opto coupelr output volt

Discussion in 'General Electronics Chat' started by z3ngew, May 12, 2013.

  1. z3ngew

    Thread Starter New Member

    Apr 18, 2013
    Hello everyone,
    I am trying to design a circuit that controls the state of a 12v relay using a microcontroller, the relay is operating using different source than the microcontroller, i've used an opto-coupler as an interface between input and output signal, but, the problem is that the opto-coupler is not transmitting the 12v needed for the relay to operate.
    In the attachments there is a schematic of the circuit, View attachment opto.zip

    Note: the optocoupler used is PC817

    Thanks in advance,
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Last edited: May 12, 2013
  3. altisclimax

    New Member

    May 12, 2013
    Maybe you can try to put the relay at the upper part (collector part).
  4. crutschow


    Mar 14, 2008
    Did you read the opt coupler data sheet? The maximum input current is 50mA and the minimum transfer ratio is 50% so the worst-case maximum output current is 25mA.

    You circuit shows a 330Ω resistor in series with the opto input which gives about 11mA with a 5V signal so the worst-case output current is about 6.5mA, likely not enough to pull in the relay.

    Bottom line is you probably need to add a transistor gain stage at the opto output to provide sufficient current to pull in the relay.
    z3ngew likes this.
  5. sheldons

    Active Member

    Oct 26, 2011
    use any led/transistor type opto with this circuit here to switch your relay
    Last edited: May 19, 2013