Help with opamp config circuit

Discussion in 'General Electronics Chat' started by hasie, Mar 20, 2012.

  1. hasie

    Thread Starter Member

    Sep 28, 2009
    35
    0
    Hi guys.

    Im trying to build a small voltage supply for valve controllers which are very old. No info available for them. I have a old circuit which was built in the 1980s to help. But these are very old and dont work properly anymore.

    The circuit should have a -12v -> +2v output.
    I have +15v and a -15v available on the circuit.

    Im at a point where im struggling as I dont know op amps to well.(Apart from the very basic inverting and non inverting, comparator functions)

    What I have done is code a AVR to do all the functions(ramp and voltage control by potentiometer, and other unrelated functions to switch solenoids etc). I do this with PWM. Which I then send through a filter and it gives a very steady 0 - 5v analogue DC output.

    Would it be possible for anyone to help me or either put me in the right direction as to how I would convert this signal to a linear range -10v to +2v?

    My first thought was to just PWM and switch with a mosfet, but because the range crosses 0v, im unsure how to do that. So I thought OpAmps must be better, but as mentioned Im not too clued on opamps except for the very basic principles.


    Many thanks, any help or info much appreciated! :)

    hasie
     
  2. jimkeith

    Well-Known Member

    Oct 26, 2011
    541
    100
    What is the DC resistance of the valve armature coil?
    From that you may determine the current requirements.
    This is possibly a biased magnet design--hence the odd voltage requirement.
     
  3. hasie

    Thread Starter Member

    Sep 28, 2009
    35
    0
    I Actually dont know. I just got the old unit which is a standing old op amp type circuit with oscillator i guess.

    Current requirements not really a issue as the goes into the controller which actually drives the valve.

    I think I got sorted.
    Texas instruments have this very handy calculator!


    Will keep you posted.
    Many thanks
     
  4. jimkeith

    Well-Known Member

    Oct 26, 2011
    541
    100
    Try this circuit
     
  5. hasie

    Thread Starter Member

    Sep 28, 2009
    35
    0
    Thanks.

    I will give that one a try aswell.

    Currently Im using the attached.


    Many thanks :D
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,049
    659
    Your circuit has positive feedback, which will not do what you want. I suspect that your drawing just has the op amp inputs swapped.
     
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