Discussion in 'The Projects Forum' started by GARYN, Sep 30, 2009.

  1. GARYN

    Thread Starter Member

    Sep 4, 2009
    Hello all, can one of you experts possibly tell me if you connect up OPEN COLLECTOR OUTPUTS in PNP or NPN configuration. I was connecting up a Mitsubishi Invertor today and used volt free contacts from a relay to signal the invertor run signal instead of directly from the PLC Transistor outputs. And i used the volt free contacts on the invertor output to signal beacons instead of the available Transistor outputs.
    This is because as a Mechanical Eng with limited Electrical Experience the Transistor connections confuse me....but as i like to further myself could somebody possibly answer about OPEN COLL. being wired up in NPN, PNP configuration or both?
    I know if you use PNP you put 0v direct to the load and switch the positive and vice versa with NPN.
    Any easy to understand explanations and possible connection diagrams would be greatly appreciated, Thanks in advance.
  2. Wendy


    Mar 24, 2008
    Depends mostly on your power supply, I think. Open collector was originally for TTL, but I can't swear to that, in which case it was NPN and a positive power supply. Two open collectors tied together effectively make a NOR gate. If the power supply is a negitive polarity you need PNP.

    Nowdays they turn both transistors off, effectively switching the device out of the circuit.
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    Both NPN and PNP open collector outputs exist.

    NPN are common, both in logic circuits and higher voltage types such as Darlington driver ICs.

    PNP are more often used in industrial gear, such as proximity switches and PLC ouputs, where an input, relay or lamp needs a positive drive signal.

    Some older industrial stuff uses NPN, but this generally considered unsafe for control circuits as a ground fault turns things On instead of Off (or blowing a fuse).

    To remember which is which, NPN switches On to Negative (0V), while PNP switches On to Positive.

    Note that some types of gear have a switch or jumper to connect the common of the inputs to either 0V or 24V, so you can chose how to drive it. I'd always use the PNP mode (Switched Positive) option if possible.