USB 5V Step Up to 12V

Discussion in 'The Projects Forum' started by LordStDennis, Jul 7, 2011.

  1. LordStDennis

    Thread Starter New Member

    Jul 7, 2011
    I've tapped into my USB Port to extract 5V+ and 5V-, then lit up my USB Night Light circuit that I designed. Then, a strip of 60 (3 per section) LED lights came in from Hong Kong and they require 12V for operation.

    Now, I could just buy a dang PS, but really, it is a lot more fun to do what I am doing. I've looked at a number of circuits, including the ICL7660-MAX1044, LT1073, and also - but not limited to - the NE555N Timer Chip with the PWM circuit in the datasheet. (I have a hundred of these chips, fresh from China!) I've also seen this same circuit in various forms via Google.

    So, what way do I go? Cost is always a factor and the circuit with the least number of chips/components is the best.

    ~ LSD
  2. simo_x


    Dec 23, 2010
    Do the LEDs have a voltage forward of 12V? How much current they consumes totally?

    For step up I thought about MC34063, but LM2577 would be a good choice too, it have greater output current than the first one.

  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    I'm just wondering what you'll do with 60 LED's hanging all over your laptop. But your pleasure is what it is...
    Generally speaking a USB port can supply 5V at a max of 500mA.
    Now since you provided us with insufficient details about the LED's power requirements (their current). I can make an assumption. The 60 LED's, broken into 3 LED sections are probably wired as such. Their foward voltage is probably 3.0V - 3.4V and they have, or should have a series current limiting reistor with each section. Assuming, again, that the LED foward current (If) is 20mA on section requires 12V and 20mA to operate. Given that there are 20 sections (60/3 = 20) then the total current necessary would be 20mA x 20 = 400mA.

    Now, since you will need to boost the 5V to 12V you are going to loose much of the 500mA that you have from the USB port in conversion losses you will have no way near the 400mA you need at 12V to opperate all the LED's. I'd be willing to guess that you might only be able to run 10 sections.

    Other options may still possible, but without a picture of these LED's and their operating specifications, it would be difficult to offer much assistance.

    simo x,
    Although a chips specs suggest a possible current output it ultimately depends on the input power and the chips efficiency to provide that output.
    In an ideal world Power in = Power out, in the real world we know that there are power losses such as heat. If P=IV then the Power in is 5V(.5A) =2.5W But the power out we need is 12V(.4A) = 4.8W. Then the real world kick in and we may have 3.8W The current we are left with is 3.8W/12V = 317mA, short of the 400mA needed. So even if the chip spec says it is possible to provide up to 1A of current you will never get 1A if you do not have the power to privide it on the input.
  4. simo_x


    Dec 23, 2010
    Correct. I didn't make calculus as you did, omitting important details.
  5. iONic

    AAC Fanatic!

    Nov 16, 2007
    No mate, just 4th grade math, at least back in the 60's it was, and these days it's probably 3rd grade math. By the way, I did pass calculus, was even exempt from the final. Today, however, it's as much a mystery as tomorrow is. Wasn't trying to belittle you, just offering a bit of insight, and I've had my share of insight given to me here as well. Be thankful that it's free!

    @ LordStDennis,
    I've read of ways some have hacked a USB connector to obtain larger currents from the US port. This would enable you to step-up the 5V to 12V and have the assumed 400mA that you need to run all the LED's. But, you'll have to ask yourself if it's really worth the trouble. If it is, then you ought to be able to find the answers via an internet search. I prefer not to help you hack something I have not already tried myself with the fear of aiding in the destruction of your laptop...
  6. SgtWookie


    Jul 17, 2007
    Good job, iONic. ;)

    Just a bit further tho....
    400mA @ 12v = 4.8 Watts.
    So, let's say you could build a boost converter that was 90% efficient. So, 4.8/90%= 5.333... Watts required.
    A typical USB port is limited to 500mA @5v for desktop/tower computers, and about 1/2 that for laptops. So, the desktop/tower has 2.5 Watts available, less than 1/2 required - and a laptop is less than 1/4 of what's required.

    Now what you COULD do, if you have a desktop or tower case, is to get a molex wye adapter (power splitter) to plug into your computer power supply's harness; the black wires are ground, yellow is +12v, red is +5v. That would have plenty of power for your LED string, and the parts count roxxors - ONE!
    stahta01 likes this.