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

    Active Member

    Oct 26, 2011
    539
    99
    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

    Active Member

    Oct 26, 2011
    539
    99
    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
    7,050
    656
    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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