A PLC output module switching a 24 V DC solenoid valve 20 times per minute will cycle 10 million times in a year. A relay output rated for 100,000 operations at 2 A resistive load will fail within two weeks at that rate — not because of a manufacturing defect, but because electromechanical contacts have a finite physical life determined by the arc energy they extinguish on every cycle. A solid-state output switching the same load will reach 100 million operations without degradation. Yet the solid-state output introduces a different problem: it leaks a small current in the off state — typically 0.1–2 mA — that can keep a high-impedance load partially energized or prevent a safety relay from dropping out. This article compares the two output technologies for general-purpose PLC and controller applications, focusing on the failure modes and load compatibility factors that determine which technology fits a given circuit.
How Relay Outputs Work — and How They Wear Out
A PLC relay output is a miniature electromechanical relay — a coil that pulls an armature to close or open a set of contacts. When the contacts open under load, the current flowing through the circuit does not stop instantly; it forms an arc across the separating contacts that sustains current flow until the contact gap is wide enough to extinguish it. This arc vaporizes a microscopic amount of contact material on every operation. Over tens of thousands of cycles, the contacts erode, the contact resistance increases, and eventually the contacts either weld closed or fail to make reliable electrical connection.
The wear rate depends on the switched voltage, current, and load type. A DC load is harder on contacts than an AC load because the DC arc does not self-extinguish at zero-crossing — it sustains until the contact gap is physically wide enough to quench it. An inductive load (solenoid, contactor coil, brake) is harder than a resistive load because the collapsing magnetic field generates a voltage spike that reignites the arc. The E+H RLN42 NAMUR isolating amplifier with relay output addresses one aspect of this challenge by providing line fault detection alongside the output, but the underlying relay contact wear physics apply regardless of the specific device.
How Solid-State Outputs Work — and Where They Leak
A solid-state output uses a semiconductor — typically a MOSFET for DC loads or a TRIAC/SCR for AC loads — as the switching element. There are no moving parts, no contacts to arc, and no wear mechanism tied to switching cycles. The only aging factors are thermal cycling of the semiconductor junction and voltage stress from transients. The published lifetime for a solid-state output is essentially the semiconductor junction lifetime — measured in decades under normal operating conditions.
The tradeoff is off-state leakage. A MOSFET in the off state has a drain-source resistance in the megaohm range, not an open circuit. A small leakage current — typically 0.1–2 mA for DC solid-state outputs, 1–5 mA for AC TRIAC outputs — flows through the load even when the output is commanded off. For a 24 V DC solenoid drawing 500 mA when energized, 1 mA of leakage is 0.2% of rated current and harmless. For a high-impedance input on a safety relay or a small pilot light, 1 mA may be enough to keep the device partially on, causing a safety function to fail to de-energize or an indicator to glow dimly when it should be dark. The load compatibility check for solid-state outputs is not voltage and current — it is whether the off-state leakage current is below the load's minimum holding or perception threshold.
Load-Type Compatibility: When Each Technology Fits
| Load Type | Relay Output | Solid-State Output | Reason |
|---|---|---|---|
| DC solenoid valve, >100 mA | Good below 1 Hz | Excellent at any frequency | SSR eliminates contact wear; leakage harmless |
| Small relay or pilot light, <50 mA | Good | Caution — check leakage | Leakage current may prevent dropout |
| AC motor contactor, 230 VAC | Good | Good (TRIAC) | AC zero-crossing benefits both types |
| High inrush (lamp, capacitor) | Good within rating | Excellent | No contact welding from inrush |
| Safety function (emergency stop) | Required (galvanic isolation) | Allowed with pulsed diagnostics | Relay provides air-gap isolation |
| Low-level signal, <10 mA | Poor — contact oxidation | Good | Relay needs min wetting current |
Minimum Switching Current: The Relay Specification That Gets Overlooked
Relay contacts have a minimum wetting current — the smallest current that can pass through the contacts while maintaining a clean, low-resistance connection. For gold-plated signal relays, this can be as low as 1 mA. For standard silver-alloy power relay contacts in PLC output modules, the minimum wetting current is typically 5–10 mA. Switching a 2 mA loop-powered sensor signal through a standard relay output results in gradually increasing contact resistance over time as oxidation builds up on the contact surface — a failure mode that takes months to develop and is difficult to diagnose because the contact resistance drifts rather than failing abruptly. Solid-state outputs have no wetting current requirement and switch microampere-level signals reliably. For low-current signal switching, solid-state is the technically correct choice regardless of cycle rate. Browse PLC output modules for both relay and solid-state options, and see compact PLCs with integrated I/O.
Relay outputs forgive what the circuit throws at them — mixed voltages, reverse polarity, inductive kickback — but they wear out with every operation. Solid-state outputs never wear out from cycling, but they leak. The selection is not about which technology is better; it is about which failure mode your specific load can tolerate.
