isolate output from input using optocoupler

Discussion in 'General Electronics Chat' started by karas, May 18, 2015.

  1. karas

    Thread Starter Active Member

    Sep 8, 2011
    View attachment 86000 I am trying to isolate input(0-5v) from output(V4) by using optocoupler but the circuit is not linear I need to make V4 the same like (0-5v), i need to do the circuit without using transformer so is there any suggestion to make the circuit work
  2. Reloadron

    Active Member

    Jan 15, 2015
    If you want to optically couple a linear signal you should use a Linear Optocoupler. Do a Google of "linear optocoupler" to get started.

    karas likes this.
  3. karas

    Thread Starter Active Member

    Sep 8, 2011
    thanks so much
  4. DickCappels


    Aug 21, 2008
  5. crutschow


    Mar 14, 2008
    Driving a standard opto coupler with a current rather than a voltage will significantly improve the input to output linearity since it eliminates the non-linear input voltage to current relationship.

    Below is the LTspice simulation of such a circuit.

    Op amp U3 is configured as a current driver for the opto input, generating a current proportional to the voltage input.

    As you can see the output is quite linear with respect to the input.
    The simulation results are somewhat idealized of course, and the real linearity will depend upon the actual characteristics of the opto coupler used, but hopefully will still be adequate for your application.
    The linearity degrades at higher operating currents due to reduction in the CTR of the opto but it should be fairly good if the maximum current is kept to no more than a few mA (2.5mA in this simulation).

    R3 will likely need to be tweaked to get the desired voltage output depending upon the gain of the particular opto isolator you have (you could make it a 5kΩ pot).
    A higher value increases the gain.

    The opamps are rail-rail types.

    Linear Opto.gif
    Last edited: May 20, 2015
    AnalogKid likes this.