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venfree/Wiki/KNX Power Supply

KNX Power Supply

The KNX power supply provides the TP bus with 29 V DC SELV and delivers operating current to all bus participants. At least one power supply is required per line; with many devices, two can operate in parallel.

Calculating the Current Budget

Each device typically draws 3–15 mA from the bus (check datasheet). Displays, couplers and active devices may require more. The sum of all device current draws must remain below the rated capacity of the power supply: standard power supply 160 mA, larger variants 320 mA or 640 mA.

With 40 devices × 10 mA = 400 mA, a standard power supply (160 mA) is insufficient. Use either a 640-mA supply or two 320-mA supplies. Power supplies may be operated in parallel, but should be from the same manufacturer and type or use a decoupling choke.

Key Facts

  • Bus voltage: 29 V DC SELV (no-load ~31 V)
  • Standard power supplies: 160 mA, 320 mA, 640 mA
  • Min. device voltage: 21 V (KNX spec)
  • Two power supplies: same type recommended
  • MDRC format: 2–4 DIN rail units (TE)
  • Power supply with diagnostic CO reports short circuits on the bus

Related Articles

KNX TP – Twisted Pair
KNX TP is the most widely used transmission medium. A twisted pair simultaneously transmits power (29 V DC SELV) and data (9,600 baud), all devices on a line share the same cable.
KNX Topology
KNX TP networks are hierarchically structured: devices form a line, lines are grouped into areas, areas are connected via the backbone. This structure supports up to 14,400 devices in a pure TP network.
KNX Bus Voltage
KNX TP devices are powered directly from the bus, the bus cable simultaneously transmits data and energy. The bus voltage is SELV (Safety Extra Low Voltage) and harmless to persons.
KNX Line Coupler and Area Coupler
A KNX line coupler (LC) connects two lines and filters telegrams, only telegrams with destination GAs in the filter table are forwarded. It reduces bus load and provides galvanic isolation.

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