What DC-OK / Power Good really means

In industrial control cabinets, DC-OK (often called Power Good) is a status signal that tells you: “the power supply output is within an acceptable range.” Depending on the model, this status may be provided as a relay contact (dry contact) or as an electronic output (active/transistor).

Practically, you should treat DC-OK as a monitoring signal, not as a power output. Your PLC uses it to decide when to alarm, log an event, or trigger a controlled shutdown sequence. When DC-OK transitions to a fault state, it often means undervoltage, overload/derating, or loss of input power (depending on PSU design and hold-up time).

The important takeaway: DC-OK is most valuable when you implement actionable alarms (fault vs warning) and keep a history (counts + timestamps). That history is what turns “a blinking light” into a maintenance signal.

Identify your DC-OK signal output type (before you wire)

Start with the PSU datasheet label. Common outputs look like: DC-OK (relay), DC OK 13/14, Power Good, Alarm, or OK. What you’re trying to determine is: (1) dry contact vs electronic output, and (2) default state (NO/NC or active-high/active-low).

Dry relay contact (Form A / Form C) behaves like a switch. You supply the PLC input voltage through the contact. This is the most forgiving in mixed systems because it is polarity-agnostic.

Active/transistor DC-OK is an electronic output referenced to 0 V (often current-limited). These are efficient and fast, but you must wire them exactly as the datasheet shows—especially if the signal is “active-low” (normal = high, fault = low).

Wiring pattern A: DC-OK relay contact → PLC digital input

This is the most common and most robust method. You wire the DC-OK relay contact like any other dry contact: route a fused 24 V (or the input module’s reference supply) through the DC-OK contact and into a PLC digital input channel. When the PSU is healthy, the contact closes and the input turns ON (or vice versa if you choose NC logic).

Two practical choices: (1) “Healthy = ON” (preferred) so broken wires trend toward an alarm state, and (2) “Alarm = ON” if your plant standard requires active alarms. Either is fine—as long as you document it and label the terminal blocks.

Don’t use DC-OK to switch loads. Treat it as a signal contact only, and let the PLC input do the sensing. If you need to drive a relay or interlock circuit, drive it from a proper output module or safety-rated device.

Tip: label the DC-OK pair at the PSU, terminal blocks, and PLC input. When someone is troubleshooting at 2 a.m., those labels are worth more than any drawing.

Wiring pattern B: active DC-OK output → PLC DI (watch polarity)

Some industrial power supplies provide DC-OK as an active switching output referenced to 0 V. The advantage is that the signal is clean and fast. The downside is that you must follow the terminal mapping exactly: DC-OK and 0 V must share a proper reference with the PLC input circuit.

If the datasheet says active high, the signal sits high during normal operation and drops low during undervoltage. If it says active low, the logic is inverted. Either way, wire it so the PLC sees a stable ON/OFF state and then implement the alarm logic in software (next section).

Practical rule: keep DC-OK wiring in the same wire duct as other low-level control wiring, avoid long parallel runs with contactor coils, and terminate 0 V cleanly to reduce nuisance events.

Alarms to implement (PLC logic that reduces downtime)

Wiring is only half the job. The real win is choosing alarms that tell maintenance what to do next. A practical approach is a three-tier model: (1) fault, (2) warning, (3) maintenance.

Fault (immediate action): DC-OK drops and stays low beyond a short debounce window (e.g., 300–1000 ms). This usually indicates a true undervoltage or PSU shutdown. Latch the alarm and capture a timestamp.

Warning (action soon): DC-OK “blips” multiple times in a shift/day. Count events and raise a warning when the count exceeds a threshold. This catches borderline load spikes, loose terminals, or supply issues before they become a hard stop.

Maintenance (planned work): trigger when warnings keep trending up (week-over-week), or when the PSU reports derating/temperature stress. This is where you prevent downtime instead of reacting to it.

Predictive maintenance from DC-OK events (what to log)

DC-OK becomes a predictive tool when you stop treating it as a single alarm and start treating it as a stream of events. Even if the machine keeps running, a rising count of DC-OK drops is often the earliest sign of: loose terminals, overloaded 24 V distribution, aging PSU components, or a noisy/unstable upstream supply.

Three metrics that are easy to implement: (1) Drop count per shift/day, (2) time-to-DCOK after power-up (longer times can indicate stress), and (3) correlation with other events (big valve actuation, contactor pull-in, heaters, etc.).

If your system uses power supplies with richer diagnostics (e.g., configurable monitoring or IO-Link), you can go further by collecting operating data and threshold alarms for derating, over-voltage protection events, and operating hours—so maintenance is scheduled before a failure stops production.

Commissioning checklist + common mistakes

• Verify the signal type: relay contact vs active output, and confirm NO/NC or active-high/low.
• Match DI polarity: confirm whether the PLC input is sinking or sourcing and wire COM correctly.
• Debounce in PLC: start with 300–1000 ms for fault validation; tune based on your process.
• Keep signal wiring clean: route DC-OK with control wiring, avoid long runs next to coils and VFD output cables.
• If DI diagnostics are enabled: some systems require a parallel resistor across mechanical contacts to avoid false wire-break alarms.
• Document everything: label PSU terminals, terminal blocks, and PLC input tag names consistently.

If you’re building redundant or parallel 24 V architectures, confirm how your specific PSU defines DC-OK (some designs intentionally prevent a parallel unit from “backfeeding” the DC-OK signal). In those cases, treat DC-OK as “this PSU is OK,” not “the bus is OK,” and build your alarm strategy accordingly.

Related internal read: Planning DC Power Architecture for Industrial Control Cabinets

When to get a wiring review (fastest path to fewer nuisance alarms)

If DC-OK is already wired but you’re seeing nuisance alarms—or if you’re designing a new cabinet with uptime requirements—an external wiring review usually pays for itself quickly. We can help you choose the right signal type, confirm PLC input topology, and deliver a buildable alarm matrix that maintenance can actually use.

(External references used for engineering context: manufacturer DC-OK behavior and monitoring concepts are documented by vendors such as Beckhoff, Phoenix Contact, and PULS.)

• Beckhoff DC-OK relay contact behavior
• Phoenix Contact QUINT signaling examples
• PULS DC-OK relay contact behavior

FAQ

Is DC-OK the same as Power Good?

In most industrial DIN-rail PSUs, yes: both indicate output status within an acceptable range. Always confirm the exact thresholds in your PSU datasheet.

Should I wire DC-OK as NO or NC?

For monitoring, many teams prefer “Healthy = ON” so open-circuit wiring trends toward an alarm state. Use plant standards and document the choice.

Why do I get false DC-OK alarms?

Common causes are mismatched DI sourcing/sinking wiring, noisy 0 V reference, long runs next to coils/VFD wiring, or insufficient debounce logic in the PLC.

Can I switch a relay coil or load with DC-OK?

Treat DC-OK as a signal contact. Use PLC outputs or dedicated interlock devices to switch loads, and follow the PSU’s contact rating if you must interface with anything other than a PLC input.

What’s the best “predictive” metric from DC-OK?

Track drop counts and trend them over time. A rising rate is often the earliest indicator of loose terminations, overload, or marginal supply conditions.