# LED Amperage handling.



## X-CalBR8 (Jun 3, 2001)

Just a quick question. Does anyone happen to know about how much amperage an average Nichia LED can handle at it's rated voltage of 3.6volts? Are most all white LEDs about the same when it comes to amperage handling or do they vary greatly? I'm concerned about an experiment that I will be working on soon that might send too much amperage to the LED even though the voltage would be within limits.


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## Chris M. (Jun 4, 2001)

Is it me or is the board screwy today?

Anyway.... an everyday white LED, if fed a constant voltage of 3.6 volts, would take about 20-25mA. 

But if you wanted to increase the current flowing through the LED, the voltage accross it would also go up. LEDs are essentially current driven devices. If you hook one up to a constant current laboratory power supply, the voltage accross the LEDs pins will vary according to the strength of that current. However in a similar way, if you hook one to a constant voltage power supply the current it draws will vary according to the strength of the voltage. The trouble you will find there though is that as the LED heats up, it will draw more current. More current means more power dissipated in it, means more heat, more current, yet more heat.....and hence thermal runaway which eventually leads to its death. Constant current supplies however, do not lead to thermal runaway. As the LED heats, the voltage accross it will drop a bit but since the current it draws is constant, it won`t draw more power and keep on heating up. In fact the power will go down since power equals voltage multiplied by current. Volts goes down, and so does the power. 


So anyway, in answer to your question, you could probably push a Nichia LED to 40 or 50ma with enough cooling, but it will not be 3.6 volts at its terminals, it will be nearer 4 volts. The two are related.


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## X-CalBR8 (Jun 4, 2001)

Chris M.: Thanks for taking the time to help me out with this little brain boggler. I'm just not 100% sure that I understand what would happen under the circumstances that I would be placing the LED. So if I were to use Zener Diodes to build a power supply that had an exact voltage of 3.6 volts but a very high current, would the LED only draw as much of that current as it needs or would it draw all that was available and burn itself out? Sorry to be so troublesome, but I hate to blow a pricey LED just by experimenting to find out what will happen, when others probably have already made this mistake, and I could learn from their mistake and not make it again myself. Thanks again for any and all help.


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## Chris M. (Jun 4, 2001)

_So if I were to use Zener Diodes to build a power supply that had an exact voltage of 3.6 volts but a very high current, would the LED only draw as much of that current as it needs or would it draw all that was available and burn itself out? _

Well, if the voltage was set at exactly 3.6 volts then the LED would pull only what it wanted. I`m not sure what you mean by high current? Do you mean a high avaliable current? I have a couple of laboratory type power supplies here that _can_ deliver up to 5 amps at whatever preset constant voltage. But if I set it to 3.6 volts and hook a white LED to it, the LED will only pull about 20mA. The fact that the power supply _can_ deliver 5 amps doesn`t necessarily mean it _will_. It just means that if I wanted to hook a ton of LEDs to it in paralell, I could do and it could sustain the current- all those LEDs x 20mA, or so, each. The voltage can stay at 3.6 volts. If I tried to pull more than 5 amps the voltage would reduce to keep the total drain at 5 amps.


I`m guessing this is for your super-capacitor light, right? Capacitors can deliver hundreds of amps in a short circuit, but hook a 3.6 volt LED to a cap charged to 3.6 volts and it will not dump hundreds of amps through that LED vapourising it. It`ll be OK. Keep the voltage steady and the LED should be OK. Don`t overdrive it from a constant voltage source though or you`ll get that thermal runaway like I mentioned before, and cook it. 3.6 volts should be fine, since it`s within its specified operationg voltage / corresponding current it won`t overheat.


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## The_LED_Museum (Jun 4, 2001)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by X-CalBR8:
*Chris M.: Thanks for taking the time to help me out with this little brain boggler. I'm just not 100% sure that I understand what would happen under the circumstances that I would be placing the LED. So if I were to use Zener Diodes to build a power supply that had an exact voltage of 3.6 volts but a very high current*<HR></BLOCKQUOTE>

If your power source were to never exceed 3.6 volts, regardless of how much current it can deliver, the LED current will never exceed 20-25mA.
You can put an LED directly across the terminals of some electric welders and they just barely glow!





However, if you put the same LED across an itty bitty 9v transistor radio battery, it will always die, often with a loud bang and a puff of foul smelling smoke.


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## X-CalBR8 (Jun 4, 2001)

Thanks Chris and Craig for the helping hand. After pondering your answers and pontificating on Ohm's law for a good long while (almost blew a brain fuse while doing that. LOL) I think I finally understand what you guys have been trying to explain. The amperage is strictly controlled by the amount of voltage and resistance. From the figures you guys supplied, I used trusty Ohms Law to calculate what the resistance of a Nichia LED must be. If at 3.6 volts, a LED pulls .020-.025 Amps, then using Ohm's Law R=E/I (R=resistance, E=Volts, I=Amps) this comes to R=3.6V/.020A-.025A = 144-180 Ohms. 

Was that cool or what?



I had been trying to figure out what the resistance of a Nichia white LED must be and then it hit me that Ohm's Law held the answer all along. So now that I know what the resistance of the LED is I should be able to figure out how much amperage it will pull at different voltages. There must be more to this though because at 5 volts I figure an amperage draw of 27-35ma (I=E/R) and I'm almost sure it must be higher than that. At 6 volts it would come to 33-41ma. That can't be right, or why would LEDCORP mess with putting a resistor in their 6-volt LED bulbs? The resistance must vary inside the LED depending on heat or amount of voltage applied or something else because there would be no danger of running a LED at 33-41ma would there? So why the resistor in the 6 volt LEDCORP bulb and what size resistor is it? Do they only use the resistor to make the light burn longer or is it there to protect the LED itself? Has anyone ever put 6 volts to a Nichia white LED and measured the actual MA draw so that I could see if it matches up with my calculation of what it theoretically should be? Here we are at another conundrum again. Any ideas on this one?


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## napalm-2002 (Jun 5, 2001)

i also wonder about what xcalbr8 says.will an led pull only the amperage that it needs or will it pull all that it can and burn out.

its all very rudamentary but ive been away from the theory for a long time.


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## Go Go Gadget Flashlight (Jun 5, 2001)

Quick response, got to get to sleep.

LEDs aren't governed by a linear response. 

Your calc is right for 3.6v at 20ma. But an LED is a semiconductor, not a resistor, right? It's current demand goes up exponentially with voltage I believe.

6 volts would definitely fry an LED, and it would happen at much higher current than stated.


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## X-CalBR8 (Jun 5, 2001)

Gadget: That's what I was afraid of. So how big of a resistor does a person know to put on a LED at a certain voltage without just using purely trial and error and possibly frying a LED or 2 in the process? For example, how did LEDCORP know what size resistor to put in their 6-Volt LED bulbs? Btw, has anyone taken apart one of these to see what size resistor that is? Also, does a LED's voltage change according to the total available amperage? Like say the difference between a Photon overdriving a LED and a larger battery overdriving it. The small litiums won't burn it out at a certain overdriven voltage but the larger battery with the same voltage would because it has more available amperage. How exactly does that work? Any ideas on these new issues? Again, thanks in advance for any help figuring these things out.


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## Badbeams3 (Jun 5, 2001)

This is beyond my level but I`ll try...2 2016`s cannot maintain 6 volts at say...50ma for more than a nano second...it drop off instantly to say...4.8 volts...and within a few minutes drops further...where it holds steady for a while.

Four D size batt would drop only slightly with a 50ma load...say 5.8 volts...and stays there.

You could take a 4.5 volt submarine mega amp batt (if there was such an animal) and run the led with no problem.

But a little 9 volt batt will ruin it.

It is more voltage the ruins an led

Of course an led`s draw changes with voltage...at three volt it draws very little maybe...5ma...at 4.5...maybe 50ma...at 6 maybe 120ma...and your headed for trouble.

A led will only draw so many amps at any given voltage...whether it`s 4.5 volts from D batts or 4.5 volts from the mega sub batt...does not matter...the amp draw will be the same.

A good heat sink will allow you to run higher volts as it is the heat that really ruins the led...but of course there is a limit as to what you can do...if you could keep the internal cup cold with nitrogen you might be able to run 12 volts...bright as hell. 

Our member have found that it is better to run 2 led`s at 40ma than one at 80ma...more light...the increase from 40ma to 80ma in a single led does not double the light output...but running 2 led`s at 40 each does...well to some extent anyway.

At least this is how I have come to understand it from reading these forums...I really should let others answer this...I really don`t know for a fact.


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## Go Go Gadget Flashlight (Jun 5, 2001)

What Chris said.





Well put.


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## X-CalBR8 (Jun 5, 2001)

Thanks for the quick replies. Someday I hope to be a LED expert like you guys. I'm just learning one step at a time, both on the forum and I've also dug up my old electronics books and am pouring over them like mad. I figure I'm gonna need all the help I can get before I get done with this project and get it the best way it can be. LOL. Now I know kinda what Peter feels like when designing different Arc models.





Chris: Out of pure curiosity, I have to ask, where did that formula originate? Is it something that was discovered to work out of trial and error, or is it based on other electronics formulae? Also, does that formula work for multiple LEDs or is it something that only works well for a single LED? Would I just use a separate resistor for each LED or would it be better to use one resistor and run several LEDs off of it? One last question, is it better to wire LEDs in series or parallel when using multiple LEDs? Using separate resistors would probably be best to keep from pouring too many Amps into a single resistor, right? Sorry for asking so many questions, but there is just so much to learn when it comes to the wonderful world of LEDs and modern Electronics.


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## X-CalBR8 (Jun 5, 2001)

Chris: After looking at your formula for a while, I think I see what you did. Neat use of Ohm's Law there. I'm still curious about the optimum way to wire multiple LEDs though. How does everyone else usually do it?


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## Chris M. (Jun 5, 2001)

_Neat use of Ohm's Law there. _

Yes, that`s where it derives from however I admit I actually copied it out of an electronic components catalogue years ago and it just sort of stuck in my mind.


As for connecting them up, well it depends on your power supply and size of the array. If you can hook 2 or more in series to the supply (because of the voltage), do it. Use a small resistor in the series chain to cut down any excess voltage. Otherwise you`ll have to use paralell. For a large array, I reckon you`d get away with one big resistor there. If an LED in the array failed or was damaged, the resulting alteration in the load won`t change too much so the rest of the LEDs won`t get too much more voltage. You can then change out the dud LED in your own time.

A small array of 3 or 4, best have one resistor each. If one was damaged or failed for some reason, the change in load would probably be significant enough to overload the rest, you may not have time to replace the dead one. Of course LEDs shouldn`t fail if they are treated right- take care of static discharge which damages them, and watch for overheating and you should get a lifetime out of them. Dropping your light out of a 3rd floor window however, is a different story. LEDs aren`t totally indestructable if stuck just-so...





Generally speaking series is best if you can have the higher voltage needed (eg, running from a mains supply through a transformer), but for portable battery applications paralell is unavoidable (a resistor each is the safest way but is inefficient) unless you want a constant current inverter in there to boost the volts. And then the increase in efficiency you get by omitting the current limiting resistors , might get cancelled out by losses in the inverter.

So it depends on your application. Your opinions may vary of course and I`ve not done enough LED mods to have a preferred method from experience.


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## X-CalBR8 (Jun 5, 2001)

Chris: Thanks for the helpful info. You confirmed a lot of what I thought to be right, but it always pays to get a second opinion. You know the more I think about it, the more I'm convinced that if I'm going to loose efficiency through using resistors, that I would be much better off with a regulated power supply such as comes with the Princeton Tec Matrix headlamp. If only I could get the plans to build something as nice as that. It works from .8volts to 3volts with almost constant light brightness. If I could build a neato circuit like that, then I could use a single 2.5volt 2700Farad super cap and easily have a light that would burn brightly all night long then be charged right back up by solar power the next day. If I could build a circuit that could burn at full brightness from 1volt to 2.4volts range then I would have it made and could get right down to building this thing. What exactly would you call a circuit like that and has anyone found plans for a similar circuit that I might be able to modify to suit the purposes of this project? I'm aware of Satcure, but most everyone seems to think that it could stand to be a bit brighter. Also, does it operate over such a wide range of voltages as the Matrix headlamp circuit does?


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## Chris M. (Jun 6, 2001)

_So how big of a resistor does a person know to put on a LED at a certain voltage without just using purely trial and error and possibly frying a LED or 2 in the process? _

Here`s the easy formula to calculate your resistor value.

Resistor value= (Supply voltage- LED voltage) / desired LED current in Amps.

So from a 12 volt supply, to run a 3.6 volt LED at 20mA...

Resistor = (12-3.6) / 0.02 = 420 ohms.

40mA LED current, from 6 volt supply... (assume the LED voltage stays the same, it wont affect the value too much and normal variations in resitor values probably have more effect)

(6-3.6) / 0.04 = 60 ohms.

Simple.


_It is more voltage the ruins an led_

Actually it is more like the power. Power = volts x amps. A 9 volt battery can supply much more power than a couple of 3 volt lithium cells. Power leads to heat generation and heat kills LEDs by cooking the die, vaporising the gold wirebond and melting the resin body.
If you can keep the LED cool enough you can push a huge current through it. Dip one in liquid Nitrogen and I hear you can put an amp through it- and it will shine like a flare! But in a confined space next to other LEDs or a source of heat, even 40ma could fry one or severely reduce its life.


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## X-CalBR8 (Jun 14, 2001)

Just a quick question. When figuring out the wattage handling for a resistor going to a LED, do you go by the voltage that is actually dropped by the resistor or the total voltage going to the resistor? I know I should know this, but for some reason, I just can't remember.


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## Go Go Gadget Flashlight (Jun 15, 2001)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by X-CalBR8:
*Just a quick question. When figuring out the wattage handling for a resistor going to a LED, do you go by the voltage that is actually dropped by the resistor or the total voltage going to the resistor? I know I should know this, but for some reason, I just can't remember.*<HR></BLOCKQUOTE>

I got this wrong in a previous reply a while back, so I'll go for the "make-up" points on this one and try to get it right this time.





It's the dropped voltage you are concerned with. For example:
If you have a 4.5v supply and you want to limit the voltage to 3.6v, you have to drop 0.9v across your resistor. 
Say your single LED draws 25mA at 3.6v. For a resistor to drop 0.9v at 25mA, it's value would have to be 0.9v/.025a = 36 ohms.
The power for this is 0.9*.025 = 0.0225W. Well within the range of a 1/4W resistor and not too inefficient.

As always, try it out before you solder it in permanently.


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## X-CalBR8 (Jun 15, 2001)

Thanks for the info Gadget. As always, it's much appreciated.


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