Requirement for adding diodes when switching inductive loads.

The latest Charms hardware manual shows a diode around the solenoid coil when connecting to a 24VDC isolated CHARM module. My understanding is this is to protect the solid state switch from the high reverse voltage spike caused by opening the inductive coil circuit. Older versions of the hardware manual did not show these diodes being needed. From those I have surveyed there seems to be a large number of installations that are not using these diodes. While the cost of the diodes themselves is low, the labor to add them to every solenoid on a DCS conversion project could add up quickly. My question to this forum is are you actually installing these and if not have you had any issues? FYI - the typical diode called out for this is a 1N4007.

4 Replies

  • Thanks for your question Dennis,

    This issue has been the same from the very first M-series DO card, so I assume it may have been left off the CHARMs hardware manual by mistake and I guess this is why it was added at a later state. The diode, sometimes referred to as a kickback or recirculating diode, is there to protect the CHARM from the iductive load generated in the solenoid. R-C (resistor-capacitor) snubbers can also be used with the same effect as diodes.

    I also want to mention that it is important that the diodes are insalled in the field, close to the solenoids, to stop the reversing voltage before it travels down cables in cable trays and possibly transfers to other instrument wires nearby.

    I am not a solenoid expert, but I beleive there are some solenoid valves that actually have the diodes build in.

    I would also like to hear from other users as to what solutions have been deployed on projects that have solenoids with inductive coils.

    Best regards,
  • Rune, are the CHARMS enclosures going to meet IP66 as earlier specified?  I hear that they are now only available in IP54.  We need minimum IP65 for our external CHARMS CTOs.
  • In reply to David Tudor Evans:


    Yes, we are testing the 12 CHARM I/O Junction Box to be IP66 and NEMA 4X and the PDS will update to reflect this as soon as certifications are received.

    Best regards,
  • Hi Dennis,
    Per KBA NK-1400-0115 revision March 6, 2018.
    This failure mode has not been experienced in the field but has the potential to occur at any time under the following conditions:
    A. When there is reverse polarity on the DO 24 VDC isolated CHARM power supply input power and there is a short
    circuit on the output.
    NOTE: Output circuit with snubbers installed DO NOT protect against reverse-polarity power input failure and
    the installed snubbers will appear as a short circuit to the output circuit.
    B. When removing or inserting a DO 24 VDC isolated CHARM while energized with an inductive load, with other DO
    24 VDC isolated CHARMS on the same baseplate which are also connected to inductive loads, while any of the
    inductive loads do not have snubbers installed for kick-back protection.

    Preventive Action
    3. If the end user wants the ability to remove the DO 24 VDC isolated CHARMwhile energized, AND if snubbers
    were not installed on these DO 24 VDC outputs connected to inductive loads, THEN snubbers should be installed
    on all DO 24 VDC isolated outputs with inductive loads on that baseplate.

    Final Action
    All inductive loads connected to DO 24 VDC isolated CHARMs should have adequate snubbers installed at the field
    device to prevent this issue.
    As Rune indicated, some inductive devices already have snubbers installed in them.
    Therefore asking what other people have experienced may not be a great source of assurance.
    Likely not the answer you were looking for though.

    Sal Salamone PE MBA | Application Engineer

    Novaspect Inc. | 4415 W Lawrence St. | Appleton | WI | 54914 | USA

    An Emerson Impact Partner