Current Source

Discussion in 'The Projects Forum' started by yehdev_cc, Jul 28, 2010.

  1. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    hi everybody,
    I'm working on a Current source circuit that I found its design on the web, here is the design :
    [​IMG]
    the problem is that the current levels in the simulation - run by Multisim - is very lower that the ones in the original design (0 - 100 mA controlled by voltage.
    so I wish I know why and how I can modify this circuit to get my desired levels.
    and how can I analyze this circuit ? and should this circuit give me practically precise values ?
    thanks in advance, an help is really appreciated.

    circuit file : http://www.mediafire.com/download.php?s3gjz5ck20mqg0x
     
    Last edited: Jul 28, 2010
  2. sage.radachowsky

    Member

    May 11, 2010
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    I don't understand that circuit. It's way too complicated for a current source.

    Try the attached circuit. The input will set the current seen by the load, at a ratio of 1V = 100 mA.

    Sage
     
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  3. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    Dear sage,
    thank you very much for your help.
    This circuit consists of two parts : a current sense circuit ( the upper part which consists of the op-amp and MOS transistor), and a voltage regulator ( the lower part which consists of an op-amp and PNP transistor)
    The current sense circuit functions as a feedback for the voltage regulator, it senses the current in the output and returns a signal to the regulator to either increase or decrease the output current accordingly.

    For your circuit, Rsense is 10K. Should there be any consideration on practical implementation with discrete components ?
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    The circuit will not work with an LM2904 dual opamp because its inputs are near the supply voltage but the max input common-mode range for an LM2904 is 1.5V less than the supply voltage.
     
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  5. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    Thank you very much, I didn't notice this.
    But even for 1.5v input - which is still in the proper range (0 - 3.5v ) - its output current still much lower than the desired levels.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The current sense resistor is connected to the positive supply. Then the input of the upper opamp is also connected to the positive supply where it will not work.

    I couldn't find a datasheet for your extremely old PNP power transistor and for the odd power Mosfet.

    Your load is 100k ohms so the max current is a little less than only 50uA.
     
  7. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    I'm not sure I got your point correctly, would you please give further explanation or suggest modification ?
    thank you very much.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    1) Use opamps with a input common-mode voltage range that includes the positive supply voltage.
    2) Use a newer PNP power transistor that has a detailed datasheet so we can see if the weak output current from the opamp is enough to drive it for 100mA output.
    3) Use a common Mosfet that has a detailed datasheet so we can see if its gate works with the small voltages in your circuit.
    4) Show a suitable load resistor on your schematic like 47 ohms.
     
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  9. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    Thank you very much for the detailed answer, I really much appreciate it.
    However, I have some questions :

    I have several data sheets for various opamp(s), but they don't mention a range for input voltage, only CMRR .. is that what I should consider ? if yes, then how ?

    I intend to use this circuit for calibration, should I use a little bit higher resistor ?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    No.
    The common-mode rejection ratio has nothing to do with the very important allowed input common-mode voltage rating that is on every opamp datasheet that I have seen.
    A TL081 or TL071 opamp has inputs that work at the positive supply voltage but their minimum supply is 7V, not 5V.
    Use a modern Cmos opamp that has inputs that are "rail-to-rail".

    A higher value load resistor causes a lower max output current.
     
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  11. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    I checked TL071CN, and it has input voltage common mode range, that's great ...
    now I have got another question (I'm sorry if I'm asking so many questions, but I really need to understand these points) :

    - If the datasheet of the op-amp I'm using, does not mention this value, why should I consider it ?

    - The value in the datasheet is +/- 11, so this should cover my desired range which is ## ? (I don't know it)

    - why should I look for 'rail to rail' input opamp ?
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Do not consider it because it is a very important missing spec for your circuit to work properly.

    Your range of the voltage across the 0.2 ohm current-sense resistor is from +5V down to +4.8V when it has a current of 100mA.

    Because its inputs work perfectly when they are at 0V, +5V or anywhere in between.
     
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  13. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    Thank you very much, I'll re do the simulation with these changes.
    Now, If you're to going to pick a circuit for a current source, would you use this or the one provided in this post by 'sage.radachowsky' : http://forum.allaboutcircuits.com/showpost.php?p=263799&postcount=2
    ? I like to know your opinion if you don't mind ...
    I'll implement it with discrete components ...
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    His circuit will not work since the Mosfet is a source-follower with a huge voltage loss and a very low max current.
    I fixed it.
     
  15. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    Great, but this way it'll function as a current sink not current source, isn't it ?
     
  16. sage.radachowsky

    Member

    May 11, 2010
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    Audioguru, I do appreciate the way you've redrawn the circuit. I think it would work the original way, given that there are MOSFETs that conduct with very low Rds(on) even at gate voltages of 2V and surely 3V or 4V, but I didn't specify that in the circuit. Yours is a better design anyway.

    Also, thanks for specifying that the op amp must do rail-to-rail input. That is definitely required because I've powered it from positive rail to ground, and some critical feedback is pretty close to ground.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    If you want a current source then use an opamp with inputs that sense voltages near its positive supply like a TL081, and use a P-channel Mosfet with the load between its drain and 0V.
    The control input goes to a lower voltage for more output current.
     
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  18. sage.radachowsky

    Member

    May 11, 2010
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    Here is a simulation of the simple current clamp circuit.
    This time I specified components.

    You can see that the current seen by the load is well regulated to 50 mA as set by the 0.5V input to the op amp.

    Note that this op amp (mcp6022) is rail-to-rail input and output. Also note that it is limited to 5.5V power supply, so it won't work with 12V supply.

    The MOSFET is simply one with a relatively low turn-on Vgs, so that the op amp can drive the gate as it needs to.

    Also note that the load -- whatever it is -- needs to be able to "fit" within the voltage limits imposed by the power supply voltage minus the voltages used by the sense resistor and the current-regulating device, which is here the MOSFET. So, in this case, 50 Ohm load will work but 100 Ohm load will not. A 50 Ohm load "uses up" 2.5V at 50 mA, but a 100 Ohm load "uses up" the whole 5V and the circuit can't operate, so it settles on a lower current of 45 mA as possible (because the MOSFET goes into full conduction and total resistance is about 110 Ohms from 5V to ground).
     
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  19. yehdev_cc

    Thread Starter New Member

    Jul 25, 2010
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    I much appreciate your help, thank you very much.
    This seems great, but the strict condition on the load resistor makes it less useful for me, may I use another opamp ?

    I tried it, but something went wrong, the value doesn't change correctly with the control voltage. with 10 ohm Rsense, it doesn't change at all ..
    please find the circuit in the attachments ...
     
  20. sage.radachowsky

    Member

    May 11, 2010
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    Yehdev, the limitation is that you *can't* put more current through a resistor without a higher voltage drop.

    The bottom line is that you must define your requirements...

    * What is the maximum load resistance you want to test?
    * What is the maximum current you want to test? (You said 100 mA.)

    From those two things, you will be able to determine how high of a supply voltage you need, to be able to send that much current through a resistive load.

    You'll need the voltage to be enough to send through the load, plus a little more for the current sense resistor.

    If you want to use a supply voltage higher than 5V, then yes, you can change the op amp to another one... and be sure to choose one with rail to rail inputs.

    Or, you could also simply keep the 5V supply for this op amp and draw the load supply power from 12V or higher. That would work, too, because the sensing and control all works below 5V.

    I added a simulation of that last solution.
     
    Last edited: Jul 29, 2010
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