Types of Interface Relays: Plug-in, Slim and PCB
What are the different types of interface relays? Interface relays split into three physical formats — plug-in relays on DIN-rail sockets, slim relays 6-6.2 mm wide for high channel density, and PCB-mount relays soldered directly onto a board — plus an electrical split between electromechanical (EMR) and solid-state (SSR) switching, all built around the isolate-and-amplify function defined in IEC 61810 and IEC 60947-5-1. Pick the wrong format and a 32-channel I/O rack runs out of DIN-rail width, or a solid-state part gets specified into a load it cannot switch without a heat sink. This article covers plug-in miniature relays, slim relays, PCB relays, power plug-in variants, and the EMR/SSR split, then how panel layout and I/O count drive the choice between them.
Plug-In Relays on DIN-Rail Sockets
The plug-in relay is the default format in a control panel: an 11-pin or octal relay body that pushes into a matching DIN-rail plug-in relay and socket system, held by a spring clip. The socket, not the relay, carries the screw or push-in terminals and the printed wiring diagram. Schneider's RXM (2 CO / 4 CO) on RXZE sockets and ABB's CR-P and CR-M ranges on their matching sockets are the two lines stocked here. A clip-on LED and suppression module (diode for DC coils, RC or varistor for AC) sits on top of the socket, separate from the relay body.
Field replacement is the reason this format dominates: pull the relay, snap in a spare, and the panel is back in service without re-terminating a single wire. That single property is worth more than any coil-power spec when a maintenance electrician is standing in front of a tripped panel at 2 a.m.
Slim Interface Relays for High-Density I/O
Slim relays compress the same electromechanical function into a 6-6.2 mm wide housing so dozens fit side by side next to a PLC rack. Schneider's RSL and ABB's slim CR-P variants both target this footprint, usually with 1 CO or 2 CO contacts and push-in terminal options to speed up wiring on a panel with 64 or 96 discrete points. The trade-off is contact rating: slim bodies rarely exceed a few amps, so they suit signal-level and small-coil loads, not motor starters or large contactor coils.
What we see in the field: panels retrofitted from a relay output card to transistor outputs plus external interface relays almost always end up with slim types, because the channel count per meter of rail is the constraint, not the switching current.
PCB-Mount Interface Relays
PCB-mount relays solder directly onto a circuit board with no socket at all. They show up inside OEM control boxes, motor-starter modules, and building-automation controllers where the manufacturer wants the lowest unit cost at volume and controls the enclosure design end to end. ABB's CR-P range includes a PCB-footprint variant alongside the DIN-rail socket version, so the same relay family covers both mounting styles.
The cost of that lower unit price is serviceability: a failed PCB relay means desoldering and rework, or replacing the whole board, not pulling a part out of a clip. Specify PCB-mount only where the enclosure itself is treated as the replaceable unit.
Power Plug-In Relays: When 2-4 CO Isn't Enough Current
Above the miniature and slim ranges sit power plug-in relays — Schneider's RPM and RUMC, ABB's higher-current CR-M variants — built for loads a standard interface relay can't carry: larger contactor coils, small motor loads, and heater circuits switched directly rather than through a second stage. Contact arrangements run 1 CO to 4 CO, same as miniature relays, but the AC1 and DC current ratings step up, and the socket footprint grows to match.
Some panel builders default to a power plug-in relay everywhere "to be safe," but oversizing the contact set doesn't buy anything if the coil driving it is still a 24 VDC, few-mA PLC output — see relay coil voltages for how the coil side is rated independent of the contact side.
Electromechanical vs Solid-State: A Different Axis of "Type"
Physical format is one axis; switching technology is another, and the two are independent. An electromechanical relay (EMR) has a coil and a moving armature that closes a real metal contact — it handles AC or DC loads with the same part and gives a visible, testable air gap when open. A solid-state relay (SSR) switches with a triac or MOSFET behind an opto-isolated input, has no moving parts to wear or bounce, and switches fast enough for high-cycle or PWM-style loads, but leaks a small current when "off" and needs heat-sinking as load current climbs.
Both formats exist as plug-in and slim types; PCB-mount SSRs are common in OEM equipment for the same volume-cost reason as PCB EMRs. Choosing between them is covered in full in electromechanical vs solid-state interface relays — in short, match the technology to switching frequency and load type, then pick the physical format from the sections above.
Formula: Relay Coil Power — Source: IEC 61810, coil-rating basis
Pcoil = Vcoil x Icoil
| Symbol | Description | Unit |
|---|---|---|
| Pcoil | Coil power consumption | W |
| Vcoil | Rated coil voltage | V |
| Icoil | Rated coil current at Vcoil | A |
Coil power scales with format: a slim interface relay typically pulls a few hundred mW, a power plug-in relay's coil can approach or exceed 1 W. On a PLC output card with a total current budget across all channels, that difference decides how many outputs can be driven directly versus through a buffer stage.
Matching Relay Type to Panel Layout and I/O Count
Start from the constraint that actually bites: DIN-rail length, contact current, or field-service policy. A panel with under 20 discrete outputs and a maintenance team that swaps parts rather than boards almost always lands on standard plug-in miniature relays. A panel with 60+ digital points crammed next to a PLC rack needs slim types or it won't fit the enclosure. A high-volume OEM product with a sealed housing and no user-serviceable parts is a PCB-mount candidate from day one.
This depends on whether the load itself needs amplifying at all, or just isolating — a 24 VDC solenoid within the relay's contact rating only needs isolation, while a contactor coil at 110 VAC through a 24 VDC PLC output needs both isolation and amplification, which is the core job described in what an interface relay is and how it works.
Contact configuration also narrows the field: a single control point only needs 1 CO, but an interlock circuit driving two separate downstream loads from one command needs 2 CO or more. That selection logic is broken out fully in relay contact configurations.
| Criteria | Plug-In (DIN-Rail) | Slim | PCB-Mount |
|---|---|---|---|
| Typical width | ~15-22 mm per socket | 6-6.2 mm | Board footprint only |
| Field replacement | Yes, pull and swap | Yes, pull and swap | No, soldered |
| Typical CO count | 1-4 CO | 1-2 CO | 1-2 CO |
| Best fit | General panel control, low channel count | High-density PLC I/O racks | OEM equipment, sealed enclosures |
| Terminal type | Screw or push-in on socket | Push-in common | Solder pins, no terminal |
For everyday panel work, browsing stocked interface and control relays by contact count and coil voltage is faster than starting from a datasheet search — the format decision above narrows the list before a single part number gets checked.
Frequently Asked Questions
What is the difference between a plug-in and a slim interface relay?
Both mount on a socket and are field-replaceable. The difference is width and current: plug-in miniature relays run wider (room for up to 4 CO) while slim types compress to 6-6.2 mm at the cost of contact rating, trading current capacity for channel density.
Can a PCB-mount relay be replaced without desoldering?
No. A PCB relay is soldered to the board with no socket, so replacement means desoldering the part or swapping the whole board assembly. That is the trade-off for its lower unit cost in volume production.
Are power plug-in relays the same as contactors?
No. A power plug-in relay is still an interface-class device rated for coil, solenoid, or small-load switching, not motor loads under IEC 60947-4-1. For motor circuits, see contactors instead.
Do slim interface relays support both AC and DC coils?
Yes, within the electromechanical versions — coil voltage is a separate spec from physical width. Both AC and DC coil options exist in slim housings; check the specific coil voltage code on the part.
Which type should I use for a high-density PLC output rack?
Slim interface relays, generally with push-in terminals, since the constraint is DIN-rail length per channel rather than contact current. Standard plug-in relays are the better choice once channel count drops and field-service speed matters more than density.
Conclusion
Three physical formats — plug-in, slim, PCB-mount — and two switching technologies — EMR, SSR — cover essentially every interface-relay application in a control panel. The choice isn't about which is "better"; it's about which constraint governs the panel: rail width, field-service policy, or unit cost at volume. Get the format right first, then work through coil voltage, contact configuration, and coil suppression as detailed in the rest of the interface and coupling relay engineering guide.
