LEDs wired in parallel, current draw doesn't increase

[unbwogable]

I'm stumped. I've got 24 (off brand) Luxeons epoxied (thermally conductive) on the outside of a glass globe, which is filled with mineral oil (for heatsink purposes). That said, I'm trying to make sure I've got enough of a heatsink to keep them all lit, so I've got a multimeter wired in to see current draw from my variable power supply. Each LED is rated for 3.5 volts, 700ma. If I turn on 1 LED at 3.25 volts, it draws 540ma. If I add 3 more LEDs, they draw 600ma total. I've got a thermometer in the oil, and it shows about 3-4 degrees above room temp, so I increased the voltage to 4 volts, and the total current draw is 1100ma (275ma per bulb), and oil temp doesn't increase too much. What am I missing? I spent a bit on these LEDs so I would prefer not to blow them up. This circuit should be pulling 2800ma, am I right? Unfortunately, I don't have a light meter, so I can't tell how much this thing is putting out.

 

[unbwogable]

I may have solved this issue. In my short mindedness I failed to notice that my factory-set bench power supply (thrift store freebie!) is only capable of 1A on the variable side. I put a heavy duty 3W rheostat inline and plugged it into the 3A, non-variable side and slowly increased it, and I can safely get each LED to draw 500ma without burning up, and the oil temp only gets up to about 87, and holds steady. Yay! Now to see what happens when I turn on the other 20 LEDs... I just realized that I'm going to need a very powerful current regulated supply.....

 

[Linger]

I've re-read this and I'm missing something. So I appologize for bolding ignoring some of your questions, but know I did give it the old college try.

1-Leds give some warning before they go. Namely, they turn angry blue. This is the phosphor burning up (or, something, it's just bad). If they go blue, kill the power, they'll recover (it's blue because of the heat, not permanent damage.) If you ignore the blue, well...
2-touch them. If they you can take the heat, they aren't too hot.

Most importantly,:welcome:

 

[Linger]

The oil is a really interesting comment and I'm curious about what your light looks like. If forced to guessed, I would presume some religious iconography.

At any rate, glass isn't the best conductor of heat. Of an order of magnitude less than copper, or even aluminum. While you're in the design stage perhaps pay attention not just to the temp increase in your mineral oil, but also the temp at the thermal junction of the led's themselves. Hate for them to be cooking on the wrong side of the heat-sink. Now I presume you've thought of this but I had an image of fracturing glass sending hot oil rolling out and was compelled to say as much.:shakehead

Where does the oil transfer heat to? It can only 'sink' so much

 

[unbwogable]

It's part of a steampunk lamp. This monster will be fitting inside a larger glass globe (which is frosted to hide the hardware). I'm considering filling that outer sphere with oil as well, since it is non-conductive. I looked everywhere I could for a aluminum/brass/copper sphere in the correct size, and could find one anywhere. That outer globe should (hopefully) contain it if it fractures, which I will thoroughly test while it is on my bench instead of on the wall.

 

[yellow]

high current --> big parts
low vurrent --> small parts
why not wire the led in series?
Might be easier to get a higher voltage / low current plug-supply, than one that gives 4 V / 20 A...


PS: not exactly sure on that oil thing.
What is the gain?
low surface, high weight, high cost, low rugged ... as compared to aluminium.
(but I must confess, I can not even think about the mass a 24-led "light" is best to have)
The increased "thermal mass" might need some time to heat up, but what if the heat is there?

 

[lumen_eater]

pictures please ! this sounds awesome. why do you need a perfect sphere for the inner part ? the outer part is going to be frosted right? so you could just buy 24 small heat sinks and hold them in place with wires or something than fill the thing with oil. you will also need a radiator which could be done very steampunk style ^^ i have the feeling this is going to be awesome

 

[uplite]

Yes, pictures please!

Some suggestions:

• use constant-current drivers
• use series wired LEDs
• use lots of brass for heatsinking

Why?:
http://tigerhawkt3.com/welcomemat/ledfaqs.htm#6
http://tigerhawkt3.com/welcomemat/ledfaqs.htm#9
http://en.wikipedia.org/wiki/List_of_thermal_conductivities

go steamboy!

 

[unbwogable]

I haves pictures!

This is the sphere of impossibility, as I've come to call it. It will sit on the bottom of the device.

This is the slightly finished product (sitting in the upright position - glass goes on the bottom).

This has a few extra embellishments (still a work in progress)

Those glass-like tubes on the side of the last one are actually PEX tubing, and have a white automotive-grade LED in each end (for the mad scientist look). I'll post a finished product when it actually is a finished product.

Suggestions, tips, ideas are all welcome.

lumen-eater: I really should have considered that. I didn't think about it!

yellow: I figured if people can submerge entire computers in the stuff, I can surely add it to my light

 

[blasterman]

Suggestions, tips, ideas are all welcome.

Bare LEDs, especially cool-white / white ones driven harder than 40 lumens or so are obnoxious to look at when bare. I currently have several dozen older Cree warm-whites that are only 35-40 lumens at 350mA, but have the most gorgeous, kill for it, rosy-pink warm tone I've never seen in newer emitters.

I've also fiddled with several artsy designs, but just can't come up with anything neat because when viewed bare they just give you a headache. They aren't bright enough however to be diffused well.

The sphere is a cool idea, but I don't want your design to suffer the same problem. Maybe perhaps gets some small pieces of frosted glass, or take some frosted paint spray to bare glass, and ring your design with chunks of it?

 

[lumen_eater]

are there pictuers of the outer sphere ? sorry to say this but the glas sphere looks like a thermal nightmare if your not going to use oil in the outer sphere for extra cooling but if you do it might work

 

[uplite]

yellow: I figured if people can submerge entire computers in the stuff, I can surely add it to my light

I think yellow's point was that mineral oil sucks for cooling. :thumbsdow

Dunking your computer parts in oil makes sense only for the novelty/shock value. If you want good cooling, aluminum heat sinks + decent airflow are 10x-100x better.

But...steampunk is all about appearance. If you want to make this really steampunk, you could pump a low-boiling-point refrigerant (CFC, ammonia, etc) thru your heatsinks, and condense it in a glass globe where everyone can see the "steam" vapors. :thumbsup:

More pictures as you build it, pleez!

-Jeff

 

[bshanahan14rulz]

CCFL in the frosted tubes!!!


BTW, you aren't using off-brand luxeons, you are using name-brand Crees. Win!

 

[unbwogable]

Well, I finally powered up the Cree's (thanks bshanahan14rulz), and they aren't nearly bright enough. So, I've switched to a dimmable compact fluorescent, and it looks great. I'll post pictures once I actually install it. Thank you everyone for your input!!

 

[Linger]

interesting you didn't find them bright enough. At a guess, not enough current per emitter.
I say that b/c Cree is an industry leader. If you have a spare one, try it connected to a single lithium primary (3.0v) or 3x AA's in series. (basically, give it 3.5v. Wear sunglasses, and make sure it's pressed to a piece of metal (heatsink) and only test it briefly. (alternatively, if you can isolate one on your globe, hit it with 3.5v (1000ma), it should really sing for you.