How do you select the right interface relay for a PLC output? Match four things in order: the output's electrical type (sourcing, sinking, relay, or triac), the coil voltage and current draw of the relay against the PLC card's per-point rating, the contact rating against the actual load, and the socket/terminal style against the panel's wiring standard. Get the coil match wrong and the output card overheats or fails silently over months. Get the contact match wrong and the relay welds shut or burns through in the first few thousand cycles. This guide walks through PLC output types, coil sizing, contact sizing, contact configuration, socket choice, and channel density, then closes with a brand comparison and five common selection questions.
Start With the PLC Output, Not the Relay
Most selection mistakes begin at the wrong end of the circuit. Engineers pick a relay they used last time and hope it fits. Start instead with what the PLC output card actually does electrically.
A transistor output card is either sourcing (switches the +24 VDC rail to the load, load returns to 0 V) or sinking (switches the 0 V return, load is fed from +24 VDC permanently). A relay output card has its own dry contact and needs no coil-side matching at all — it just needs a contact rated for the interface relay's coil current. A triac output switches AC directly and has a small leakage current even when off, which can be enough to hold a very low-power AC coil partially energized.
What we see in the field: panels wired with sourcing-output PLCs driving relays selected for sinking logic still work, because the coil doesn't care which side is switched, but the wiring gets confusing for whoever troubleshoots it in five years. Keep the polarity convention consistent across the panel.
Match Coil Voltage and Current to the Output Card
Coil voltage has to equal the PLC output's supply rail, almost always 24 VDC on modern I/O. Coil current is the number that gets skipped, and it's the one that causes field failures.
Formula: Relay Coil Current Draw — Source: general electromagnetic relay sizing per IEC 61810
Icoil = Pcoil / Vcoil
| Symbol | Description | Unit |
|---|---|---|
| Icoil | Coil current drawn from the PLC output | A |
| Pcoil | Rated coil power (datasheet value) | W |
| Vcoil | Rated coil voltage | V |
A typical 24 VDC miniature interface relay coil sits at a few hundred mW, which works out to roughly 15-40 mA. That looks trivial against a transistor output rated 0.5 A per point — until you add inrush and the fact that many output cards share a common current budget across all points on the same connector, not just a per-point limit. Sum the coil currents of every relay on a shared connector and check that total against the card's group rating, not just the individual point rating.
This depends on how many other loads share the same 24 V bus rail as the PLC output card. A rail sagging under a VFD control supply or a solenoid bank can drop coil voltage below pull-in on the far relay in the chain, even though the PLC's own output logic reads fine.
Size the Contact for the Load, Not the Signal
The signal side is 24 V and milliamps. The load side is where the relay actually does work, and it's rated separately for AC and DC because the physics of interrupting current differs sharply between the two.
AC contact ratings benefit from the natural zero-crossing every half-cycle, which helps the arc self-extinguish. A typical miniature interface relay contact might carry 6 A at 250 VAC, AC1 (resistive). DC has no zero-crossing, so the same physical contact breaks a fraction of that current at a given DC voltage, and the derating gets steeper as voltage climbs. A contact happily switching 6 A at 24 VDC may be rated for well under 1 A at 110 VDC.
Inductive loads — contactor coils, solenoid valves, small motors — also demand a look at the AC1/AC15 or DC1/DC13 utilization category, not the raw resistive AC1 number. A relay sized purely on resistive-load current will wear out fast on an inductive coil load because the inrush and the collapsing-field spike both stress the contact far beyond steady-state current.
Pick Contact Configuration and Count
Most PLC interfacing needs one change-over contact per output point: 1 CO (SPDT) switches the load and, if wired to the second throw, can feed back a wire-break or contact-weld status to another input. 2 CO (DPDT) covers two independent circuits from one coil, useful for interlocking two loads from a single PLC bit. 4 CO exists on power plug-in relays for cases where one signal has to fan out to several isolated circuits at once.
Don't over-spec contact count as a default. A 4 CO relay in a socket sized for it costs more, takes more DIN-rail length, and leaves unused poles that someone eventually miswires during a panel modification. Match count to the actual number of circuits the point needs to touch.
Choose the Socket and Terminal Type
The socket, not the relay, usually sets the practical current and wire-gauge limit, and it's the part that stays in the panel for the relay's entire service life while the relay itself gets swapped when it wears out.
Screw terminals are familiar, re-torquable, and easy to inspect during a maintenance walk-down. Push-in (spring-cage) terminals wire faster on a large I/O rack and hold torque under vibration without a re-tightening schedule, which matters on mobile equipment or near rotating machinery. Neither is universally correct; match it to how the panel gets maintained, not just how it gets built.
Decide on Slim vs. Standard Width for Channel Density
A control panel with 32, 64, or 96 discrete I/O points cannot afford a standard 12.5-17.5 mm relay per point — the DIN rail runs out before the enclosure does. Slim interface relays at roughly 6-6.2 mm width exist specifically to sit one-per-channel next to a PLC card without doubling the panel's footprint.
The trade-off is accessory ecosystem and contact count: slim types are usually limited to 1 CO or 2 CO and a narrower coil-power range than standard plug-in relays. For a handful of high-current or multi-pole points, mix in standard-width relays on the same rail rather than forcing every channel into the slim form factor.
Frequently Asked Questions
Does the interface relay's coil voltage have to exactly match the PLC output voltage?
Yes, within the relay's rated tolerance band. Most PLC discrete outputs run 24 VDC, so a 24 VDC coil is the default. Mixing a 12 VDC or 48 VDC coil onto a 24 VDC output either fails to pull in or over-drives the coil and shortens its life.
How do I know if my PLC output can drive the relay coil?
Compare the relay's rated coil current, calculated from coil power divided by coil voltage, against the output's per-point current rating and, just as important, its total current budget for the connector group the point belongs to.
Do I need a freewheel diode if the relay socket already has one?
No — if the socket accepts a clip-in suppression module and one is fitted, a separate diode across the coil terminals is redundant and, on some designs, wired backwards can short the output. Check which side of the coil the module protects before adding your own.
Can one interface relay switch both an AC load and a DC load?
Only if its datasheet lists ratings for both, since AC and DC breaking capacity are separate specifications for the same contact. Check the DC figure at your actual DC voltage, not the AC1 headline number, before assuming it covers a DC load.
Should I choose screw or push-in terminals for a PLC interface rack?
Push-in terminals wire faster on racks with many single-point connections and hold torque under vibration; screw terminals are easier to re-torque and inspect during routine maintenance. Either is valid — match it to how often the panel gets opened after commissioning.
Brand Comparison for PLC-Facing Interface Relays
| Criteria | Schneider Zelio RXM | ABB CR-P | Slim (RSL-type) |
|---|---|---|---|
| Typical CO count | 2 CO / 4 CO | 1-4 CO | 1 CO / 2 CO |
| Coil voltage range | 24 VDC and AC options | 24 VDC and AC options | 24 VDC common |
| Terminal style | Screw, push-in variants | Screw, push-in variants | Push-in common |
| Width per channel | Standard plug-in width | Standard plug-in width | ~6-6.2 mm slim |
| Best fit | General control interfacing | General control interfacing, wide accessory set | High-density PLC racks |
Both Schneider Zelio (RXM, and the slim RSL family) and ABB (CR-P, CR-M, CR-U) cover the same job at the series level: plug-in relays with LED and suppression modules on screw or push-in sockets. The practical differences show up in socket keying, accessory range, and how far down the slim-width line each brand takes its density options — check the current interface and control relays catalog for what's stocked before locking a design to one series.
Conclusion
Selection order matters more than any single spec sheet: PLC output type first, coil voltage and current second, contact rating against the real load third, contact count and configuration fourth, then socket and terminal style, then width for density. Skip a step and the failure shows up months later as a marginal coil, a welded contact, or a card that quietly throttles current across a connector group. For the broader picture of how these relays fit into a panel, see the interface and coupling relay engineering guide, and for the isolation and amplification role specifically, read how PLC interface relays isolate and amplify I/O. For coil-side detail, see relay coil voltages explained and relay contact configurations. Where an electromechanical relay isn't the right fit for the switching frequency, compare it against electromechanical vs. solid-state interface relays. When the load is a motor rather than a signal-level device, the part you actually want is a contactor, not an interface relay.