle, and the flat spot on the LED corresponds to the line across the tip of the triangle in the schematic.

[wannabeafarmer]

Thanks neanderthalman, that clears it up a whole lot. Now, I recall reading somewhere that you need resistors, or something, to limit the current. How do you determine what resistor? Also, just to make sure I'm on the right track...you determine the total number of leds that can be run on a power supply by the amp rating of the power supply, right? You add mult. the amp draw of each led, by the total # of leds, and it has to be less that the amp rating of the power supply, correct.

 

Thanks for all you help!

[neanderthalman]

You usually do need current-limiting resistors, also called dropping resistors. What I did was wire this array in a manner that made resistors unnecessary. This only works if you're making an array of identical LED's, or at least have the similar electrical characteristics.

 

If you wanted to power a single 4V LED with a 12V power supply, you would have to have a resistor. Lets assume that you want to have 20mA of current through the resistor, which is typical. The LED needs to have 4V across it, so the resistor needs to have 8V across it, so that the total voltage matches the power supply. Using Ohm's Law, the resistance you need is found by the formula R = V/I. The resistor you need is 400 Ohms. Resistors are typically only available in common sizes, so you go to the next highest resistor, which I beleive is 430 Ohms.

 

What I didn't like about this is that only 1/3 of your power is being used by the LEDs, and the other 2/3 of the power is being wasted as heat in the resistors. This means that you need a power supply 3X as powerful, and you're dumping heat into your tank.

 

The solution I found was to put three LEDs in series, so that the total voltage is 4V + 4V + 4V = 12V. This eliminates the resistors, and now that 2/3 of your power that was being wasted as heat in the resistors is instead being used by the LEDs.

 

That way, you can power 3X as many LEDs with the same power supply as before, and get 3x as much light for the same power consumption. I was getting 5600 lux at 8" with an 8W array. If I had used resistors, I would have only had about 1860 lux.

 

To determine the size of power supply needed, you need to determine the total current draw of your array. If you wired 48 LEDs with resistors, you would need a power supply that can provide 48 x 20mA = 960mA. If you wire the LEDs in groups of three, eliminating the resistors, the same current is used by each group of three. You would have 16 groups of three, and each group draws 20mA. The power supply is much smaller, needing to provide only 16 x 20mA = 320mA.

 

This is the reason I wired my LEDs in groups of three.

[Blind Tree Frog]

Within a branch, each LED will have a slightly different I-V curve. Since the current through each LED will be the same, they will each have slightly different voltages. The effects of this are minor, as the only effect may be a slight variation in light intensity, so long as the current and voltage remain within safe values. LEDs are not identical, but are close enough.

The problem isn't the intensity in the light the problem is that you have nothing controlling the current and it can change.

 

So if LED 1 starts heating up and the resistence drops the current will increase. Well now the current on that string is increasing and the currents on the other strings starts dropping. This (in theory) just snow balls and ends up frying LED's. This is why the resistor is there to control the current as it will be more consistent then the LED.

 

But we'll see how it goes. I'd wager you don't have enough LED's in the array to really so you'll probably be fine.

[neanderthalman]

The current through one branch does not have any effect on the current through another branch. In the case where an LED stops working, the other two LEDs in the same branch stop working.

 

LEDs have an internal resistance that is represented by exponential I-V curve. That means that the resistance is not constant, but it is predictable. Resistors have an internal resistance that is represented by a linear I-V curve, which means resistance is constant. The resistance of an LED and the resistance of a resistor are both affected by temperature.

 

The I-V curve of a resistor controls the current through the resistor. Similarly, the I-V curve of an LED controls the current through an LED.

 

Your theory doesn't make sense, even with a resistor. If a temperature increase in an LED decreases the resistance, then the current through the LED will increase. The current through the resistor will also increase. The increased current will heat up the LED and the resistor even more, and the resistance of each drops even more. The same snowballing will happen.

 

The size of the array doesn't matter either. Each branch of three LEDs is independent of all other branches. The current in one branch has no effect on the current in the other branches. This could be an array of 30 LEDS or 30,000 LEDs, and the current in one branch will not affect the current in another branch.

 

My array is still doing fine, and the proof is in the pudding....

[Blind Tree Frog]

Originally posted by neanderthalman

The current through one branch does not have any effect on the current through another branch.  In the case where an LED stops working, the other two LEDs in the same branch stop working.

 

No each branch controls it own currents, but the total circuit current is affected.

 

Not sure what I was thinking when I wrote that other post.

 

 

 

 

The size of the array doesn't matter either.

and as I said, as you have a small number of LED's you probably won't see any problems. The point I was getting at was that the more LED's you are using, the more chances for flawed or out of spec LED's you have.

 

 

Regardless, I'm bowing out of this discussion. I'll stand behind what I said originally of this should be interesting since there are no resistors controling the current and leave it at that.

[neanderthalman]

I just picked up my leds from the post office today, and it turns out that he sent me the wrong white LEDs. They're a lower voltage, but are 12,000 mcd instead of 10,000 mcd. Because of the lower voltage, I'm going to run them in series of four with a 12V power supply, so that they each have roughly 3V.

 

I say roughly because the blue LEDs are now a slightly higher voltage than the white LEDs, which makes it more difficult to determine what the voltage will be across each LED. I'm going to make up a test fixture tomorrow with two whites and two blues in series, put 12V across it and make sure that the current is appropriate.

 

I have also decided to stick with my original design and mount the LEDs in holes drilled in acrylic. I looked over the data sheet and they have a maximum operating temp of 80 C, which is like 180 F or so. My current array is running at about 90F, so I shouldn't be overheating the LEDs on the larger array, especially if I ventilate the hood.

 

I'll try to post some pics of the new test fixture tomorrow. The new hood still needs to be made as well...probably should have been doing that while waiting for the LEDs, oh well. At least I've already got my power supply, from my old 486 . A computer power supply should be outputting pretty damn close to 12V, so I won't be blowing up my array this time.

[mushroom head]

Cool!

[taekwondodo]

Damn - I just threw away to ATX cases with supplies (really old) a couple of months ago...

 

- Jeff

[neanderthalman]

Yeah, it's a lot tougher to use a newer computer power supply. Old computers had a power switch on the front of the computer that interrupted the 120VAC from the wall to shut down the power supply.

 

In a newer computer, the power switch was replaced by a power button. A portion of the power supply is always on, powering a small circuit on the motherboard. That circuit waits for an input from the power button, and when it gets that input, it tells the rest of the power supply to turn on. That's why newer computers are able to turn themselves off after you shut down, whereas older computers would display a screen telling you it's safe to shut down the computer. You could use a newer power supply if you can find out what wire turns on the rest of the power supply.

 

One thing I also noticed was that the supply has minimum current output requirements. I'm not sure what this means, but the 12V output has a minimum of 1.6A output. Fortunately my array should draw about 4A, well over the minimum current output. The max current is 9A, so I'm good there as well.

[lichatha]

Neanderthalman if you need old PC power supplies check Computer-Tek here in Orillia, they've got tonnes of old pc crap laying around. It's not crap but it's old to me. And while you're here you can always drop off some Zoo's

 

Gerald

[neanderthalman]

Thanks for the tip lichatha, I'll keep that in mind if this power supply doesn't work. I havent built the test fixture yet, but I did pick up the wood for building the hood.

 

I also stopped in at a local Princess Auto to see what they had for ventilation fans, and I found an awesome PC exhaust fan that I'm going to use. I'm at work, so I can't post any pictures. When I get the test fixture built, I'll post some pics of it and the fan.

 

My tank is also overheating at the moment, but I picked up a second fan that I'm going to mount to the back of my tank. It should increase airflow enough to keep temps stable. For now a desk fan is doing the job.

[neanderthalman]

Alright, I slapped together four LEDs in series, and it's no real surprise that I'm gettting about 3V across each, and about 20mA current. Sounds like the full size array is a go this weekend.

 

Here's a pic of the new test fixture.

 

I'm also going to fit this exhaust fan in the hood above the LEDs.

 

Its designed to fit in a PCI/ISA slot in the back of a computer, but it's powered by a drive power connector. I'm going to straighten the attached metal bracket and use it to mount the fan in place. The LEDs are such a low profile and the fan is such a low profile that the hood isn't going to be overly tall once it's done.

[fritzthewondermutt]

Have you considered putting a "diffuser" under the leds to smooth out the lighting, (overcome the focus of the leds)? A piece of acrylic sanded with 800 grit or steel wool ought to do the trick. Should even out the light without eating too much of it, (no, scuffing up the front of the leds doesn't work for those of you might want to try it).

 

Thanks, by the way for all the info. You have saved me lots of hours and dollars in experimentation.