# milliamp overdrive decision



## consorts (Apr 23, 2011)

I have this LED;
http://www.eos-led.com/en/Standard-LEDs/Cree-5mm-LED-C503C-WAN-CBADA151-Cool-White.html
power supply is 12volt DC, Diode forward voltage 3.2-4.0, 20-100ma range
this LED will be under water that is 24c all the time (LED won't heat up water),
shrink wrapped in silicon, so LED heat can't damage jacket or connections.

So I want to overdrive this LED, but don't have experience on how hard to push it.
I know more power will give more light and heat, but decrease the LED's life.
I'm OK with that, so if I go from 100,000 hours to even 10,000 hours - that's fine.

so now I have to pick what resistor to use (at Radio Shack) the choices are;
20ma 470ohm 0.5w
30ma 330ohm 0.5w
60ma 100ohm 1.0w

I already tried 470ohm, and while the bulb runs cool, the light it gives is lame,
and I don't think 10ma more is going to be noticeable.
so if I crank it up to 60ma (on a 20-100ma rated LED) what penalty will I pay
in terms of heat generation and life expectancy?
for instance, I'm fine if it's around 50 Celsius (same as hot tap water)
and the LED loses over half it's life expectancy.

so what say you experienced LED devils from overdrive hell?

*oh, one important question;
which resistor will generate less Heat while limiting my 12v power source;
100ohm 1.0w
1,000ohm 1.0w*


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## consorts (Apr 23, 2011)

here's a picture of the 1.0w left and 0.5w right assembly.
(notice the left silicone jacket contains a fatter resistor)
the left 1.0w LED is warmer and the 1.0w resistor is hot,
hot as in around what I guesstimate to be 50 Celsius.

Radio Shack does not have 1.5 or 2.0w resistors,
so should I be satisfied with 1.0w? or find a higher
wattage resistor on ebaY to push this LED furthur?


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## Walterk (Apr 23, 2011)

I feel there are more options for resistor values, when you combine them in series and/or parallel. (see Wiki)
Can't help you, just wondering what are you building that you like to go into this much engineering for this underwater led?


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## consorts (Apr 23, 2011)

Hi Walterk, thanks for showing an interest. I wish more members here would chime in.
I'm trying to light a water fountain feature from within the nozzle itself, so the light
travels and bends with the water. that's why I must provide the most light in the
narrowest assembly possible as the radius of the nozzel pipe is under 15mm and 
needs room for the water to run past. 3mm are too dim, and SMD's are too fat, so 
5mm is the size I'm working with, and that bullet shape bulb makes it easier for me
to keep it waterproof. I know there are higher >40,000mcd 5mm LED's on ebaY,
but I don't trust those Asian seller claims, and would rather overdrive a CREE where
I can see the specs independently, not trust the word of an overseas LED reseller.
However, if anyone has a 5mm suggestion better than my C503C-WAN-CBADA151
I would be open to suggetions. I would have gotten the C503*D*'s but Digikey does
not sell single quantities of them. I ran a heat test on my 1w assembly by leaving
it a few hours in a cup of warm water - it stayed room temperature, so obviously
when I submerge this assembly, additional heat won't be an issue.


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## Lynx_Arc (Apr 23, 2011)

5mm LEDs heatsink through the leads and even watercooling them won't get rid of the heat internally very much. The design is not good for overdriving at all but they will take overdrive. I am thinking the 100ma max is including about all the overdrive they can handle and I am guessing that the returns for overdriving them even above 40ma could be not worth the long term output of them, they may dim fast at even 60-80ma which would have you replacing them every few months for perhaps 10-20% more output. so if you have a 12v power source at perhaps 8.2v to drop (~3.8v to LED) that would need about a 200 ohm resistor 1/2 watt or two 400 ohm 1/4 watt in parallel.


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## consorts (Apr 23, 2011)

Lynx_Arc said:


> 5mm LEDs heatsink through the leads...



Lynx, thank you very much for considering my application.
I agree a 1/2 watt resistor would be appropriate, but as
you can see from the photo above, I'm tempted to push
this LED even if it shortens it's life.

Your comment though, has inspired some other thoughts as follows;
on the 1w assembly, the fact that the Resistor is hotter than the LED
may not be due in an inappropriete resistor, rather that it doubles as
the headsink from the LED leads, so the heat's origin is actually the LED.

if that were the case, then some sort of narrow copper tube may help
heat sink such a small assembly, or perhaps I should just fill the tube
this assembly is in with thermal grease so the silicon jacket itself can
more evenly disburse the heat to all that cold water rushing by. does
such a narrow heatsink exist - basically something that can jacket a
thin wire, where the wire is hot and the jacket helps radiate the heat.
I know copper comes in tinfoil like sheets - should I try applying that,
or with 1-2mm of cavity space, perhaps the grease itself will suffice.

_I am aware that the thermal grease would have to be ceramic/silicone
based, not anything with metal/carbon, or it will conduct electricity too._


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## JohnR66 (Apr 23, 2011)

I did test the Cree C503C series white LEDs. At 30 Ma they lasted around 4,500 hours until intensity dropped to 70%
http://www.candlepowerforums.com/vb/showthread.php?264326-5mm-LED-fade-testing-continued
Driving them harder, say 40ma or above will degrade the phosphors and epoxy above the die and the LED may last no more than a few hundred hours. These LEDs along with the Nichia GS are the brightest single die 5mm LEDs I'm aware of.

You are correct to be weary of the ebay cheapos. The so called 55,000 mcd "super bright" LEDs rated 100,000 life, rated only 15,000 mcd on my meter and fell below 70% of initial output in less than 100 hours. JUNK!!!

If longevity is needed, pushing 5mm won't get you there. You need to look into higher power LEDs and steer clear of the junk.


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## consorts (Apr 23, 2011)

JohnR66 said:


> At 30 Ma they lasted around 4,500 hours until intensity dropped to 70%



excellent work John, thank you so much for sharing your findings with me
so it would seem using a Radio Shack 30ma *330ohm 0.5w* may be as hard
as I should be pushing this LED. I'm glad my ebaY instincts were correct,
and confirmation that these CREEs are as bright a 5mm form available.

I'm still tempted to push my luck with a 100 or 1000 ohm 1watt resistor,
so tell me, given my 5mm tube housing the wire posts and resistor, what
can I do that will dissipate more heat in order to postpone LED degradation.
keep in mind the assembly will already be cooled by 24 Celsius under water.
or does the current itself degrade the LED, not the heat such current creates.
I assume your tests were done with no special heat sink, just LEDs in the air.


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## Lynx_Arc (Apr 23, 2011)

consorts said:


> excellent work John, thank you so much for sharing your findings with me
> so it would seem using a Radio Shack 30ma *330ohm 0.5w* may be as hard
> as I should be pushing this LED. I'm glad my ebaY instincts were correct,
> and confirmation that these CREEs are as bright a 5mm form available.
> ...


 
not a bunch you can do to heatsink 5mm LEDs other than connect them to larger copper wire. A larger resistor would dissapate more heat easier or locating it a distance from the LED so no heat would interfere with it. 
Personally I would recommend something like a cree XP-G heatsinked with an optic to get the light dispersion you want from it. You could run it from 20ma to 1 amp easily with proper heatsinking and perhaps with a proper driver you may even be able to put an adjustable resistor so you can crank the light up/down as desired.


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## consorts (Apr 23, 2011)

Lynx_Arc said:


> to heatsink 5mm LEDs connect them to larger copper wire.
> A larger resistor would dissapate more heat easier or
> locating it a distance from the LED so no heat would interfere with it.



well, thus far I did in fact locate the resistor an inch away from the LED to keep their heat dissipation fields further apart. the idea of using a thicker wire gives me an idea... I'm using a stranded 22 gauge wire since that's all I need to deliver the current while snaking it through a 4mm tube, however if I cut a few inches of solid 12 gauge wire and applied it between the connections of the 22 gauge wire and the negative LED post, and the positive LED post to the resistor and the positive leg of the 12 volt power wire, then maybe that extra copper can help heatsink and dissipate the LED and capacitor heat where it will be most intense. it will be a tricky soldering job, but certainly worth pursuing. anyone care to comment on using thermal paste to move some of the heat from the internal assembly components to the surface of the silicone jacket where the cold water will carry the heat away?


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## Lynx_Arc (Apr 24, 2011)

consorts said:


> well, thus far I did in fact locate the resistor an inch away from the LED to keep their heat dissipation fields further apart. the idea of using a thicker wire gives me an idea... I'm using a stranded 22 gauge wire since that's all I need to deliver the current while snaking it through a 4mm tube, however if I cut a few inches of solid 12 gauge wire and applied it between the connections of the 22 gauge wire and the negative LED post, and the positive LED post to the resistor and the positive leg of the 12 volt power wire, then maybe that extra copper can help heatsink and dissipate the LED and capacitor heat where it will be most intense. it will be a tricky soldering job, but certainly worth pursuing. anyone care to comment on using thermal paste to move some of the heat from the internal assembly components to the surface of the silicone jacket where the cold water will carry the heat away?


 
If you get anything that creates heat far enough away from the LED and have either a circuit board or thick enough wire connected to the LED trying to do more is a waste of time as the leads of the LED itself do not have a lot of surface area to pull excessive heat from overdriving away from the LED itself. It is like wrapping yourself in a plastic bag and putting a fan on your feet to cool you off. If you have a fever it just won't do a lot to help you (overdriving).


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## Walterk (Apr 24, 2011)

Have you read http://www.candlepowerforums.com/vb/showthread.php?89607-White-LED-lumen-testing
ou prbably have realized testing should be done with the powersource and cable lengths that you will use finally, so these parameters won t change afterwards.

I think it is easier to build something based on glass-fibre leads to bring light into the tube. Or the synthetic variant.


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## consorts (Apr 24, 2011)

Walterk said:


> glass-fibre leads to bring light into the tube. Or the synthetic variant.



thanks for that link Walterk
I agree if this were some commercial application - fiber optics would be best,
but I'm just an silly DIY'er trying to jazz up my indoor pond/aquarium, that's all.

Lynx - I get your point; the LED post it too thin a heat sink pipe for any more
sinking beyond to matter much - especially If i'm cooking the LED on overdrive,
thus generating more heat than the LED post was designed to ever channel.

*so can anyone comment on using thermal paste inside the tube to help move
more heat 1mm from the wiring to the waterproof jacket. is it worth a bother?
I mean, it can't be any worse than just leaving it an air cavity.*


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## Walterk (Apr 24, 2011)

Even when DIY-jazzing up things then the best solution might be a strand of fiber... 
You are designing micro, while even with our macro leds we can't put figures on things we like to control. 
I would just test to see how it goes, cranking up amps every week and buy some stock of the Leds for spare. 

The heat is in the bubble, and the only way out are the leads. Would it harm the fish to leave the first half inch of the posts of the Led open in the water? 
With that low current I don't see any problem as the water isnt that conductive. (Assuming you don't have a saltwater pond...) 

If there was such an affordable good heat transferring material I like to know so I will follow your quest .
And do a search on thermal transfer of different metals and thermal glue, you will see that bossing up the posts wont help much.


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## consorts (Apr 24, 2011)

Walterk said:


> leave the first half inch of the posts of the Led open in the water?



I suppose if the posts are aluminum, that may be an option worth considering.
I'm not concerned about 1 watt maybe leaking out into a freshwater aquarium.
besides, this tank has too many snails, so it might act like a bug zapper, LOL.

since my assembly will be inside the nozzle, nothing living can get in anyway.
however it will upset the water flow resistance dynamics a bit, which may
effect how the nozzle performs in terms of shaping the out flowing water.

this is the paste I was going to order;
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=260766402387
it's a USA shipper, has no metal so won't conduct electricity,
and the reviews I read on mobo modder forums are favorable.


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## JohnR66 (Apr 24, 2011)

You will need to use the Nichia GS LED. Most LEDs including the Crees use a steel lead frame. Steel is not as good a thermal conductor as copper. The GS uses a copper lead frame. Putting a heat sink as close to the die cup lead (cathode) as possible will be the best. 5mm leds are not ideal for overdrive as the thermal paths are not optimized in any way.

Bad news. Nichia GS LEDs are harder to come by and have an "angry" blue center to the beam. After testing many LEDs I have an unproven throry that it is not only the heat that degrades the white LED. It's the strong light radiation coming from the very small die that degrades the phosphors and epoxy above the die which contributes to lumen loss. Heat is generated from the die and from the conversion losses in the phosphors that generate more heat. the top side of the die has no good thermal path out of the package.

If you use more than 1 LED, it is better to use 3 LEDs in series and drop the rest of the voltage with a resistor. Dropping 8.8 volts in the resistor and the remaining 3.2 in the LED is kind of a waste. 3 LEDs in seres will leave about 2.4 volts to drop in a resistor. 120 Ohm resistor will give you 20ma. If your 12 DC volt supply is not regulated, consider that the voltage may be much higher at low currents. It could be over 16 volts!

Sounds like your needing 1 LED, but not clear.


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## doctaq (Apr 24, 2011)

copper is a bad thing for invertebrates and in high enough doses anything


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## VegasF6 (Apr 24, 2011)

Would tinning the legs with solder at least keep them from corroding when exposed to the water?

There really is no point to whatever you are trying to do with that heatsink paste. Ideally with those leds you don't want to cut the legs, you don't want to encase them in an insulator as they can't work that way, (think of how your body sweats to cool itself. If that sweat can't evaporate it won't work. You are putting your leds in a jacket.) and you don't want your resistor so close. Put it (the resistor) at your power source instead.

Also, going way back to the top, your use of the words 1 watt assembly and talking about "pushing the led" confuses me. Pardon me if I am mistaken, but I have seen this with other poster before. Do you believe the use of a half watt resistor vs a 1 watt or 2 watt is going to affect the amount of current the led sees?


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## consorts (Apr 24, 2011)

VegasF6 said:


> Put it (the resistor) at your power source instead.



another useful idea, thanks




that will help me keep the
tip assembly thinner - too!

sorry I'm such an electronics newbie.
I'm sure all you 1,000 post veterans
must be having a good laugh at this.

instead of debating the merits of each idea,
I'm just going to incorporate them all 
I should have the components later this
week, and will show you what transpires.


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## VegasF6 (Apr 24, 2011)

Everyone starts with 1 post. Many of them know way way more about electronics then I ever will, since I am just self taught. I just wanted to se if you understood the basics, and that the wattage of a resistor has nothing to do with the amount of current your circuit will use (well, essentially anyhow). It is only the resistance (ohmic rating) you are concerned with. And welcome.


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## Oznog (Apr 24, 2011)

I'm not sure how you'd prevent contact with the water. With any conductive path at all, electrolysis will quickly erode one side.

The package itself may not survive water contact indefinitely. The acrylic case itself should be waterproof, but water contamination may creep up the leads.

In short I don't think an immersion plan with work long-term. For kicks, something to show off for awhile and forget about, it'll work ok.


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## Lynx_Arc (Apr 25, 2011)

If the water isn't salt water tinning the LED legs probably would not be necessary.. you could just use some heat shrink on them. Most likely any corrosion would take many years to become an issue and by then you would probably have replaced the LED with a better one or even a higher power assy.


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## consorts (Apr 25, 2011)

VegasF6 said:


> It is only the resistance (ohmic rating) you are concerned with.



Vegas, it took a while to sink in, but I think I get it now - how the resistor wattage above 1watt is pointless,
and it's all in the ohm rating how intense (or how many milliamps) the resistor will allow to flow to the LED.

since I'm using a 12volt source and a maximum 0.1amp LED, I can't use more than 1.2watts of power.

as for what ohm rating delivers what intensity; at the .02amp minimum rating on the LED I would 
need 425ohm since 12volt-3.5volt(of the LED) = 8.5volts then divided by .02amps or 20 milliamp.
using the same formula, the maximum intensity LED light possible, would be a 85ohms resistor

now I see why buying a 120ohm 2w resistor was pointless - it's all in the ohm rating on <1w resistors,
so the 100ohm 1w resistor I'm already playing with, is about as far as I should push this type of LED.

just thinking aloud here, so be patient with my crude math;
_tomorrow I'm going to experiment with a ∞-25ohm 3watt rheostat that Radio Shack sells.
I'll put the 100ohm 1w resistor between the 12v power source and the rheostat, then
use the rheostat to increase/decrease the resistance further to the LED like a variable
resistor, giving me ability to enjoy a "dimmer" on the LED and while that 100ohm resistor
keeps me from blowing it, since 100ohm+25ohm delivers 65ma to the LED, which according
to John will shorten the LED's lifespan, but not melt it, so that could be the short duration
"show off" intensity, but during normal prolonged operation display, I would adjust the 
rheostat to a more comfortable ~400 ohm setting for this LED's preservation. another
approach I could use is a 4-way switch with a ~400, ~200, ~100 ohm resistor on each 
switch post (and not use the 100ohm/1w at the 12v source) but I like the dimmer idea
better than having a "low,medium,high" intensity switch to my LED._

*Guys, do us all a favor and wind up the discussion on exposing the LED posts to Water,
because it's not gonna happen. I've already moved on to other approaches to dissipate 
the heat coming off the LED posts.*


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## VegasF6 (Apr 25, 2011)

Bravo, sounds like you more or less have a grip on figuring resistance now. You can certainly use a pot in series with your main resistor. FYI you can buy a logarithmic taper pot or just a linear. I am assuming the one you are looking at is linear. You will probably find you are using a small portion of the travel of your pot, in it may not feel quite like you want, I.E. just because you are in the middle of the travel, it doesn't mean you will be at middle brightness level. Supposedly a log pot will help correct that, I haven't tried it. Wouldn't mind hearing your results though.

Now, I was going to tell you don't be so concerned about driving the LED at 65 mA, since that is well within the specs you posted in the first post. 
_"Diode forward voltage 3.2-4.0, 20-100ma range"
_But, since then, I took a look at the data sheet. Absolute max continous current for that diode is 30 mA and even that will reduce it's life compared to the suggested 20 mA. 100 mA is the peak rating for pulse driven circuits only. Note: Pulse width ≤0.1 msec, duty ≤1/10. 

Granted if it fails you are only out .56 euro but, I think you really are asking too much of that poor little led. 

And finally, I did want to ask you about your 12V power source. Do you have to do it that way? I am guessing that is also the power source for your pump, and that's why? It just seems a shame to waste so much power in the resistor. Perhaps you would rather have 2 more LEDs somewhere else in your design and a smaller resistor.


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## consorts (Apr 25, 2011)

VegasF6 said:


> I did want to ask you about your 12V power source. Do you have to do it that way? I am guessing that is also the power source for your pump, and that's why? It just seems a shame to waste so much power in the resistor. Perhaps you would rather have 2 more LEDs somewhere else in your design and a smaller resistor.



I join in your concern about energy efficiency, you'll be pleased to know the following;
I'm tapping the 5v/12v power supply line coming out of a nearby personal computer,
so basically when the PC is running anyway, so will this fountain LED. I also have
a cheap 110v:12v 500ma AC/DC adapter I use to power car accessories indoors,
should I ever want to power the fountain LED separately. The reason I tapped
the 12v instead of the 5v line from my PC is because I may add 3 more spotlight
LEDs to form a sort of tripod of light to further enhance the fountain display, but
I'll wait first to see how well the water is illuminated from within the nozzle alone.
This summer I may also be adding 12v fans to evaporative cool the tank water.
It is already powering a 30 SMD strip light bar under my computer desk hutch.
Oh, and to be even more energy efficient, the fountain is not running off a pump,
rather it's the water outlet for the aquarium canister filter, so it has to run anyway.










you see the trick to this display is to illuminate the water without the lights 
being much of a glaring distraction. if I simply wanted to light up the water,
I could have spent $9 on a waterproof umbrella light and called it a day, 
but what self respecting DIY'er would sell out to the Chinese like that...


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## Walterk (Apr 26, 2011)

There are lights in faucets and showerheads.
Don't know how it looks in real.
Show some pics once youve got some set-up running ?


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