MOSFET Control

Discussion in 'General Electronics Chat' started by Tim Saunders, Jun 17, 2009.

  1. Tim Saunders

    Thread Starter Member

    Apr 28, 2009
    10
    0
    Hi
    I'm building a circuit like so:

    8-bit DAC > OP-AMP(as a buffer) > MOSFET > 555 circuit > Servo

    I use the MOSFET to control current which in turn controls timing.
    Since the MOSFET is only useful from 4V (threshold voltage) to 5V and the DAC puts out 0V to 5V, I lose 204 degrees of accuracy. How could I solve this problem?

    -The DAC is home made so I could modify it, it's a R-2R setup.
    -It's a power MOSFET, is it possible to get low threshold voltage FET?

    It seems like there should be a simple solution, but I just can't think of it!
    I'm interested in learning so if this whole thing seems like a stupid approach (maybe a MOSFET is not the right tool) please tell me.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Can you post the schematic of the circuit? It's really hard to guess what is going on.
     
  3. Tim Saunders

    Thread Starter Member

    Apr 28, 2009
    10
    0
    All I really need to know is how I can change a 0V-5V signal to a 4V-5V signal. I'm using the MOSFET like a variable resistor. Let's see how my drawing skills are:
     
    Last edited: Jun 17, 2009
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Tim,
    Create/save your image in .PNG format. .jpg format is too lossy. MS Paint can save in .png.

    Make sure the image is under 100k before uploading it.
     
  5. Tim Saunders

    Thread Starter Member

    Apr 28, 2009
    10
    0
    Sorry it looked fine on my screen. Here is the PNG image:

    [​IMG]

    Is it better? I could draw it in LTspice if that's better
     
  6. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    Add these two resistors to your OpAmp.
     
  7. AdrianN

    Active Member

    Apr 27, 2009
    97
    1
    Last edited: Dec 6, 2009
  8. Tim Saunders

    Thread Starter Member

    Apr 28, 2009
    10
    0
    Geezz, Of coarse! Lol. I don't know why I couldn't think of that!
    Hey thanks alot eh, I really appreciate it.
     
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