# LED current draw



## Who's Me (Aug 29, 2006)

I built my first attempt at a surface mount circuit board, It is a 5v 100mA regulator. I used all salvaged components so the values aren't optimal but it works and I proved to myself I could solder such small parts.
The blue light on the output wires is a bulb from an old stereo drawing 45mA but I am not sure how to figure how much current the LED is consuming, I would use my meter but it is so tiny I already killed one trying to solder wires to it.
The resistor is marked 390 ohms (I didn't measure actual value) and I don't know anything about the LED except that I got it off of the keypad of an old Motorola cellphone. 
My output voltage measures 4.91v I get 2.83v across LED and 2.06 across the resistor feeding it.
is this enough information to get the current drawn by LED? 


P.S Please criticize my design if you see anything wrong or that could be improved. (Remember that the components were salvaged but I do create the board layout)


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## etaf (Oct 2, 2003)

this formula should help you work out the current 
http://en.wikipedia.org/wiki/LED_circuit
just rearrange the formula for current

about 10mA

theres quite a few interesting online calculators for this sort of thing
http://ledz.com/?p=zz.led.resistor.calculator

when i played at this, only had a sliderules, logbooks and just getting affordable calculators  

maybe misleading but, that does look bright


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## JohnWill (Oct 19, 2002)

Sounds like only about 5ma through the resistor, so that shouldn't be too much. Note that some of the really small surface mount LED's probably only require a couple of mils to function.


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## Noyb (May 25, 2005)

Depends on which LED is being measured.
If memory serves me correctly .. The bottom LED style could usually handle up to 25ma max


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## Frank4d (Sep 10, 2006)

I zoomed in on the photo and made a mental schematic. It looks like the 390 ohms is in series with the on-board LED, and agree the current is about 5 mA.

The LED connected to the red and black wires appears to be connected to the 5 VDC output of an LDO type regulator with no current limit resistor.

Edit: this LED probably has a built in resistor or current source.


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## Who's Me (Aug 29, 2006)

Thank you for the input. I had already been to the wiki link and got confused but the online calculator was easy enough so I bookmarked the link to it. I was asking because I wanted to know how many of those little LED's this thing could light. The online calculator also has LED's in series and parallel so I can figure the best resistor needed for however many LED's
It looks like I could run 20 - 30 of them depending on current and I was only thinking of using about 6 - 10 so I know it should handle that. 
Yes it is very bright, I wanted a 470R but could not find one.
The light on the output wires is an incandescent bulb from a car stereo (not sure of its intended voltage but it draws 45mA @ 4.91v) I was just using that one to load test the circuit overnight.
Thank you again for the help.


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## ARTETUREN (Dec 2, 2007)

Who's Me said:


> My output voltage measures 4.91v I get 2.83v across LED and 2.06 across the resistor feeding it.
> is this enough information to get the current drawn by LED?


Hai, .. yes this is enough - *OHM law* I (current) = U (voltage) / R (resistance)

Blue led have higher voltage threshold than others coloured leds.


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