Discussion in 'The Projects Forum' started by anoman38, Apr 8, 2012.

  1. anoman38

    Thread Starter New Member

    Dec 28, 2011
    I'm currently using FOD2711A optocoupler as an isolation between the PWM generator and the rest of the circuit. I connected the optocoupler as shown bellow. My problem is that the output voltage is not the normal logic signal. At frequency less than 5KHZ it works nice since the output voltage is a logic signal with 10V peak but in the case of high frequency the Vout looks like a capacitor charging voltage( goes up with delay). Attached to you Can my circuit. you please help me.
    Last edited: Apr 8, 2012
  2. rfredel


    Jan 23, 2011

    I think, the Load-resistor of the transistor is to high, Look for the datasheet at page 10. There you see that the voltage gain quickly goes down. It's better to use a resistant with 100 or 500 Ohms.



    PS: Sorry, if my English is not so good
  3. anoman38

    Thread Starter New Member

    Dec 28, 2011
    I have tried as you mentioned but problem still unsolved.
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
    1. What exactly did you try? What resistor value?

    2. You are driving the LED with a current near its maximum rating , decrease it to 10mA.

    3. Then consider for the collector resistor calculation a CTR of 100%.

    4. What is connected to the output (apart from the resistor)? Maybe some capacitive load?

    5. At what frequency do you want to drive the opto?
  5. anoman38

    Thread Starter New Member

    Dec 28, 2011
    Dear praondevou

    The input resistor is selected to be 350ohm, so the LED current is 10mA. The CTR is considered 100%. the collector resistor is 700ohm. I'm checking the output voltage of the optocoupler without connecting the output pin to any load. Just connect the oscope between the pins 2 and 3. I couldn't find a better output signal. The switching frequency is 40KHZ.