# Brighter LED's = REALLY HOT Resistors.



## niblique71 (Dec 2, 2009)

To the wonderful brainiacs in this forum.

I'm working with some new and far brighter LED's from a different manufacturer. I did the Calculations and (I believe) have the correct wattage and Ohm resistors, but they are getting hot. REALLY Hot!! Not hot enough to melt PVC but Close. Since these are much brighter LED's, I just wanted to use one per circuit. But that could be the problem. I noticed that the resistor wattage calculations went down as I calculated 2 per circuit and then 3 per circuit. The Ohms also went down with the required wattage.


Here are the Values
10mm LED
12v power supply
forward voltage 300ma
voltage drop is 3.4

The calculator says a 2W 33ohm resistor. 
I first tried it with a 3W 39ohm resistor

Both got hot as heck and the 3W actually discolored to a crispy brown over 3 hours.

So the question is, would I be better off using 2 or 3 per circuit?? I want to enclose them as I have done in other fixtures. I use LED's because the DON:T get hot... So this perfect setup I was hoping for (1 superbright LED per spotlight) is becomming less desireable

Did I do something wrong?? I've been building LED for years using the 5mm and never experianced anything like this.


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## David_AVD (Nov 9, 2012)

Using a lower voltage to start with is one solution. The dissipation (voltage drop x current draw) is going to be high with your circuit no matter what you do. Using several resistors to spread the heat out may be feasible for you.

A proper (switching type) constant current driver would result in much less wasted energy if you really need to use one LED from a 12V supply.


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## niblique71 (Dec 2, 2009)

UPDATE: The 2W 33ohm (The correct one) is Hot but tolorable when out in open air. They both got way overheated when I placed them in a small cpvc tube which is what I was hoping to do. I guess I won't be able to make the fixtures as compact as I wanted unless I move the resistor to another location outside of the LED chamber.

Is there any reason I couldn't put the resistor at the other end of the circuit??

Is there a better resistor to use that might not get to hot??

The LED itself is working just fine after 2 hours and is Not hot at all.


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## David_AVD (Nov 9, 2012)

You can't avoid the heat with a simple linear circuit like that. There has to be heat dissipated somewhere. You can move it, share it over several (lower value) resistors in series, etc, but you can't eliminate it.


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## niblique71 (Dec 2, 2009)

David_AVD said:


> Using a lower voltage to start with is one solution. The dissipation (voltage drop x current draw) is going to be high with your circuit no matter what you do. Using several resistors to spread the heat out may be feasible for you.
> 
> A proper (switching type) constant current driver would result in much less wasted energy if you really need to use one LED from a 12V supply.


I bought some Disc type LED's 3w and 5W last year and used those types of drivers, but the LED's themselves got REALLY hot. Bummer.

The 10mm LEDs I purchased are SOOO Much brighter compared to my originals from 2 years ago. I don't mind using 2 or 3 of the new ones per circuit if you think that will result in more desireable heat dissapation. I am trying to achieve various levels of brightness throughout the haunt. So having a few eye blistering spotlights with 3 of the 10mm's won't be bad...for the few that I want to make. So would that help the heat dissapatoin??

I also bought a bunch of the newer 5mm's from the same manufacturer. They are still 3X the brightness of the asia enginner ones and I plan on using them in the traditional 3 per circuit setup. I doubt I will have heat issues with those resistors, at least I hope Not.

Thanks for the info...


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## Otaku (Dec 3, 2004)

I agree with David_AVD, you need to reduce the input voltage. Batteries may not be the best solution due to the high current draw. I had that problem with using a pair of 15K mcd 10mm LEDs in a single spotlight fixture, they drained a 9 volt battery in about an hour.


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## niblique71 (Dec 2, 2009)

Here's my setup and why I hope I can keep 12V.

I use a computer power supply at 12V and have 3 legs of 12ga landscape lighting wire, a total of 700 feet worth, to feed all of my lights and there are already 170(ish)of 'em wired for 12v. 
So

Would it help for heat to use 3 LED's per circuit?? The calculator dropped the wattage of the resistor to 1 watt when I calculated with 3. 

OR

Is there a cheap and simple circuit I can make to reduce the voltage in each fixture? I have a few LM7805s. Or I can buy some really cheap drivers from one of the overseas merchants.

or

Since the Resistor seems to be "OK" in the ambient air, can I put the resistor in line in the wire with perhaps in a separate enclosure?? It feels like I can go back to my original size 1" tubing or [perhaps larger just to give it some room to heat up. It only got SUPER HOT when I placed it in a 1/2" CPVC tube. As I mentioned I was just trying to get some ultra sleek and super bright LED's. I don't mind giving up the "Ultra Sleek" part if I can Not critically overheat the circuit.

BTW... Thanks for helping everyone... I am learning ....slowly LOL


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## David_AVD (Nov 9, 2012)

Any linear means (7805, resistor, etc) you use to reduce the voltage from 12V to 3V is going to generate the same amount of heat. That's just the way it is.

The key to your problem is minimising the difference between the supply voltage and the LED forward voltage. Putting 2 or 3 LEDs in series will certainly reduce the heat in the dropping resistor.

You could use 3 LEDs (in series) and reduce the LED current (increase resistor value) so that the combined light output is more what you want. This will also reduce the resistor heat as a side benefit.


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## niblique71 (Dec 2, 2009)

Thank you David and Gary, I had a feeling that using 3 in series could be a possible solution. Now I have to recalculate and buy some new resistors for 3X. I still want to test a few single 10mm's in larger enclosures with a heat sink I've been thinking about. I am aware that the energy has to go somewhere. there aren't any shortcuts with energy. As I mentioned earlier the main issue was that I can't fit a single LED into a small tube now because of the heat. But 3 in a larger enclosure would more closely resemble a true spotlight and could ultimately be a better solution anyway.

Thanks for your patience. I'm sure I'll have more questions.... 


Like... Should I get a 2 Watt resistor when they call for a 1 Watt?? or would that hold more heat and not dissapate it efficiently enough?? Or would the Greater mass dissapate more heat??

"Geerz this Niblique guy is a PIA!!"


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## David_AVD (Nov 9, 2012)

Zombie got ya tongue? lol


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## pshort (May 6, 2008)

There are switching-type regulators in the same package as a 7805 (I've forgotten the part number). Using them would reduce the heat involved in reducing the voltage from 12V down to 5V, although the heat in reducing the voltage from 5V down to 3.4V would still remain. They're quite a bit more expensive than a generic 7805 regulator. Still, that removes perhaps 75% of the excess heat.


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## corey872 (Jan 10, 2010)

You mentioned a computer power supply? That should have a 5V rail as well- and would be a much better match for your 3.4V LED. Then you would need a 1W, 5.6 ohm resistor. Computer supplies also have a 3.3V rail - which would be ideal, if you could stand just a slight drop in brightness, you wouldn't even need a resistor. (though I have seen some supplies actually push almost 3.4V on that rail.

The resistor can really go anywhere, so if you want to leave it exposed somewhere along the line - even right at the power supply, you should be OK.

Is there any possibility your original 'hot parts' have been mislabeled somehow? The resistors should be at least 3W, not sure why a calculator would recommend 2W - but then you say the 3W resistor was even hotter than the 2W


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## David_AVD (Nov 9, 2012)

corey872 said:


> Computer supplies also have a 3.3V rail - which would be ideal, if you could stand just a slight drop in brightness, you wouldn't even need a resistor.


Using an LED with a "stiff" supply rail without a current limiting resistor is not recommended. The tiniest of voltage fluctuations can see the brightness vary a lot or if too high, kill the LED in short time.


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## HomeyDaClown (Oct 3, 2009)

It's just in the math. You always want to use a larger wattage resistor than calculated to keep things cool. LED calulators are a good tool but assume that the resistor can easily dissipate the calculated wattage without excess heat. In any case the calculated wattage is heat that has to be dealt with in any voltage dropping situation.

One led: 
12 volts in and 3.4 volts out = 8.6 volts drop across the resistor and the power dissipated will be 8.6 x .3 amps = 2.58 watts (why your original resistor setup got so hot) a 5 watt resistor would have done the trick with just getting warm to the touch.

Three leds in series:
12 volts in and (3 x 3.4) or 10.2 volts out = 1.8 volt drop across the resistor and the power dissipated will be 1.8 x .3 amps = .54 watts (a one or two watt resistor will be fine)

I take the voltage drop across the resistor and mutilply that by the current going though it as a starting point for the wattage value, then double it and find the next higher wattage resistor to do the job.


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## hpropman (Jul 27, 2008)

Sorry I am late to the party but Homey beat me to the answer. The math never lies when in doubt go with a larger wattage resistor it will not get as hot.


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## niblique71 (Dec 2, 2009)

HomeyDaClown said:


> It's just in the math. You always want to use a larger wattage resistor than calculated to keep things cool. LED calulators are a good tool but assume that the resistor can easily dissipate the calculated wattage without excess heat. In any case the calculated wattage is heat that has to be dealt with in any voltage dropping situation.
> 
> One led:
> 12 volts in and 3.4 volts out = 8.6 volts drop across the resistor and the power dissipated will be 8.6 x .3 amps = 2.58 watts (why your original resistor setup got so hot) a 5 watt resistor would have done the trick with just getting warm to the touch.
> ...


Before I comment on the quote above, Thank ALL of you for your help. Every reply was insightful and very helpful. I'm so glad I can count on my forum friends to help me as I help others in areas that I am familiar with.

With that, Homey's description above cause my brain to go CLUNK

In all of the descriptions of what goes on mathmatically in an LED Circuit. This was the first time it was ever explained with such clarity and simplicity as to what is actually happens in an LED circuit. For the first time it all made sense.

I never fully 100% knew what forward voltage was... Till now. Now It's so clear.

So as a result Voltage drop makes sense. The use of resistors make sense and the Fact that 3 in series run cooler and more efficiently also makes sense.

Wow,,, Sometimes ya just never know when the light bulb (No pun intended) in the brain will go on. Major epiphany(Sp) for me. THANK YOU for you insightful description.

On a side Note, I found a bunch of 5 Watters online and ordered them. But Now I realize that running a single LED at that milliamp rating is very inefficient. I might have to create an interesting solution.


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## hpropman (Jul 27, 2008)

Greg,

What about runnign 3 leds in series with one resistor but with each led in its own tube. In other words run one of the power wires through the resistor and then through 3 light fixtures each with on led inside. It is the same circuit as the ones we made in the bottle caps just with more distance between the leds and the resistor. Or you can make a 3 led spot using the larger tube like the 1 1/2 PVC leds spots that I make (see the link below) you can fit 3 10 millimeter leds in there easy.

http://www.hauntforum.com/showthread.php?t=30656&highlight=multi+led+spots


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## niblique71 (Dec 2, 2009)

niblique71 said:


> Now I realize that running a single LED at that milliamp rating is very inefficient. I might have to create an interesting solution.





hpropman said:


> Greg,
> 
> What about runnign 3 leds in series with one resistor but with each led in its own tube. In other words run one of the power wires through the resistor and then through 3 light fixtures each with on led inside. It is the same circuit as the ones we made in the bottle caps just with more distance between the leds and the resistor. Or you can make a 3 led spot using the larger tube like the 1 1/2 PVC leds spots that I make (see the link below) you can fit 3 10 millimeter leds in there easy.
> 
> http://www.hauntforum.com/showthread.php?t=30656&highlight=multi+led+spots


ROFLOL You are an Uncanny person and we think very much alike. That was my "Interesting" Solution. To get the resistor away from the 3 spotlights to obtain a few super slim and super high power LED's in groups of 3. It's a "Gangly solution" but for certain circumstances, it would work just fine.
You and I Really do need to get together for a Thinktank Session. Whenever you want to Play in the Picaxe sandbox, I'm ready willing and able.


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## dustin2471 (Oct 27, 2009)

wanting to subscribe to this post, after reading the tutorial (Awesome by the way!) I kow I am going to need to reference this post.

Thank You!


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## imax (May 22, 2006)

With those high-current LED's, you really need a special circuit to drive those little buggers.

You will need a power supply that can handle the load and manage the heat. Here's an example product that will help get you in the right direction:

http://www.ledsupply.com/03021-d-i-1000.php

Note that there are differences in these drivers based on your voltage input requirements, and often they can drive more than one LED so don't let the sticker shock get ya - it may be more feasible than you initially think. There's also subtle differences in the technology used (Buck vs Boost) in these things, so take care when selecting.

Also, high wattage LED's almost ALWAYS need some sort of heat sink to keep them from getting too hot an just unsoldering themselves. If you have an exact model of your LED's or a datasheet, that would be a good place to start figuring out all the details.

-- I


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