Power Supplies for Building and Home Automation

Building automation puts power supplies in a distribution board next to occupied rooms, on a system that is expected to run for the life of the building. The electrical requirements are modest compared with an industrial cabinet. The constraints that decide the choice are usually spatial and acoustic instead: how many modules are left on the rail, how much the board hums at two in the morning, and how much the system draws when the building is empty.

A KNX installation adds a requirement that has no industrial equivalent. The bus carries data and power on the same pair, so the bus supply is a system component with its own rules, and it is not interchangeable with a general-purpose 24 V unit.

A KNX bus supply is not a 24 V supply

KNX twisted-pair segments carry both the data and the power the devices run on. The supply feeds the pair at around 30 V DC through a choke, and that choke is the part that distinguishes a bus supply from an ordinary one: it presents a high impedance to the telegram frequencies so the bus signalling is not shorted out by the supply's own output capacitance. Connecting a general-purpose 24 V DIN rail supply to a KNX pair does not work, and the reason is the choke rather than the voltage.

Bus supplies are rated by the current they deliver to the bus rather than by wattage — the units in the KNX category include a 640 mA supply in the KNX-20E-640 and a higher-current option in the KNX-40E family. Sizing is arithmetic on a device schedule: every KNX device declares a bus current draw in its datasheet, usually a handful of milliamps, and the segment total has to fit inside the supply's rating with margin for the devices added during fit-out.

Segment topology imposes its own limits on cable length and on the distance between the supply and the furthest device, and those are independent of the current budget. A segment that passes the current calculation can still fail on length, which is the more common mistake on a large floorplate.

Auxiliary 24 V alongside the bus

Bus power runs the devices' own electronics. It does not run the things those devices switch. Motorised valves, blind motors, door hardware, room controllers with displays and most third-party sensors need their own supply, and drawing that from the bus is the quickest way to make an installation unreliable.

That auxiliary rail is an ordinary job for a DIN rail supply, and the DIN rail category covers it from compact units upwards. Two selection criteria differ from the industrial case. Module width is at a premium in a residential board, so the watts-per-module figure is worth comparing directly. And because the board is often inside or adjacent to living space, a supply whose fan or transformer is audible in a quiet corridor will generate complaints that no amount of electrical performance offsets.

Lighting control inside the building system

Where the automation system also dims the lighting, the driver has to speak the control system's language natively. Translating between protocols with gateways works, but it adds a component, a failure mode and a commissioning step to every circuit.

The KNX category includes drivers that sit directly on the bus and need no gateway at all. They divide by what they are driving:

• Constant-current drivers — the LCM-KN and XLC-KN families — for LED modules and engines specified at a fixed current.
• PWM controllers — the PWM-KN and SPWM-KN families — for constant-voltage LED strip and tape, where dimming is done by modulating a 12 V or 24 V supply.

Where the lighting is not on the bus, the wider LED driver category covers the mainstream dimming protocols, including DALI variants. The LED lighting page goes into the choice between protocols in more detail.

One point catches people out on refurbishment work. PWM dimming of LED strip modulates the supply to the strip, so the strip's own flicker behaviour is set by the PWM frequency, not by the driver's internal switching frequency. In a space with cameras or where occupants spend long periods, that frequency is worth knowing.

Standby power, and what an empty building costs

A building automation system runs continuously for twenty years or more. The load at three in the morning is small, but it is never zero, and a supply's no-load and light-load consumption is multiplied by 8,760 hours a year and by however many boards the building has.

Two figures are worth reading on the datasheet. No-load input power is what the supply draws with nothing connected. Efficiency at low load matters more than peak efficiency, because a supply sized with headroom for a fully fitted-out floor spends most of its life at a fraction of that load, and the efficiency curve of a switching supply falls away at the bottom of its range.

This is also the argument against oversizing out of caution. A supply at 15% load is running in the least efficient part of its curve, and the wasted energy becomes heat inside a closed board.

Retrofit work and the cable you inherit

New-build installations get the cable the design calls for. Retrofits get whatever is in the wall, and the choice of supply often has to accommodate it rather than the other way round.

The recurring constraints are cable cross-section that limits how much current can be delivered without excessive drop, distances longer than a new design would allow, and existing back-boxes that constrain where equipment can physically go. Working out the voltage at the far end of an existing run, at the current the new devices will actually draw, is a five-minute calculation that prevents a return visit.

Where a run cannot carry the current, a local supply near the load is usually cheaper than pulling new cable — which shifts the question from one large central unit to several small distributed ones, and changes which products are appropriate.