The Problem
Digital output (DO) cards were found sourcing approximately 117V on outputs that were not energized. The PLC logic showed the output as OFF (0), but a voltage measurement at the terminal showed near-full voltage present on the output pin.
Why This Happens
This is typically caused by capacitive coupling or leakage current through the output cards solid-state switching elements (triacs or transistors). When outputs are densely packed and some outputs on the same card are energized, the non-energized outputs can pick up induced voltage through internal capacitance.
In some cases, the end device connected to the output has a very high impedance (e.g., an indicator light with a tiny load, or an open circuit). High-impedance loads cannot sink the leakage current, so the voltage builds up to near-supply levels.
Impact
- End devices may partially or fully energize when they should be off — solenoid valves may chatter, indicator lights may glow dimly, relays may buzz.
- During commissioning, this creates confusion — “the PLC says its off but the valve is cracked open.”
- Safety concern if outputs control critical devices.
Solutions
- Add a bleeder resistor across the output terminal (parallel to the load). A low-value resistor (e.g., 10kΩ) provides a path for leakage current to drain, pulling the voltage down to near-zero when the output is off.
- Verify the load impedance — if the connected device draws very little current, consider whether the output card type is appropriate (relay output vs. solid-state).
- Check the card specifications for stated leakage current in the OFF state. Compare this against the minimum load requirement.
- Use relay-type output cards where ghost voltage is unacceptable — relay contacts have true galvanic isolation with no leakage path.