12 V led strip on USB

Discussion in 'Power Electronics' started by joey Rogovein, Oct 17, 2016.

  1. joey Rogovein

    Thread Starter New Member

    Oct 17, 2016
    Hey everyone, So i'm new to all of this stuff and still learning on how to do this so bare with me lol.

    I have a 12v LED strip and was just thinking.
    If a PC USB port provides 5v, Can I use two USB ports connected to one cable then connected to the LED Strip?

    Would that boost the power to the led strip to 10v to power the LED Strip, It will be dimmer then normal but would power? or am I completely wrong. lol.
    Please be kind :p .
  2. ian field

    AAC Fanatic!

    Oct 27, 2012
    First thing to make clear: you have to run LEDs from constant current - like any diodes; they have a sharp knee curve. A small increase in voltage can cause a large increase in current - and failure.

    Check whether the strip has a built in current limiting resistor - removing it could have a couple of advantages.

    If they're white or blue LEDs; they drop about 3.4V at the knee curve, so one LED with a current limiting resistor can run from a 5V USB supply.

    To get 10V - you need some kind of multiplier. Since VxA=W - if you increase the volts, you won't get as many amps, and it gets worse - you have to also allow for losses.

    Probably the easiest route is a blocking oscillator/inverter. A single transistor and a simple transformer which isn't that difficult to wind yourself.

    You can get some general guidance from the plentiful online Joule thief blocking oscillators - the most common example runs from a single 1.5V cell and has the base winding straight up to Vcc - you can't do that with a 5V supply. it'll destroy the B/E junction. The base winding goes to the base as before, but the other end is AC decoupled to GND with a capacitor and fed by a resistor from Vcc to provide some start up current - varying this resistor will give you a little bit of control over the output.

    It provides narrow spikes to the LEDs, so there is only a finite amount of energy delivered - a bit of trial and error but it can be made to work quite adequately. For the extra complexity of another transistor and resistor; you can implement active current limit.........