Relays vs dimmers: which C-Bus output unit controls your lights

One of the most common questions we get from customers planning a C-Bus job is deceptively simple: “Will I be able to dim that light?” The honest answer is that it depends entirely on which output unit we wire your circuit to. In C-Bus, the brains live in the switchboard, and every lighting or power circuit is driven by either a relay or a dimmer output unit. Get the pairing right and the whole house feels effortless. Get it wrong and you’ll be chasing flicker, buzz and dying LEDs for years.

Here’s how we explain the difference to our Melbourne customers, and how our team decides what goes where when we design a board.

What a C-Bus output unit actually does

The wall switches you press — your Saturn, Neo or eDLT plates — don’t actually carry the lighting load. They’re input units. When you press a button, they send a low-voltage message across the pink C-Bus cable telling an output unit what to do. That output unit, sitting on DIN rail in the switchboard, is what switches the 230 V mains to your lights. The diagram above shows how the pink cable ties the switches and the output unit together onto the one network.

There are two families of output unit, and the difference between them is the whole point of this article.

Relay output units: full on, full off

A relay output unit is exactly what it sounds like — an electrically operated switch. It connects the load to mains (on) or disconnects it (off), and that’s it. No in-between. Relays are robust, run cool, and don’t care what’s plugged into them.

We reach for relay channels whenever a circuit doesn’t need to dim, including:

• Power circuits — GPOs, appliances, pumps, anything you just want switched.
• Exhaust and ceiling fans (fan speed is handled separately, but on/off control is a relay job).
• Non-dimmable LED downlights and battens — and there are a lot more of these around than people realise.
• Fluorescent and other fittings that simply aren’t built to dim.

Relays are forgiving. The main thing we watch is the inrush current and the load type — a bank of cheap LED drivers switching on at once can hit a relay hard, so we size and stagger accordingly. But there’s no “mode” to set and no flicker to chase. If a circuit doesn’t need brightness control, a relay is almost always the right call.

Dimmer output units: varying the brightness

Dimmer output units do the clever bit — they vary how much power reaches the load so you can set any level from off to full. In C-Bus terms each channel responds to a Group Address and drives to a Level between 0 and 255, with a programmable ramp rate so lights fade up and down smoothly rather than snapping.

Most C-Bus dimmers we install are multi-channel DIN-rail units. The L5504D2 family is the workhorse — four channels in the one module, rated at roughly 2 A per channel. That 2 A figure matters a great deal, and we’ll come back to it.

If you want the detail on how channels map to Group Addresses and how we set ramp rates, our C-Bus lighting guides walk through it, and the programming articles cover the Toolkit side.

Universal dimmers and the leading-edge / trailing-edge choice

The modern dimmers we fit — the universal types such as the L5504D2U and the newer SpaceLogic 5504D2D — can run in one of two phase-control modes. This is the single most important setting for getting good dimming, so it’s worth understanding what it means.

• Leading-edge (forward phase) chops the front of each mains half-cycle. It suits traditional, heavier loads — iron-core (magnetic) transformers, old halogen setups, and some larger electronic loads.
• Trailing-edge (reverse phase) chops the back of each half-cycle. It’s gentler and is what most modern LED and electronic drivers want. It dims smoothly, runs quietly, and is far kinder to LED electronics.

The catch: each output is set to one mode at a time. You can’t mix leading-edge and trailing-edge loads on the same channel, and you can’t have it switch automatically per fitting. So when we design your board we group lights by what they need and assign channels accordingly. Schneider Electric publishes mode and load guidance for their dimmers worth a look — see the Clipsal site for the current C-Bus range.

What goes wrong when the pairing is off

Choosing the wrong output unit, the wrong edge mode, or an underrated channel is behind most lighting complaints we’re called out to. The usual symptoms:

• Flicker and shimmer — almost always wrong edge mode, or LED drivers that need a minimum load the dimmer can’t see at low levels.
• Buzzing or humming — either from the dimmer module or the fitting, again usually a mode mismatch (leading-edge driving electronic LEDs is a classic offender).
• Dropout at low brightness — the lights cut out before they reach off because the load is too small for the channel to control cleanly.
• Premature LED failure — the wrong waveform stresses cheap drivers, and they die early.
• Overload tripping — too much load on a 2 A channel. Add up your driver wattage honestly, including inrush.

This is why we always ask for the actual fitting and driver specs before we finalise a board design, rather than guessing. A $9 downlight and a quality dimmable one behave very differently on the same channel.

How we decide relay vs dimmer on a real job

Here’s the rough thought process our team runs through for each circuit:

1. Does this circuit ever need to dim? If no — power, fans, non-dimmable fittings — it’s a relay channel. Done.
2. If yes, what’s the load type? Modern LED with electronic drivers → trailing-edge universal dimmer. Iron-core transformers or older halogen → leading-edge.
3. What’s the total load on the channel? We add up the wattage and keep it comfortably under the ~2 A per-channel rating of an L5504D2-family dimmer, leaving headroom for inrush.
4. Group like with like. Lights that share an edge mode go on the same dimmer module so settings are clean and predictable.
5. Confirm dimmability with real fittings. We check the fitting is rated dimmable and, ideally, test before committing the whole house.

All of these output units, relay and dimmer alike, mount on DIN rail in the switchboard and clip onto the same pink C-Bus network. That’s the beauty of C-Bus — once the cable’s run and the units are talking, reassigning what a switch does or tweaking a ramp rate is all done in software, no fishing wires through walls. If you’re still planning your system, our getting started guide sets out how the pieces fit together.

The short version

Relays switch things fully on or off and are perfect for power, fans and non-dimmable lights. Dimmers vary brightness, are usually four-channel DIN-rail units around 2 A per channel, and need to be set to the correct leading- or trailing-edge mode for the load — trailing-edge for most modern LEDs. Match the unit and the mode to the fitting and your lighting will be smooth, quiet and long-lived.

If you’ve got flickering LEDs, a buzz you can’t pin down, or you’re just trying to work out what your C-Bus board needs before a renovation, give us a yell — we sort this kind of thing for Melbourne homes every week. Reach the DUKE team via our contact page and we’ll point you in the right direction. — Adam and the DUKE team.