Powering circuit via usb [newbie]

Discussion in 'General Electronics Chat' started by peck68, Dec 25, 2009.

  1. peck68

    Thread Starter Member

    Nov 27, 2009
    Hi - i have been playing around with led's and strip boards now for about a week, and i want to extend my "learning" as you wish so i can use the wonders of usb.

    I have been wanting to make a project that will be powered by usb, however - can some body tell me what components i will to harness usb power and any other stuff i would need to know. I just want it to power a small load like a few leds and a 1.5-3v motor (for sake)

    Also, what is the power output of a usb?

    Last edited: Dec 25, 2009
  2. iulian28ti


    Dec 4, 2009
    an usb has a voltage of 5V
    maximum output is 500mA
    that means a max of 2.5watts
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    That is only true if your device tells the USB host via USB protocol that it needs high current. Otherwise you can draw only 100mA or the host will shut the port down.
  4. peck68

    Thread Starter Member

    Nov 27, 2009
    Ahh thanks


    So i go and try a little demo on yenka ( i just check it to make sure nothing blows up before hand lol) and i decided to place 16 30mA LEDs in parallel.
    (500mA / 30mA = 16.6')

    Slight issue, i tried everything and anything - repeating the calculations (of my very limited electronics experience) but the LEDs just KEEP blowing up :confused:

    I don't know whether it is having a hissy fit with me or i'm doing something wrong, can somebody make sure it is right or wrong?

    I used 82Ω resistors with each LED (2.5v, 30mA) and by working out (5-2.5)/0.03 equalled me with 83, so it has to be it! :(

    Still learning this stuff though :)

    This is just a case for 5v supply, obviously i wouldn't use 16 leds on the go lol
    Last edited: Dec 25, 2009
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    Your calculations may be okay, but most LED's like something more like 10 ma, so a 330 ohm resistor is more in keeping.

    Those LED's in parallel all pull the same current. So figure on 16 X .01, or 160 ma. The 30 ma solution pulled 480 ma.
  6. PIC_User


    Sep 22, 2008
    I use “USB power” to program microcontrollers and the circuits connected to them, during the development stage.

    Advice: Always use a powered USB hub.;)

    Not just to get FULL power from the USB line, but to avoid taking out your computer’s USB port, if something shorts out.:mad:
  7. peck68

    Thread Starter Member

    Nov 27, 2009
    Ahh right thanks

    So one last question slightly unrelated but anyhoo - is it safe to power an LED @ 50mA, it's max rating is 100mA (its a 20000mcd lol)? Cause maplins (where i'm hoping to get it from) says its typical output at 20mA, where i thought i could push it that much further and still hopefully be safe :)

    Also 2x 50mA led = 1 unit load of the USB power = no kaboom to computer even with usb 1.1 :) (which is why i want to bump it up from the 20ma)
  8. Audioguru

    New Member

    Dec 20, 2007
    Your LED might survive 30mA if it is kept cool. But you have many LEDs so they concentrate the heat then get too hot and fail. Never use the absolute maximum continuous current.

    The maximum current might be higher if it is for very short duration pulses so it doesn't have enough time to get too hot.

    I don't think Maplins knows who manufactures their stuff so detailed datasheets are not available.

    An LED with an output of 20,000mcd probably is a dim one in a tightly focussed case. The very narrow beam cannot be seen unless it is shining directly at you. I like wide angle bright LEDs.
  9. peck68

    Thread Starter Member

    Nov 27, 2009
    I took your advice - downsized and went for some 4x 8000mcd (20° - i prefer it to be a sort of torch :)), which draws only 80mA for the lot of them, and a 4 pole switch so i can then switch to 2x UV leds, which only draws 20mA.

    The UV were just novelty lol - a little pricey on a first project, but hopefully it does me good (LEDs alone are going to cost me £20 :()