Problem with D to A converter for DC motor drive.

Discussion in 'The Projects Forum' started by lam58, Mar 5, 2014.

  1. lam58

    Thread Starter Member

    Jan 3, 2014
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    0
    I'm trying to design a digital to analogue converter and voltage controller for a DC motor to be driven by an arduino (pictured below). For the D to A converter (summing amp) I'm using a 741 op-amp, the issue is I can only use a 6V battery for the VCC rail voltages on the op amp. However, the arduino supplies 5v per output and I have 6 outputs from the arduino, hence if more than one output is used the op amp saturates/doesn't work how I want it to. I tried adding a voltage divider to reduce the signal voltage into the positive terminal of the op amp however it still saturates. Does anyone have any hints to help me get over this issue. I can only use the equipment I have quoted.
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi lam,
    I have run your DAC in LTSpice , you do not require the 14/1k divider on the input to the 741, it gives a +5Vout when all the 6 Inputs are High and 0Vout when all inputs are Low

    I suspect that your problem is the power output OPA supplies are 'floating' ie: there is no Common connection between the two OPA's.

    E.
    If you want to see the simulation, ask.:)
     
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  3. lam58

    Thread Starter Member

    Jan 3, 2014
    69
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    Yeah, could you show the simulation it would be much appreciated. :D
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi,
    The first image is 'raw', the different value resistors are applying a different load to the 4024 counter used as a digital source, so there are unwanted 'steps' in the Vout.

    eg: the 1K resistor loads the PIC pin more than say the 16K pin, so the PIC pin input drive voltage will be lower for the 1K, 2K etc,, do you follow that.?

    The second image I have added some loading on the digital inputs in order to show the problem you may encounter when using the arduino , ie: unequal pin loading.

    E
     
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  5. crutschow

    Expert

    Mar 14, 2008
    13,002
    3,232
    Higher resistor values, such as increasing each by a factor of 10, will minimize the loading effect.
     
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  6. lam58

    Thread Starter Member

    Jan 3, 2014
    69
    0
    Thanks I'm re-testing again today, let you know how it goes. :)
     
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