1. Computer problem? Tech Support Guy is completely free -- paid for by advertisers and donations. Click here to join today! If you're new to Tech Support Guy, we highly recommend that you visit our Guide for New Members.

LED current draw

Discussion in 'Do It Yourself (Not Computer-Related)' started by Who's Me, Apr 25, 2010.

Thread Status:
Not open for further replies.
Advertisement
  1. Who's Me

    Who's Me Thread Starter

    Joined:
    Aug 28, 2006
    Messages:
    247
    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)
     

    Attached Files:

  2. etaf

    etaf Moderator

    Joined:
    Oct 2, 2003
    Messages:
    65,408
    First Name:
    Wayne
  3. JohnWill

    JohnWill Retired Moderator

    Joined:
    Oct 19, 2002
    Messages:
    106,418
    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.
     
  4. Noyb

    Noyb Trusted Advisor Spam Fighter

    Joined:
    May 25, 2005
    Messages:
    21,001
    First Name:
    Jay
    Depends on which LED is being measured.
    If memory serves me correctly .. The bottom LED style could usually handle up to 25ma max
     
  5. Frank4d

    Frank4d Retired Trusted Advisor

    Joined:
    Sep 10, 2006
    Messages:
    9,126
    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.
     
  6. Who's Me

    Who's Me Thread Starter

    Joined:
    Aug 28, 2006
    Messages:
    247
    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.
     
  7. ARTETUREN

    ARTETUREN

    Joined:
    Dec 2, 2007
    Messages:
    248
    Hai, .. yes this is enough - OHM law I (current) = U (voltage) / R (resistance)

    Blue led have higher voltage threshold than others coloured leds.
     
  8. Sponsor

As Seen On
As Seen On...

Welcome to Tech Support Guy!

Are you looking for the solution to your computer problem? Join our site today to ask your question. This site is completely free -- paid for by advertisers and donations.

If you're not already familiar with forums, watch our Welcome Guide to get started.

Join over 733,556 other people just like you!

Thread Status:
Not open for further replies.

Short URL to this thread: https://techguy.org/919108

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice