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TINA/Spice/LM741: LM741 for Constant Current Source

Sheldon Maciel
Intellectual 400 points
Part Number: LM741
Other Parts Discussed in Thread: TINA-TI, , LM321

Tool/software: TINA-TI or Spice Models

Hello,

I am designing a constant current source (technically sink in my case) and wanted to make it as simple as possible, accept a large input range, and be inexpensive while being adjustable for different currents by changing gain resistors and the MOSFET. I designed and simulated it in TINA-TI and got it to work for my needs, but when I simulate it on a breadboard (used a different MOSFET, but I didn't see mine in TINA) it didn't hold a constant current.

I am attaching my schematic, simulation results, and spice model to make it easier to help.

I am not opposed to changing the op-amp if there is a better instrument op-amp, etc. I just choose that one because it seemed like it could work and was inexpensive.

Thanks in advance,

Sheldon Maciel

LM741_ConstantCurrent.TSC

  • 0
    Intellectual 400 points

    I modified the design slightly and amplified the voltage from the shunt with another LM741:

    But now that I have 2 op-amps, do you think it is a good idea to just use a better single op-amp?

    Thanks,

    Sheldon

  • 0 in reply to Sheldon Maciel
    TI__Guru**** 164796 points

    Sheldon,

    The input range for LM741 does not include ground (pin 4). Datasheet recommends 3V higher than ground for input.

    Change op amp search to include rail to rail "IN to V-" checkbox
    www.ti.com/.../products.html
    LM321 is the low cost device.

    You should also consider a bypass cap for op amp supply and add option for frequency compensation in the output lopp.

  • 0 in reply to Ron Michallick
    Intellectual 400 points
    Hi Ronald,

    Thanks very much for your reply!

    I can't believe I overlooked that! I'll be to read the datasheet more carefull next time!


    Thanks for the LM321 suggestion for an op-amp. I'll need to simulate it and double check it works (I think it will). Great idea, yeah I need to add a bypass cap (100nF should work).

    Yes, I'll try doing the frequency compensation. Do you mean I need to modify my schematic or do I need to do that in the search option?

    Thanks,
    Sheldon