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current and current consuming devices

Discussion in 'Electronic Basics' started by panfilero, Aug 20, 2008.

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  1. panfilero

    panfilero Guest

    I just had a couple of questions about current... can I device draw
    too much current for itself? I mean... if I hook up a device that
    uses 1amp to a power supply without a current limiting control will
    that device just use what it needs? So basically will a 12V 2Amp dc
    power adapter work for all devices that require less than 2Amps?
     
  2. GregS

    GregS Guest

    You had it when you said drawing less than 2 Amps. Turn on surge current can cause problems.
    its a major issue driving 12 lamps. Sometimes the supply will shut down.

    greg
     
  3. Bob Eld

    Bob Eld Guest

    Yes
     
  4. Which question are you nasering?
     
  5. panfilero

    panfilero Guest

    yeah I'm not sure either.... are you answering "can a device draw too
    much current for itself?" and saying "yes" that it can?

    Tell me, does current just depend on a devices impedance? so I can
    know how much anything is going to draw by just measuring the things
    resistance and knowing the voltage I'm about to put across it?

    I have keyboards and guitar pedals and such that all require 9V
    adapters at different voltage ranges... so I can just buy a 9V adapter
    which exceeds their current ranges... say the require 500mA and I buy
    a 3AMP power supply.... then I should be good to go right?


    but I guess this is my main question here: "can a device draw too much
    current for itself?"

    I try to build an analogy to all this stuff in my mind.... I picture a
    board laying flat with some kind of jack on one side that lifts that
    end making the board angled... on one end of the board I picture
    marbles.... when the board is flat that is equivalent to no voltage
    and the marbles dont move, if you raise on end of the board the
    marbles start to flow down the board.... you are increasing the
    voltage... and the marbles flowing is the electrons.... but my problem
    in my analogy is that the electrons (marbles) are only present if
    there is a device that wants them.... if there's no device then they
    wont flow even if you lift the board... I guess you could put a little
    door that slides up on one end of the plank or in the middle of the
    plank and if its completely shut that's infinite resistance and as you
    lift the door open and marbles pass under it you are decreasing your
    resistance.... so the door only accepts the flow of marbles it needs
    no more no less....
     
  6. It depends on the device. A linear resistance/impedance will draw
    current proportional to the voltage applied. Non-linear devices (such
    as many power converters) will draw some power to satisfy their own
    needs and more power depending on the requirements of any attached
    load.
     
  7. Usually, a device will just draw the current it needs when the voltage
    supplied to it is within its allowable input range. But you cannot simply
    measure the voltage (V) of the supply and the impedance (Z) of the load,
    and then connect them together and expect the current to be V/Z. Simple DC
    power supplies have filter capacitors and trannies with intentionally poor
    regulation, so their open circuit output voltage will be much higher than
    it will be under rated load. And most loads other than a simple resistor
    will be non-linear, so that the applied voltage will affect the impedance.
    For instance, an incandescent 12 V lamp might read 1 ohm on a multimeter,
    but only draw 1.2 amp at 12 VDC, because the filament resistance can go up
    by a factor of 10 when it is white hot.

    Musical equipment contains circuitry that can draw varying amounts of
    current, and usually will have a regulator that will allow a reasonable
    range of input voltage without problems. But if you use an adapter that is
    rated at a much higher current, it might apply a voltage that is too high,
    and the device could overheat.

    I've always used the water analogy, and I have not heard of the marbles
    analogy. I guess it's better in the case of a major circuit malfunction, to
    "be all wet". rather than "lose ones marbles"!

    Paul
     
  8. Rich Grise

    Rich Grise Guest

    Yes, if the supply is voltage-regulated. Some cheap wall warts need a
    specific load to ensure that the voltage is in spec; that depends on
    the internal resistance of the supply.

    Cheers!
    Rich
     
  9. Frank Buss

    Frank Buss Guest

    Yes, a device limits the current to what it needs. See this page for more
    information:

    http://en.wikipedia.org/wiki/Ohm's_law

    So if you connect a lamp with 10 Ohm to a 12V power supply, the current is
    12V/10 Ohm = 1.2A, even if the power supply can deliver up to 100A, because
    the power supply regulates the voltage to 12V all the time (if the current
    draw is less than 2A), and Ohm's law does the rest. Note: As other posters
    wrote, there are very cheap power supplies, which might not regulate the
    voltage for all currents. Use a voltmeter to test it.

    For non-linear devices, like ICs and transistors, the resistance may be
    more complex, but in the end Ohm's law is true all the time, so the
    resistance may change in a non-linear way, but if the device is specified
    for 12V and if the power supply can regulate it to 12V all the time, it
    doesn't matter how much current the power supply can deliver at most.

    If you want to connect multiple devices to one power supply, you can add
    the maximum needed current of each device for calculating the needed
    maximum current for the power supply. If all devices need the same voltage,
    you can connect them in parallel.
     
  10. Bob Eld

    Bob Eld Guest

    Which question are you nasering?

    All of them.
     
  11. neon

    neon

    1,325
    0
    Oct 21, 2006
    any source that say max 2amps will work for any devices that require less. The voltage however is another story too hi voltage will destroy the device too low will not allow the device to work.
     
  12. Rich Grise

    Rich Grise Guest

    A device _CAN_ draw too much current for itself, if the power supply
    supplies a significantly higher voltage than the device's rating.

    For example, if you plug a 120V toaster into a 240V outlet, it will
    draw WAAAAAY too much current, for a little while. Be sure to have the
    ABC extinguisher on hand if you do this experiment at home. ;-)

    Hope This Helps!
    Rich
     
  13. neon

    neon

    1,325
    0
    Oct 21, 2006
    That is not quite true. the inrush has nothing to do with any output load. The inrush is caused by magnetics that see it at dc and saturates until the circuits stabilize. for instance turning -on power when there is 0 volt AC is not the same as when it is at 90 degrees or 180 degres on its cycle.
     
  14. whit3rd

    whit3rd Guest

    Maybe. The problem is, that '12V 2 Amp' rating identifies one and
    only
    one point on the I/V curve (called the "load line") for your power
    adapter,
    and normal operation may wander over the load line and find
    other points. Commonly, these adapters are regulated, i.e. the load
    line is nearly constant V = 12 +/- .1V, only dropping to lower V for
    currents
    in excess of that 2A rating (never exceed 2A, and all is well).
    Some regulators require a minimum output current too.

    There is one other issue, however, that should be addressed: if you
    have
    a dozen 0.1A items that need 12V, it isn't always possible to run them
    from a single 12V supply. That's because there are no GROUND
    connection
    issues with an isolated wall-tumor adaptor, until you connect both
    a positive-GND and a negative-GND device to it. The first attempt to
    connect the grounds of those devices will short out the power supply.
    Here, again, the simple '12V 2Amp' rating doesn't tell you if the
    device needs to use negative-ground, or center-ground, or positive-
    ground
    power. Rarely, a wall-tumor may have internal ground connection
    (to the third-prong on the wall plug).
     
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