# Powering circuit via usb [newbie]

#### peck68

Joined Nov 27, 2009
73
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?

Thanks

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#### iulian28ti

Joined Dec 4, 2009
42
an usb has a voltage of 5V
maximum output is 500mA
that means a max of 2.5watts

#### kubeek

Joined Sep 20, 2005
5,587

#### peck68

Joined Nov 27, 2009
73
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

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

Edit:
This is just a case for 5v supply, obviously i wouldn't use 16 leds on the go lol

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#### beenthere

Joined Apr 20, 2004
15,808
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.

#### PIC_User

Joined Sep 22, 2008
20
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.

#### peck68

Joined Nov 27, 2009
73
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)

#### Audioguru

Joined Dec 20, 2007
11,251
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.