Driving Inductive Loads with Digital I/O

    Our range of digital I/O modules can be used to drive inductive loads such as the coils of large relays or contactors. The output stage of digital I/O modules like 40-412 include protection circuits which current and voltage limit and in the event of a thermal overload will shut down the driver.

    To avoid the generation of EMC problems in a system when driving inductive loads it is best to include a component to manage the voltage transient created when the current in a coil is interrupted.

    High Side Drivers

    Diagram of the high side driver

    When using high side drivers on closure current will build up in the coil until it reaches a steady state current limited by the coil resistance. This does not create any great problems for the driver as the current build up is limited by the coil inductance.
    However, when the output is opened the current is interrupted, the discharge of the filed creates a large negative voltage transient. Digital I/O modules like the 40-412 have an internal diode that will clamp the out (D1), but the current discharges in a large loop formed by the connection from the coil to the 40-412, then through D1 and into the system ground. This forms a significant antenna in the system and generates a transient field and disturbs the ground. However, fitting a diode, D2, across the coil avoids any discharge through D1 and into the system ground. It creates less disturbance in the system.

    Low side Drivers

    Diagram of the low side driver

    When using low side drivers the circumstances are similar, in this case when current flow in the coil is interrupted the voltage on the coil rapidly rises. For a protected digital I/O module like 40-412 the voltage is clamped by an internal clamp diode, D1, but again in doing so discharges a current into the system ground and the connection loop emits a transient field. The current loop also disturbs the power supply through the power supply decoupling capacitors, which could be close to the coil but in some cases may be more remote, increasing the amount of disturbance created.
    This can be avoided by fitting a diode D2 across the coil, the current discharge is entirely local and does not disturb system ground.

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