Control Relay Selection
Control relay selection covers choosing small interposing (ice-cube) relays used to isolate PLC outputs from field loads, switch signals between different voltage domains, or provide extra contact multiplication where a single PLC output needs to drive several independent circuits.
- Isolating PLC digital outputs from higher-current or higher-voltage field loads.
- Interfacing 24V DC control logic with 110/230V AC field circuits.
- Multiplying a single PLC output into several isolated contacts for independent circuits.
- Providing an easily replaceable interface point between control logic and simple on/off field devices.
Selection depends on coil voltage (matching the driving output), contact configuration (SPDT, DPDT, or multi-pole), contact current rating for the switched load, and whether the application needs a plug-in socket base for easy field replacement. Relay response time, contact life at the expected switching frequency, and whether the load is resistive, inductive, or lamp/motor-inrush type all affect whether a standard ice-cube relay is adequate or a heavier-duty control relay or solid-state relay is a better fit.
- Driving a relay coil directly from a PLC output without checking the output's current capability against coil inrush.
- Selecting contact ratings based on resistive load tables when the actual load is inductive or has high inrush.
- Omitting a flyback diode or RC suppressor across DC coils, causing electrical noise or output damage.
- Using a fixed relay where a plug-in socket base would make future field maintenance far faster.
- Under-rating contact life for high-cycle applications, leading to premature relay failure.
ClusterVise checks each PLC output against its connected load type in the generated I/O and BOM, and proposes an interposing relay with matched coil voltage and contact rating automatically when a direct drive isn't appropriate — rather than surfacing the mismatch only after commissioning.
| Item | Selection | Basis |
|---|---|---|
| PLC output | 24V DC, 0.5A relay output card | Standard digital output module rating |
| Field load | 230V AC solenoid valve, 1.2A inrush | Requires voltage domain isolation from PLC |
| Selected relay | 24V DC coil, DPDT, 5A contact, plug-in base | Matches coil voltage and handles inrush safely |
| Suppression | RC snubber across contact side | Reduces arcing on inductive solenoid load |
| Maintenance access | Socket-mounted for tool-free replacement | Simplifies field-level relay swap |
