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ISO1211: ISO1212 /ISO1211 LED on input Side

Thomas Donner
Prodigy 130 points
Part Number: ISO1211

Hi

In the past we attached a status LED on the input side of the 24V input ( close to the connector).
Is there a suggested position where I should connect the LED to get a clear high/low feedback?

Kind regards

Thomas

  • 0
    TI__Mastermind 49485 points

    Hi Thomas,

    Welcome to TI E2E Forum!

    Thank you for showing interest in ISO121x. May I know what is the current consumed by the LED? I hope the 24V input will be able to support the current required by both the LED and ISO121x and hope this doesn't violate IEC 61131-2 specs.

    If the current requirement is met by the source then the position of LED shouldn't affect operation of ISO121x, hence you can decide the position of LED according to your convenience. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 130 points

    Hi

    my idee wasn´t to place the LED paralell to the ISO121x, but in serial with this chip.

    Perhaps one of the three pins has can have a good behavioral in the current flow for the LED to get a good ON/OFF switching behavioral.
    But this has effects on the switching point of the output.

    What du you mean about this?


    Kind Regards

    Thomas

  • 0 in reply to Thomas Donner
    TI__Mastermind 49485 points

    Hi Thomas,

    Apologies for the delay in my response and thanks for the clarification.

    Yes, the LED can also be connected in series and the device will still be able to operate fine provided the voltage drop of LED isn't much and the LED turns ON with the current that device is meant to sink. The only difference that you would notice is that the HIGH & LOW thresholds will be increased to higher voltages by the LED voltage drop. The LED voltage drop variation with current and temperature will also vary the HIGH/LOW thresholds of ISO121x accordingly.

    Please do let us know the turn ON voltage / current of the LED and/or share the LED part number. The HIGH/LOW thresholds of ISO121x can also be designed to accommodate the average voltage drop of LED thereby minimizing the increase in HIGH/LOW thresholds from their desired values.

    Let me know if you have any questions, thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Mastermind 49485 points

    Hi Thomas,

    One additional challenge that you would face by adding LED in series is that, based on the LED turn ON voltage/current the LED may turn ON even before the ISO121x HIGH threshold voltage is reached. You may still try to find an LED whose turn ON thresholds match that of the set HIGH thresholds of ISO121x but it will be a challenge to consider all the dependencies and find an LED.

    Another challenge is that the ISO121x input is Schmitt input where the HIGH and LOW thresholds are defined while the LED may transition between ON & OFF at one particular voltage. This will also lead to wrong status indication by the LED.

    An alternative approach to accurately indicate the input status is to connect the LED on the output pin of ISO121x as given in the below diagram taken from Figure 26 of ISO121x datasheet. Even though it is connected on the output side, you could still position the LED physically to a location that is convenient to you using connecting wires. I think this is also a common practice followed in the industry as this enables accurate input status indication.

    Let me know if you have any questions, thank you.

    Regards,
    Koteshwar Rao