Relay Coil Voltages: 24VDC, 110VAC and 230VAC Explained
What coil voltage should you spec for an interface relay? The coil voltage (Un) is the rated value at which the relay's electromagnet pulls the armature in and holds the contacts over, per IEC 61810 — 24 VDC dominates PLC and DCS panels, while 110 VAC and 230 VAC remain common on legacy control circuits and panels wired straight off line voltage. Pick the wrong class and the relay either won't pull in reliably or draws power the driving output can't source. This article covers the three common coil classes, pull-in and drop-out voltage as a percentage of Un, coil power and heat load, matching coil voltage to a PLC or line-voltage control circuit, and where Schneider Zelio and ABB CR coils sit across these ranges.
Why Coil Voltage Is the First Spec to Check
Before contact rating, before CO count, coil voltage decides whether the relay energizes at all. A relay ordered with a 230 VAC coil sitting on a 24 VDC PLC output card will not pull in — the coil impedance and drive current are wrong for a DC source, and forcing it damages the output. Coil voltage is stamped on the relay body and the socket does not change it; swapping sockets never fixes a coil-voltage mismatch.
Un is also a range, not a single point. Manufacturers publish an operating window, typically 0.8 to 1.1 x Un for continuous duty, so a "24 VDC" coil is expected to hold in from roughly 19 to 26 V. Panels with sagging battery-backed 24 V rails or long cable runs need that margin checked, not assumed.
24 VDC: The PLC Panel Standard
Most interface relays sold into automation panels ship as 24 VDC. It matches the standard PLC I/O supply, sensor supply and DCS output-card voltage, so one coil voltage covers control power, field sensors and interface relays without a step-up or step-down transformer. Schneider Zelio RXM and ABB CR-P/CR-M ranges both lead with 24 VDC as the default coil, then offer AC options for panels that need them.
DC coils need a suppression element — a freewheel diode across the coil — because a PLC transistor output has no natural way to absorb the inductive kick when the coil de-energizes. Skip the diode and the transistor dies, sometimes not on the first cycle but on the hundredth. This is covered in depth in the relay coil suppression guide.
What we see in the field
Panels mixing 24 VDC interface relays with a handful of legacy 24 VAC devices on the same terminal strip are a recurring source of "intermittent" faults — the coils look identical on the DIN rail, but AC ripple on a DC-rated coil, or vice versa, shortens contact life well before the rated mechanical cycles are used up.
110 VAC and 230 VAC Coils: Where They Still Fit
Line-voltage coils still make sense where the control circuit is already wired at 110 VAC or 230 VAC and adding a 24 V supply just for a few interface relays isn't worth the extra transformer, fusing and wiring. Older motor control centers, retrofit panels, and building-services control circuits fed directly off single-phase mains are the typical cases.
AC coils need RC or varistor suppression rather than a diode, since the coil current reverses every half-cycle and a diode would simply short one polarity. The trade-off: AC coils draw a higher apparent power for the same mechanical work, and pull-in is less forgiving of low-voltage sag near the end of long branch circuits.
Pull-In and Drop-Out: The Formula Behind Coil Ratings
Two thresholds define whether a coil is doing its job: the voltage at which the armature pulls in and seals, and the lower voltage at which it releases. Both are specified relative to Un, not as fixed values, because the same relay family ships in multiple coil-voltage variants.
Formula: Pull-in and drop-out voltage — Source: IEC 61810-1, coil operating range
Vpull-in ≤ 0.8 × Un Vdrop-out ≥ 0.1–0.3 × Un
| Symbol | Description | Unit |
|---|---|---|
| Un | Rated coil voltage | V |
| Vpull-in | Maximum voltage at which the armature is guaranteed to seal | V |
| Vdrop-out | Minimum voltage at which the contacts are guaranteed to release | V |
The gap between pull-in and drop-out is deliberate — it's what stops a relay from chattering when the supply sits near the threshold. A 24 VDC coil with a supply sagging to 20 V during a motor-start inrush event is still inside the pull-in window; the same coil at 5 V is guaranteed to have dropped out. The failure mode to design against is a supply that hovers between the two, which produces contact chatter rather than a clean on or off.
Coil Power and Heat: What a Few Hundred mW Means in a Crowded Panel
Individual interface relay coils draw modestly — commonly in the 200 mW to 1 W range depending on series and coil voltage — but that number matters at scale. A terminal strip carrying forty interface relays for a large I/O count adds up to real heat inside an enclosure with no forced ventilation, and it adds up in current draw on the 24 VDC supply rail shared with sensors and PLC modules.
Formula: Coil power — Source: basic electrical power relation, applied to relay coil sizing
P = Un × Icoil
| Symbol | Description | Unit |
|---|---|---|
| P | Coil power dissipation | W |
| Un | Rated coil voltage | V |
| Icoil | Steady-state coil current at Un | A |
Sizing a 24 VDC supply for a panel with dozens of interface relays means summing each coil's steady-state draw, not just the PLC and sensor load. This is a checklist item, not a formality — undersized supplies drift low under combined load, and a coil sitting at 0.8 x Un with no margin left is one voltage dip away from dropping out mid-cycle.
Matching Coil Voltage to the Control Circuit
Coil-voltage selection is really control-circuit selection. A panel driven by PLC transistor outputs or DCS digital-output cards is 24 VDC by default — matching the field bus voltage keeps sensor, PLC and relay coil on one supply and one set of fusing. A panel switched by contactor auxiliary contacts or a line-voltage pilot circuit is often 110 VAC or 230 VAC, and forcing it to 24 VDC just adds a transformer with no functional benefit.
This depends on whether the PLC output sinks or sources current as much as it depends on coil voltage — a sourcing (PNP) 24 VDC output and a sinking (NPN) relay coil circuit need the wiring polarity checked before the first power-up, or the relay simply never sees forward voltage across the coil. See how to select an interface relay for PLC outputs for the selection sequence.
Socket and accessory compatibility
The socket itself is voltage-agnostic — it carries current and provides terminals — but LED indicator modules and suppression modules that clip into the socket are coil-voltage specific. A 24 VDC LED/diode module on a 230 VAC coil socket does nothing useful and can be damaged. Confirm the module part number matches the coil voltage, not just the socket footprint; see the plug-in relay and socket system guide for how relay, socket and accessory modules fit together.
Schneider Zelio and ABB CR Coil Voltage Ranges
Schneider's Zelio RXM and RSB ranges lead with 24 VDC and offer AC coil variants across common line voltages for panels wired off mains control circuits; the RSL slim range follows the same pattern in a narrower footprint for high channel density next to a PLC. ABB's CR-P and CR-M ranges mirror this — 24 VDC as the primary automation-panel coil, with AC-rated variants for line-voltage control circuits, and LED/suppression modules keyed to coil voltage on both. Neither brand publishes exact per-model coil-current figures that are worth repeating here; check the specific series datasheet for the coil you're speccing, since coil power varies by series and CO count within the same voltage class.
Both brands' interface relay ranges are built to IEC 61810 and IEC 60947-5-1, which is what lets a 24 VDC Zelio RXM and a 24 VDC ABB CR-P sit on functionally equivalent sockets in a mixed-brand panel, even though the socket footprints and accessory keying differ. Browse the current interface and control relays range for stocked coil-voltage options, and see the interface and coupling relay engineering guide for how coil voltage fits into the full relay selection picture alongside contact rating and socket type.
Frequently Asked Questions
What is the most common interface relay coil voltage?
24 VDC is the default in PLC and DCS panels because it matches the standard automation I/O and sensor supply voltage. 110 VAC and 230 VAC coils remain common where the control circuit is already line-voltage.
Can I use a 24 VAC coil where the panel is 24 VDC?
No. AC and DC coils are wound and suppressed differently — a DC coil needs a freewheel diode, an AC coil needs RC or varistor suppression — and applying the wrong waveform to a coil rated for the other will not produce reliable pull-in and can damage the driving output.
What happens if supply voltage drops below the relay's pull-in threshold?
Below roughly 0.8 x Un the armature is no longer guaranteed to seal fully, which can produce contact chatter, incomplete closure, or a relay that fails to energize at all under load.
Why does coil power matter if it's only a few hundred milliwatts?
A single coil's draw is small, but a terminal strip with dozens of interface relays adds meaningful heat inside the enclosure and meaningful current on the shared 24 VDC supply rail, which needs to be sized for the combined load, not just the PLC and sensors.
Do I need a different socket for a different coil voltage?
The socket itself carries current and terminals regardless of coil voltage, but LED indicator and suppression modules that clip into the socket are coil-voltage specific and must match the relay's rated coil voltage.
Conclusion
Coil voltage is the first filter in interface relay selection, not an afterthought after contact rating is chosen. Match it to the control circuit that drives it — 24 VDC for PLC and DCS panels, 110 VAC or 230 VAC where the circuit is already line-voltage — check pull-in and drop-out margin against actual terminal voltage, and size the shared supply for the combined coil load across the panel. Get the coil-voltage class right first, and contact rating, CO count and socket type follow from there.
