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Digital Input Circuit

  1. Oct 23, 2012 #1
    Please see attached schematic.

    Basically the diagram shows a single channel of a 32-channel digital input card for an industrial controller.

    My diagram represents this configuration with the 24V supply externally sourced to terminals 65 & 66.

    The question is what is the purpose of the 220Ω resistor?

    Also, the voltage between the switch (the input) and ground is less than 24V because of this resistor. So calling it a 24VDC input is not strictly true am I right?

    Finally, I am assuming the unlabelled resistor in my diagram has two purposes:

    1. a current limiter for the LED
    2. a pullup resistor for the logic circuit

    Am I correct?


    Attached Files:

  2. jcsd
  3. Oct 23, 2012 #2
    The 220 is a Current Limiting resistor - consider all of the inputs are closed at once, the draw on the Power Supply will be too high, the extra resistor on the opto-coupler LED - limits the current for that device, the opto may also be rated for different voltage.

    When the external contact ( Customer Wiring) is open, there will be a full 24V there. As a digital 24 V system this is valid - when the switch closes, you will not have 24 V across the contacts.

    All you need to worry about is the Customer Contact - the internal diagram is for reference, for example if you were using solid-state contact ( line NPN) it shows how to be connected.
  4. Oct 23, 2012 #3
    I thought the resistor R was to limit the current for the opto-coupler LED? Assuming this is the case are you also saying the the 220 resistor is to make sure the voltage rating of that device is not exceeded as well as limit the current drawn from the supply?

    With regards to the Voltage AB, I get what you are saying about the open circuit voltage being 24V, but is my formula correct for the closed circuit voltage? (again, assuming no volt drop across the opto-coupler).

    I realise these questions are unnecessary for just simply wiring the card up but I'd like to know the full details anyway just for reference.

    Thanks for helping
  5. Oct 23, 2012 #4
    When the switch is closed the voltage at the switch is between 0 and 24 V --- it is a 24V Digital Input Card - the inputs individually can not "take" 24 V INPUT - this is a standard way these cards are configured. The wiring, switches etc. that you use need to be 24V or greater.
  6. Oct 23, 2012 #5
    Can I draw your attention to the sketch on the second page of the PDF? I appreciate that it's a 24VDC input card and by externally powering the inputs I do agree the voltage across the input terminals will be 24V however in my configuration I am connecting the 'internal 24V supply' to the 'external 24V supply' which according to the manual is acceptable.

    Thus you end up with a circuit as shown in my sketch. In which case the 24V is dropped across the 220 resistor and when the circuit is closed, is split between 220 and R.

    Sorry I did not make this clear, my intention is to use the internal 24VDC supply by linking the terminals.
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