# How much current is this 3mm led seeing?



## AlexGT (Apr 22, 2009)

Hi all!

I am doing a mod to my spotlight and want to add an "always on" 3mm white led so I can find it in the dark, I want to know how much current is the led seeing with the following conditions

Input voltage: 11.1V 
Resistance: 33,000 ohms

After the voltage drop, how much current is the led actually recieving?

It looks dim but want to determine the usable time before recharge in weeks if not months.

According to an online VIR calculator I should see 0.00033 amps, is that 33 ma or 3 ma?

Thanks!
AlexGT


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## Illum (Apr 22, 2009)

R = V/I

ohms = [Vin - Vdrop]/I


if you want to drive it at 3.6V 33ma, then use a 370 ohm resistor
if 3.6V 3ma, use a 2700 ohm resistor 

a 33K ohm resistor will give you somewhere around 0.25ma...I don't think the die will even light up


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## Al Combs (Apr 22, 2009)

Ohms law without an LED gives .000336 amps. which is ≈ 1/3 of a ma or 336 micro amps. I plugged 3 volts into one of those LED calculators and got 1/4 ma. So just guessing, 3*2,200 mAh 18650's in series would be just a hair over a year. Like Illum said, I'm surprised to hear it lights up at all.


AlexGT said:


> Hi all!
> 
> I am doing a mod to my spotlight and want to add an "always on" 3mm white led so I can find it in the dark, I want to know how much current is the led seeing with the following conditions
> 
> ...


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## AlexGT (Apr 22, 2009)

Well I checked again the resistors and apparently they are really 33,000 ohms, the bands are Orange, Orange, Orange and gold. How weird to light up with less than 1/4 ma!

What voltage should It read after the resistance and before the led? I ask because I think they are bright for such a small amount of current and want to check again.

My goal is a dim kind of glow in the dark light source that is always on to help me find the spotlight in the dark

AlexGT


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## MWClint (Apr 22, 2009)

maybe the resistor wattage is too low for the load you are putting on it and extra current is trickling thru.


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## AlexGT (Apr 23, 2009)

The resistor is 1/2 watt

AlexGT


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## Al Combs (Apr 23, 2009)

LED's are the kind of device whose internal resistance "adjusts" to the available voltage. If you replaced the LED with a piece of 12 gauge solid copper wire, the load on the battery wouldn't increase significantly. As an example, your Ohms law 336 micro amps vs. 245 micro amps if you have an LED that draws ≈ 3 volts. With the 336 representing a dead short in that part of the circuit instead of an LED. If it looks too bright and you're worried it must be drawing more power somehow, it's not. The resistor is doing its job as long as it's wired in series. If by bright you mean you want it dimmer, just put a larger resistor in the circuit. Try a 47K ohm or maybe something larger until you get the desired effect. Do you have a DMM to test the circuit?

I'm not sure what you mean by, "after the resistance and before the led" in your voltage question. The how much voltage is the LED drawing depends on the Vf of the LED. They draw current. Their resistance varies with power intake and so does the voltage. Take a look at Nichia's Lamp Type LED section. They have 3 different 3 mm LED's. They all are rated for 20 ma current. I think that's pretty typical of epoxy encapsulated LED's. I downloaded the pdf for one of them. The typical voltage for this particular LED is 3.2 volts @ 20 ma.There is a section with graphs in it. The first one is Forward Voltage vs Current. At 20 ma, the Vf is 3.2 volts. It drops off to 1 ma at what looks like just over 2.75 volts. The graph next to it is Current vs Relative Luminosity. It drops in a more or less linear slope from an output of 1 @ 20ma to 0 output @ 0 ma.

That is to say your LED is putting out 1/80th of what it's capable with the 33K resistor. Assuming its nominal current is 20 ma. Remember it's not a Fauxton lighting up a keyhole 2 feet away, it's pointed at your eye.



AlexGT said:


> Well I checked again the resistors and apparently they are really 33,000 ohms, the bands are Orange, Orange, Orange and gold. How weird to light up with less than 1/4 ma!
> 
> What voltage should It read after the resistance and before the led? I ask because I think they are bright for such a small amount of current and want to check again.
> 
> ...


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## AlexGT (Apr 23, 2009)

Thanks for the explanation Al Combs!, I decided to just increase the resistance until I got a dim light suitable for "Find me mode" I settled on a resistor with 68,000 ohms, the brightness is just about right for what I want it to do (find me!), and now I know it lasts a really long time.

Anyone care to calculate runtime on a 11.1v 13,200 mah li-ion battery pack? You will be amazed at the result!

AlexGT


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## Al Combs (Apr 24, 2009)

Glad to be of help. :wave:


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