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    Adjustable voltage controlled current source with negative values

    I have a MEMS mirror which can tilt based on the input current. The current can vary between -15mA to +15mA in order to change the angle of the mirror from -15 to +15 degree. Now, I need to have an adjustable voltage-controlled current source that can also provide negative values. I prefer to use Raspberry pi or Arduino. I appreciate any help.

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    Re: Adjustable voltage controlled current source with negative values

    You need a Current Charge Pump that sources and Sinks a current. I believe this current pump will change the current linearly between -15 to 15 mA Right ?
    In this case, you should find a solution Voltage-to-Current Converter to drive this Charge Pump.



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    Re: Adjustable voltage controlled current source with negative values

    Depends on the input resistance of this MEMS (is it linear? I have never used one), but you could use maybe a resistor and a variable voltage source to create the necessary current values.
    The voltage source needs negative value too, with dual supply it is easy. If you don't have dual supply maybe a bridge-circuit from stereo potentiometer or a differentially variable voltage source could solve it.
    It would help I think if you shared more data about the MEMS and your application, how accurate tuning you need, etc.
    "Try SCE to AUX." /John Aaron/



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  4. #4
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    Re: Adjustable voltage controlled current source with negative values

    You could likely use a Howland op amp current source, since it can generate plus and minus currents.
    What is the resistance of the load?
    What power supply voltages do you have available?
    Zapper
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    Re: Adjustable voltage controlled current source with negative values

    Here's the LTspice simulation of a representative Howland current source that generates ±15mA pk constant-current into a load:
    Notice that the load current (yellow trace) remains constant as the load resistance increases (red trace) until saturation.
    For the op amp and supply voltage shown, it saturates at a load resistance of 1k ohm and above.
    The input is fully differential, and either input can be connected to ground, as desired.

    Click image for larger version. 

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    Zapper
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    Re: Adjustable voltage controlled current source with negative values

    If you need to derive a -5v supply from an existing +5v supply then this is a popular method:

    Click image for larger version. 

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    For the load you install your circuit which needs between +5 and -5v (referenced to 0v ground).

    The low-ohm resistor at the 555 output is optional although it is a good idea in order to reduce spikey jolts in the system.



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  7. #7
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    Re: Adjustable voltage controlled current source with negative values

    Quote Originally Posted by BradtheRad View Post
    If you need to derive a -5v supply from an existing +5v supply then this is a popular method:
    It can be also used to generate -12V or -15V from +12V or +15V respectively.
    Zapper
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    Re: Adjustable voltage controlled current source with negative values

    Here's a corrected schematic with a higher voltage op amp.
    The LMC6484 is only rated for 16V max.

    Click image for larger version. 

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    Last edited by crutschow; 29th April 2019 at 20:40.
    Zapper
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