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Using Voltage Regulator for current limiting - John Fileds ?

Discussion in 'Electronic Basics' started by William A. Bong, Jan 10, 2006.

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  1. I think that it was John Fields that posted a circuit using a voltage
    regulator IC such as the 7812 as a current limiting device - John or anyone
    else that could help out with this please repost it - I searched but it's
    obviously no longer on my server.

    I was thinking of using this as a current limiter to build a USB to Nokia
    phone charger - I have
    the hardware and connected it up straight, but it trips the power to my
    notebook USB port requiring a reboot to reset the USB.

    All advice and most welcome.

    Bill.
     
  2. Bobscar

    Bobscar Guest

     
  3. Pooh Bear

    Pooh Bear Guest

    There is a way to configure a standard regulator as a constant current source.
    I suspect this is what you mean. Only ever did it once for EMI reasons.

    For a positive output you need a negative regulator and you use it 'back to
    front'.

    A power resistor ( R ) is placed between Common/Sense and Out. I = R/Vref (
    Vref = 5 for 7905 for example or 1.25 for LM337 ) . DC in + goes to Common /
    Sense and the current out comes from 'In'.

    Graham
     
  4. Pooh Bear

    Pooh Bear Guest

    And then I recall that there's a way to make a shunt regulator to IIRC ! Uses the
    'opposite polarity' chip again in a wacky configuration.

    Graham
     
  5. I think allot of voltage regulators can be used like this, but the LM317
    seems one of the most popular for the task.

    http://www.google.com/search?hl=en&q=lm317+constant-current

    The datasheet is also a good reference source for cutesy circuits like
    this.

    The key is that the output pin and the ground pin will always have 1.25V
    across them. By floating the ground and connecting it to the output pin
    via a resistor, you can create a constant current source. The resistor
    is sized using Ohms law (R=E/I). So for a 25mA current source, you
    would need 50 Ohms of resistance between the pins. Be sure to calculate
    the power being dissipated by the resistor and size it accordingly. In
    this case, the power is 31.25mW so a 1/8W resistor should be plenty big
    enough. If you want to supply 1A, then you would need a resistor of
    1.25 Ohms that would be dissipating 1.25W, so use at least a 2W and
    don't burn yourself. ;-)
     
  6. Rich Grise

    Rich Grise Guest

    A current limited regulator isn't what you're looking for here - all that
    will do is make the power supply to your device droop, and it won't
    operate properly. You need either a USB with a beefier supply, or an
    auxiliary power supply, like a wall wart.

    Good Luck!
    Rich
     
  7. kell

    kell Guest

    Are you connecting your Nokia phone directly to the USB? It's actually
    not such a bad idea, in principle -- the USB is a 5 volt power supply,
    and you can charge a Nokia cell phone from a 5 volt power supply. But
    if your Nokia is like mine, it doesn't draw a steady low current when
    charging. Mine draws its power in pulses. And I'm sure these pulses
    would be enough to trip a USB, which can only put out very small
    currents.
    Try putting a resistor in series with the USB and allowing a capacitor
    to charge from the limited current coming through the resistor. Charge
    your phone from the capacitor. Start with a few dozen ohms for the
    resistor, and 1000 uF cap. With a correctly sized resistor, you may be
    able to pevent the USB from tripping and yet still get the cap to
    charge up between pulses. You should be able to make this work if the
    total power drawn by your phone, over time, does not exceed the amount
    of steady state power the USB can deliver. But there's no guarantee of
    that.
     
  8. Jasen Betts

    Jasen Betts Guest

    use the regulator to regulate the voltage in a resistor.

    usually works more economically with a 5V or lower regulator.
    as the 5V etc is wasted

    Bye.
    Jasen
     
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