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30 LED blacklight

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EEYORE

Tue Jul 25 2006, 08:43PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Ok, so i just got 30 UV LEDs on ebay...While i know these arent the best quality (30 for 3$) they do infact cause stuff to flouresce nicely. They are 1000mcd units...Cause my Autunite to glow brightly, as well as my uranium glass vacuum tubes...

So here is what I want to do. While 1 is nice, I would like to use all 30 of them. I have heard paralleling LEDs is bad, but since 30 in series would require a high voltage, I am stumped. Any ideas?If possible, Id like it to portable (able to run on batteries). Im thinking either rectified mains/dropping resistor, or some kinda boost converter?
Each LED is rated at 3.6volts, and im guessing 25mA.
Thanks!
Matt

Marko

Tue Jul 25 2006, 08:59PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I don't think paralleling is that bad, as 30 led's isn't really some huge number.

Bigger problem would be delivering enough current from ordinary batteries (3/4 amps!) they would work but loose brightness quickly as battery dies, and then various other effects would show, led's would stop to be equal in brightness and etc.

ragnar

Tue Jul 25 2006, 09:05PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

If you use separate dropping resistors for each LED and run them at 20mA (or even 15), they should parallel nicely. I'm not sure rectified mains / dropping resistor is the best idea, use a transformer, regulator, and a dropping resistor if you're running off the mains.

If you use rechargeable cells, three in series, you'll get 3.6V, but it's still advisable to use a resistor of some description. Running off alkalines at 4.5V, you calculate your dropping resistor for each LED as such:

(Vin - Vdesired) / (currentdraw)

e.g. one LED (4.5V - 3.6V)/0.02A = 45 ohms. Remember that the discharge curves of alkaline cells suck, and rechargeable batteries have fairly nice, flat discharge curves.

EEYORE

Tue Jul 25 2006, 09:07PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Firkragg wrote ...

I don't think paralleling is that bad, as 30 led's isn't really some huge number.

Bigger problem would be delivering enough current from ordinary batteries (3/4 amps!) they would work but loose brightness quickly as battery dies, and then various other effects would show, led's would stop to be equal in brightness and etc.

How about this idea...3 LEDs per string, and 10 strings total, all paralleled. This will let me use 12 volts, no?Sorta MMC style

"Hurray tesla!"
Matt

dan

Tue Jul 25 2006, 09:08PM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

You can parallel them you just need to get 30 220ohm-ish resistors. Put one resistor in series with each of the LEDs.. Then parallel all the resistor&LED strings.

I have made a large 321 blue LED array with 107 three LED strings and only used a 2ohm 20W resistor on the supply to limit the current. I have since had to replace a few of the weaker led's that fried but it does work well off of 12v and draws ~4A from a car battery. I did it that way because I didn't want to solder 321 resistors but I will probably recover the LEDs from the board and rebuild it the correct way one of these days.

p.s. If they are the cheap ones from some seller on ebay from hongkong do expect to loose a couple over time.. they aren't all made equal and there will be some that turn out to be duds. I find about 1 in 1000 dies after an hour or so.. an 1 in 100 after 10 hours. But what do ya expect for $0.01/led

rupidust

Tue Jul 25 2006, 09:30PM

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Registered Member #110 Joined: Fri Feb 10 2006, 12:23AM
Location: Banned City
Posts: 85

Decide on a working voltage, 5, 12, 24 and work from there on. If you decide 12v, use correct amount of Leds in series just under 12v and a low value current limiter resistor so that watts will not be wasted in the current limiter. Then make parallel strings of series leds.

To hell with what is heard. Have you had any problems yourself paralleling LEDs? The main issue is the effect of heat on Leds in close proximity for long periods of time. (30) 100 milliWatt Leds is 3 watts rather it be pure 30 in parallel or combinations there of. 3 watts is nothing. And of course using 30 current limiters is questionable because at 5v the limiter may waste 35 milliWatts each totaling 1.0 watts of heat but thats 1 watt over a large surface area; therefore making no difference for a one time non production product. Parallel strings of 12v (10.8v - 1.2v current limiter) may only waste 30 milliWatts per 12v string. There in lies the rational for using high voltage strings.

Paralleling large numbers of resistors with only ONE (1) current limiter may cause a problem because ONE limiter will not compensate if one or two leds were to not function causing the remaining leds to increase in current. This is not bad for large numbers because the increase is distributed across the remaining leds. If you were to have 5+ leds die on you, then you got other problems.

1000mcd are relatively weak comparted to common 5,000 and 20,000 these days. You might want to try some 5,000mcd Leds simply because the Led count will drop about 1/5 the total.

Simon

Wed Jul 26 2006, 01:47AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

When people say paralleling LEDs is bad they mean directly parallel. Making an LED resistor circuit that lights up and putting it in parallel with another is fine.

Putting LEDs in parallel directly is problematic because they are very much non-ohmic in a drifting way. They don't have fixed voltage drop per unit current relationship, so there's no way you can expect the same current to flow through the two different LEDs.

EEYORE

Wed Jul 26 2006, 01:54AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Well im done with it...Works really good!So bright though...I can see stuff florescing all around, but alot is drowned out by the visible part. Any way to filter out the visible spectrum?

Matt

Bjørn

Wed Jul 26 2006, 05:42AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

1000mcd are relatively weak comparted to common 5,000

Those numbers says nothing about the amount of light it outputs so it is a fairly useless comparison unless you have other information,

Conundrum

Wed Jul 26 2006, 09:43PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

I've had failures in el cheapo white LED's. Unfortunately it caused overcurrent in the remaining diodes causing them to also fry :(

repaired the torch as best i could (the LED's cost over Â£40) and so far it looks OK.
But I am going to add a failsafe and also a temp activated fan to keep things cool.

-A

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