Solenoid valves, contactor coils and other inductive loads store energy in a magnetic field while current is flowing. When the circuit opens, the field collapses and the stored energy produces a voltage transient. If the switching device and suppression method are poorly matched to the load, that transient can stress a PLC output, relay or surrounding electronics.
Start with the load, not the nominal current alone
Two coils with the same steady-state current can behave very differently. Before selecting an interface relay, collect at least:
- AC or DC load voltage;
- continuous current and inrush current;
- coil power or VA;
- inductance, L/R time constant or stored energy when available;
- required release time and switching frequency;
- the suppression already fitted to the load.
The relay datasheet must explicitly support the load. A current rating stated for a resistive load is not automatically valid for an inductive coil.
What the interface relay protects
An interface relay separates the PLC channel from the field load and lets the relay’s output stage perform the switching. This reduces direct electrical stress on the controller output and makes the field interface replaceable. It does not remove the need for correct fusing, suppression, wiring and thermal design.
Delcon output relays use pulse-transformer galvanic isolation between the control and load sides. Product-dependent internal protection is specified in the datasheet, but external suppression may still be required. Always follow the instruction for the selected model.
DC coils: suppression changes release behaviour
A diode connected across a DC coil limits the turn-off voltage, but it also lets coil current decay more slowly. That can lengthen the mechanical release time of a valve or contactor. Other suppression networks can permit a higher clamp voltage and faster release, but they must be designed for the switching device and application.
For example, the SLO24CR datasheet specifies a maximum inductive load of L/R 5 ms at 24 V / 2 A and a 0–3 A continuous load range. The SLO24CRA5 is designed for more demanding DC loads and specifies L/R 50 ms at 24 V / 5 A. These figures are model-specific selection limits—not general ratings for every Delcon relay.
AC coils require a different check
AC contactor and solenoid coils can draw substantial inrush current before the magnetic circuit closes. Select the relay from the coil manufacturer’s inrush VA/current and the relay’s surge-current specification. Also verify the actual switching frequency and average thermal load. A relay that survives one operation may still overheat in rapid repetitive service.
The SLO24TR is one example of an AC-output relay. Its datasheet should be checked against the coil voltage, inrush and duty cycle before use.
A practical protection checklist
- Identify whether the load is AC or DC.
- Obtain steady-state and inrush values from the load manufacturer.
- Check the relay’s load type, continuous current, surge current and inductive-load conditions.
- Choose external suppression from the required release time and the relay datasheet.
- Provide short-circuit/overload protection with a correctly rated fast fuse where specified.
- Apply temperature, grouping and mounting derating.
- Test the complete circuit under real operating conditions.
See Delcon application guidance or send Delcon the load datasheet and switching conditions for a product recommendation.