Power up a circuit with 5V DC

Discussion in 'The Projects Forum' started by logicmind, Jun 21, 2008.

  1. logicmind

    Thread Starter New Member

    May 20, 2008
    9
    0
    hi, i was wondering if any know how to build a circuit that would turn on something whenever there is a 5V or less DC voltage present, for e.g, those self power external hard drive that comes on whenever u turn on your computer
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Do you mean up to 5v?

    Are you talking about external drives that are plugged into the USB port? They're powered by the USB port's output; up to 500mA @ 5v for regular computers, lower current for laptops.
     
  3. logicmind

    Thread Starter New Member

    May 20, 2008
    9
    0
    no, i want the ouput of the circuit to be 12volts, but i want the circuit to turn on whenever there is a 5v present.

    on my external hard drive, the hard drive is connect to a adapter that provides it with 12V, there is also a usb cable going to my computer which transfer the data and also somehow tells the hard drive when to turn on and off. when i turn on my computer it turns on itself. the usb is not powering the hard drive it's the 12V adapter, cuz when i disconnect the 12v adapter the hard drives turn off.

    am thinking of using a phototransistor, from the USb cable i will get a 5v from there to light a led, and whenever the phototransistor sees that it turns on the circuit, but am not sure how to wire it to give and ouput of 12V

    a simpler solution would be to use a 5v to 12v Dc to Dc converter, but it's bothering me that i can't figure it out the other way i described. It's like i wouldn't rest until i firgure it out..lol
     
  4. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    Why would you want to cut the 12v to the hdd? Don't you said that the computer turns on and off the drive?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    You could use a relay with a 5v coil energized by the USB supply to switch mains power to your external supply, as long as the coil required less than 500mA. You might also use it to switch a TRIAC.

    A DC-DC converter would work if your circuit didn't draw much power. Going from 5v to 12v means you'd reduce the available current from the USB from 500mA to 500mA x 5/12 x the efficiency of the converter itself. You might get one around 92% efficiency. That would give you around 191mA @12V.
     
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