Need help from a Math wizzard/LED Geek

[Paul B]

If I use the copper tube, I can only run the circuit in parallel.

 

[Jimmyb819]

Paul,
How many leds do you think you can run on each length of pipe? How many pipes will be running along the tank?

Once we figure out how many leds will be in each parallel circuit and how many parallel circuits we will have then we can start designing the schematic.

 

[Paul B]

I think I can use 3 runs of copper tubing using 60 LEDs evenly spaced. So that is 20 LEDs on a tube. I can use more or less LEDs and I can also do it with 4 copper tubes. The top of my tank is only 14" across and 6' long. But there is an algae trough at the back of the tank that takes up 4" so I only have 9" of surface to light (subtracting for the width of the glass)
I am going to design the cooling "radiator" so that I can easily add to it if I need more cooling as it is impossible to figure that without experimentation.
I am having trouble getting 1/4" rigid copper tubing. I can get it but I don't think they make fittings for it so I may have to go with 3/8" tubing, which I also can't get fittings for. But I can just use that for the straight pieces and solder that to 1/2" tubing so I can use standard fittings. This will be a very cool project. I may use a car heater coil which is just a small copper radiator. If I can find one cheap, they are very small and efficient and there are plenty of junk yards here where I could possibly get a wrecked one I can fix.

 

[redfishbluefish]

Interesting idea. But I'm scratching my head on this one for two reasons:

1. Copper isn't the greatest conductor of heat. Aluminum is 2.3 times more conductive. If you could use aluminum, that would greatly assist.

2. Last I looked tubing was cylindrical (round), while the base of an LED was flat. I'm at a loss as to how you would get intimate contact of the flat LED to the round tubing to allow the heat to transfer.

Following along.......

 

[Paul B]

Actually copper is an excellent conductor of heat and much better than aluminum.:
I took this off a metal conducting chart:

Copper:Copper (pronounced /ˈkɒpɚ/) is a chemical element with the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile
metal with excellent electrical conductivity. Copper is rather supple in its pure state and has a pinkish luster which is (beside
gold) unusual for metals, which are normally silvery white. It is used as a heat conductor, an electrical conductor, as a building
material and as a constituent of various metal alloys. Copper is the best heat conducter next to silver.

Material
Bulk Conductivity (W/mk)
Silver, Pure 418.0
Copper 11000 388.0
Aluminum 6061 T6 167.0
Zinc, Pure 112.2
Iron, Cast 55.0
Solder, 60% Tin 50.0
Titanium 15.6
Thermal Grease, T660 0.90
Fiberglass 0.040
Air, stp 0.025

But besides that. Copper is much easier to solder and repair if it leaks. Aluminum is very hard to solder with regular home materials and tools. I could have done it on my job, but I am retired. Also copper tubing is very available and it doesn't rot like aluminum. Aluminum is much cheaper and lighter which is why heat sinks in electronics are made of it but cars use copper for their cooling system even though it is much more expensive because a smaller copper radiator is more efficient than an aluminum one and can be made smaller.
It is true that the tubing is round. I looked for square tubing and although it is kind of available, it is very expensive and hard to connect to anything and be water proof. To secure the LEDs to the round copper is easy. The base of the LEDs I have is less than a quarter of an inch. The thermal glue is thick and will take up the room on the sides of the led. Of course I would like the tubing to be square but I don't have the budget of NASA and must use what I can get reasonably and at a reasonable cost. I have plenty of flexible tubing and copper sheet but I want to use rigid tubing for looks. I can also solder small copper disks to the round copper tubing for a better heat transfer but I am hoping I don't need to do that. I will experiment before I build the thing. I have a Whitney Punch (metal hole puncher) and can make copper disks. But as I said, this is not going to the moon.

 

[redfishbluefish]

I stand corrected.....I looked up specific heat (I'm a chemist, what do I know!) and not thermal conductivity. Here's the thermal conductivity of some metals (k ( W.m-1. K-1 ))

Iron - 73
Steel - ~ 46
Aluminum - 210
Copper - 386
Silver - 406
Gold - 293
Platinum - 70
Yellow Brass - 85

 

[Paul B]

That's OK, I am an electrician and you should see the rats nest of wires under my tank.

 

[Jimmyb819]

Ok,
20 LEDs per parallel circuit. If you want to run them at the full 500ma then that's 10 amps per circuit, 30 amps total. What are you planning to use as a power supply?

 

[Paul B]

I don't really want to run them at full power because I am not real sure how much cooling I need. I can build a power supply or if I look around my work shop I should have a few someplace from eons ago.
Unfortunately power supplies of that size are physically large but I can stick it under the tank. What resistors will I need and where should I install them? I know I have large battery chargers that will have plenty of amps and an output of 6 or 12 volts which I can adjust. I know I built a power supply a long time ago and I think I know where it is. It is fully adjustable for DC volt output and amps.

 

[twilliard]

Hi Paul if you have a hard time with a power supply the mean well drivers are small and not to expensive.
I have a cool ac dc tube driven power supply I would send you but its heavy! (For the work bench)

 

[Jimmyb819]

Ok, the resistor you use will dictate the current through the LEDS so we need to know how high you want to run them. 300ma would be 60%. The resistor will also be using burning off the excess voltage so being closer to the led voltage would be best.

 

[Paul B]

Twillard, thanks, I think I should have some type of power supply around or I can build one from what I have. I hope anyway. I used to do quite a lot of electronic inventing in years back. But we didn't have any LEDs then.

I may even have a few smaller power supplies around if I want to use a separate supply for each length of pipe.

Jimmy. 60 or 75% sounds OK. I can build a test circuit and let it run a while to see if it blows up, goes on fire or disintegrates. These LEDs that I am using will only run about 20 seconds without a heat sink before they overheat and go out.
So what resistors will I need and where should I install them?

 

[Paul B]

I found my test power supply. OK, stop laughing, I built this thing in the 60s or 70s from parts off an old (1920ish) elevator we were demolishing in Manhattan. That is where I acquired a lot of parts. That old stuff was built very rugged and heavy. No printed circuits involved just transformers and rectifiers. It is adjustable up to 20 volts dc and 20 amps. I use it for testing. After I find the correct values, I will build a power supply for what I need.

 

[twilliard]

Your welcome Paul!

Here is a screenshot I did for you

 

[Paul B]

OK, so if I make 4 strings I can use 4 resistors, each rated for 5.6 ohms and one watt. Is that correct? And each circuit will run 15 LEDs or is it like your schematic and only run 3 LEDs?
It looks like I will need a resistor for each 3 LEDs. Is that correct? That would be no problem. Well a little problem but not insurmountable because if I can only run 3 Resistors off each resistor I will have to have 4 circuits going to each tube. That will cause a little of a problem as I am trying to make the wiring as minimal as possible being there is no where to run the wires and keep them sort of hidden.
Oh wait!!!! I see a problem. I want to wire these LEDs in parallel. Your diagram has them in series/parallel. Can you figure 15 Leds in parallel? I know this is complicated. If it were easy, I would have done it myself.
Look at my original (very technical) diagram in post #6

 

[TheEngineer]

Is that what you want? This is 4 parallel strings of 3 leds in series.

Are these LEDs on stars? How are they mounted? I'd like to do a quick calculation of how much heat you can realistically transfer to the copper tube.

 

[twilliard]

Here you go Paul you can enter your numbers and values here
http://ledcalc.com/#

 

[twilliard]

Ok like this

 

[twilliard]

Now that resistance is just an example as the Fv of the diode is not known in this example verses what you are using

 

[TheEngineer]

Looks like a good layout to me. He wants 15 per strand.

You want a power supply as close to the required voltage (3.3VDC) as possible. Anything over that and you need to burn it off in the resistor and that's going to be tons of power lost. Even at 3.3VDV if we guess the voltage drop is 2.2VDC you are going to burn off 5 watts in the resistor. That's going to generate a lot of heat too. Running a 12 VDC supply will require around 50watts