Increase current

Discussion in 'The Projects Forum' started by j3r, Jan 10, 2011.

  1. j3r

    Thread Starter New Member

    Jan 9, 2011
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    Hi all, i need to drive a motor which takes in -20mA to 20mA. I got a module from NI, NI 9269

    http://sine.ni.com/nips/cds/view/p/lang/en/nid/207638

    Basically, this module only supports max of +/- 10mA, and i can't replace it with others as no other modules from NI suits my requirement of +/- 20mA. Therefore my solution would be have a simple circuit to "step up" the current to drive the motors. The circuit would be attached to the NI module at one end, other end would be connected to the motor. I've googled and search this forum, MOSFET would be an ideal choice. But i'm not quite good in electroics, how can i go about it. Can someone reckon me provide me some MOSFET which i look into?

    MANY THANKS!!
     
  2. #12

    Expert

    Nov 30, 2010
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    Quote: The total output of all channels is +/- 20 ma. Page 12, operating instructions and Specifications. This means you can parallel 2 channels and get the current you need.

    Check it out and see if I read it correctly.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You could use (an) opamp(s) wired as voltage followers/buffers to increase the current.
    To wire an opamp channel as a voltage follower, wire the output to the inverting (-) input, and provide the input signal to the non-inverting (+) input.

    You'll need an external power supply, of course. +12v/-12v would probably work with most opamps that are designed for dual supply operation.

    L2722 dual power opamps will output up to 1A, and are quite inexpensive.
     
  4. j3r

    Thread Starter New Member

    Jan 9, 2011
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    No we can't as the MAX output total current is limited to +/- 10mA.
     
  5. j3r

    Thread Starter New Member

    Jan 9, 2011
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    Thanks. I'm looking into this.
     
  6. j3r

    Thread Starter New Member

    Jan 9, 2011
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    I saw the datasheet, do you mean cascade the 2 op amps?
     
  7. #12

    Expert

    Nov 30, 2010
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    Page 13 confirms your statement. The max total output is 20 ma, but page 13 says you can't parallel for current, you can only do series connections for higher voltage.

    Back to the drawing board!
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    What is the motor?

    Is it rated for ±20mA at a certain voltage?

    Typically, motors do not need to be current limited, only voltage regulated, which is why I ask.
     
  9. #12

    Expert

    Nov 30, 2010
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    This is the basic form you will need to use. I forgot to draw the connection from AO- to ground.

    Now to select a center tapped transformer, full wave bridge rectifier, filter capacitors, and an op-amp that will deliver at least +/- 10 volts and 20ma (as instructed by the AO terminals).

    There is room to tighten up the ripple, but motors are not very picky about that and op-amps are rather good at refusing to let the ripple through.

    It's 2:30 am at my house. I must go to sleep now. C ya 2morrow.
     
    Last edited: Jan 10, 2011
  10. j3r

    Thread Starter New Member

    Jan 9, 2011
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    0
    I don't have much info about the motor (sadly not even the dataheet :( ), basically its a thruster used in ROVs. I tested various voltages to achieve +/- 10mA as of now, the voltmeter reads +/- 1.8V.

    I understand what you mean, since the load ( ie. motor) is fixed, therefore varying the voltage will induce varying current which drives the motor at different speed / direction.

    With the current module i had from NI, i'm only able to go up to 10mA.

    The only info i had is the +/- 20mA. Sorry about the little info i have at hand.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    No, I did not mean to cascade the 2 opamps.

    Take a look at page 5 of the L2722 datasheet for some motor control ideas.
     
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