Constant current through a load independent of its resistance

Discussion in 'General Electronics Chat' started by unseensoul, Jun 29, 2009.

  1. unseensoul

    Thread Starter Member

    Dec 13, 2008
    22
    0
    I tried several different circuit configurations (ie. current mirror, howland's current source, etc) to achieve a constant current through a load independent of its resistance but no success. How is this possible? It doesn't make sense to me...

    For instance, I tried this circuit configuration - http://www.falstad.com/circuit/e-howland.html - but the current load is affected (it changes) as the load resistance is changed which it shouldn't?! What's happening in here?

    By the way, do you know any feasible circuit configuration which will provide me a constant current through a load independent of its resistance?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    I was about to answer ur post but after seeing the link..:eek:
    How do u do that man, it's really cool (Simulation thingy)
    Pls tell :p

    Rifaa
     
  3. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    In order to achieve this without violating nature law of V = IR you need a circuit that constantly raises the voltage to keep the current magnitude at where you want it to be.

    You can't violate physics. So in order to "force" some amount of current trough a circuit you need a circuit that senses the current magnitue and increases the voltage until you have achieved the current magnitude you want. Most likely that will destroy conventional circuits if they are designed to run on lower current then you are trying to force into it.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    I was playing with the circuit...coooool! :p
    refer to http://www.falstad.com/circuit/e-howland.html
    As for ur problem, there will be no current via load when u by pass it buddy.
    The current will chose the lowest resistance and since a close switch has no voltage drop, the load has no voltage across it, no V, no I in load ;)
    Get the point

    Rifaa
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    A constant current source should do that - http://en.wikipedia.org/wiki/Current_source

    Remember that I = E/R, so the load resistance has to be within limits. As R gets larger, E has to rise in order to force current. As R gets small, the ability of the source to supply current becomes limited. Both the voltage and current available to a constant current source are limited, and so therefore is the upper and lower value of the load.
     
    Last edited: Jun 29, 2009
  6. unseensoul

    Thread Starter Member

    Dec 13, 2008
    22
    0
    The circuit configurations I've used are regarded as typical constant current sources so they compensate any loss/gain within some limits, right? Well, it didn't work for me... I was trying to get a constant 5 mA which I think is within the limits...
     
  7. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    I have a schematic of a constant current sink circuit at
    http://tinyurl.com/6cbn6h

    Scroll down to the "Electronic Load" circuit. By adding cells
    you can get to hundreds of amps. Be careful of the power
    dissipation.

    (* jcl *)
     
  8. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Google it. Simple circuit is an op amp, possibly driving a power transistor. All these circuits crap out if the required voltage exceeds the power supply voltage ( usually minus a few volts ) The general idea is that you put a fixed low reistance in series with the load on the low side and use the op amp to maintain a constant voltage on it. Works on the high side if you have a high side current sense, which is a bit more complicated. There is a 2 transistor solution often used in the optput of power amps to limit the current. Good if the current need not be adjusted or very well regulated. Look at battery charges for ni cad these are normally constant current sources.
     
    Last edited: Jul 1, 2009
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