PCS PulseWorx — Relay Dimming Explained (App Note 100)

Technical application note from PCS (PulseWorx) explaining Relay Dimming — a method for dimming LED fixtures using only a standard power relay, without running separate 0-10V control wires.

What Relay Dimming Is (and Isn't)

Relay Dimming is not a power line carrier (PLC) protocol. Older and modern PLC-based automation methods — X10, Spread Spectrum, CEBus, Insteon, UPB, and IPC — all inject a signal onto the power line and can suffer from noise or attenuation that masks or creates false signals.

• X10: imposes a small 5VPP 120kHz oscillated signal onto the power line, using the 60Hz sine wave as a carrier.
• UPB (developed by PCS): uses Pulse Position Modulation — digitally encoded information sent as precisely timed electrical pulses over the power line.
• CEBus: a spread-spectrum method developed to address X10's attenuation and noise issues.

All of these methods require an extra transmitter circuit. Relay Dimming uses no transmitter at all — only the power relay that already controls the fixture circuit.

How Relay Dimming Works

The "marks" or "clicks" are placed on the load line between the relay switch and the fixtures it controls. A brief portion of the 60Hz sine wave — half of one cycle, for LED office troffer fixtures — is removed to create a mark. A second mark is made some number of cycles later; the number of cycles between the two marks IS the message. The receiving module uses a patented method to detect these marks and is highly tolerant of power line aberrations.

Because the relay's load line only feeds the fixtures on that circuit, it is impossible for this signal method to propagate backward into the building's other circuits, or for anything else on the circuit to interfere with it. When the relay is open, it is simply open — the same mechanical relay type used in occupancy sensors and relay switches for the past 20 years.

Relay Dimming works by very briefly turning the power off, then back on. Because it rides directly on the regular 60Hz sine wave rather than an injected signal, it is immune to the noise and attenuation issues that affect other power-line signaling methods.

Does It Affect the Fixture?

No. The portion of power removed is so small that LED drivers don't notice it. PCS has tested Relay Dimming with fixtures from Phillips, Acuity/Lithonia, and Columbia/Hubbell with no observed effect. Any LED fixture with a 0-10V driver is expected to be compatible with Relay Dimming.

Key Takeaways

• No transmitter circuit required — uses only the existing power relay
• Limited to fixtures on a single circuit (the relay's load line)
• Immune to power line noise and signal attenuation
• Eliminates the need to run separate 0-10V control wires from the wall switch or occupancy sensor to dim LED fixtures
• Compatible with any 0-10V driver LED fixture; tested with Phillips, Acuity/Lithonia, and Columbia/Hubbell