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I'm trying a cost-effective way of controlling conventional non-KNX relays (impulse, solid state, bistable, contactors) with KNX devices/controllers, and am not having much luck. It seems that while KNX ships actuators (relay modules) for 10-16A loads, it doesn't ship a device that has pure electrical output at 12 or 24V.

I have found that both dimmers and heating actuators are able to produce 24V output, but they are horribly inefficient in terms of both price and the amount of space they take up. I also found modules such as this (a 'concentrator') that expressly state it is not suitable for relay control (why not?).

Ideally I want an affordable, small control block with lots of low-power (sufficient to power a relay) 12V or 24V outputs that can be used to control relays. KNX is the protocol of choice, though if it works with 'cheaper' protocols such as DALI that's fine too.

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    \$\begingroup\$ What is KNX? A websearch only turns up some random transportation company's stock prices and a Los Angeles radio station. \$\endgroup\$
    – Hearth
    Feb 11 at 21:02
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    \$\begingroup\$ @Hearth looks like KNX solutions is a company ... KNX by itself is not really anything ... maybe the OP means KNX controller or something like that \$\endgroup\$
    – jsotola
    Feb 11 at 21:11
  • \$\begingroup\$ knx.org \$\endgroup\$ Feb 11 at 22:02
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    \$\begingroup\$ So you want to control relays with KNX. You might want to be more specific, you want to build a KNX relay which needa to receive KNX commands with MCU and drive relays, or you want to send KNX commands with MCU to KNX relays? And what help you need to do that? Or are you asking what to buy? \$\endgroup\$
    – Justme
    Feb 11 at 23:14
  • \$\begingroup\$ @Justme clarified \$\endgroup\$ Feb 12 at 7:28

3 Answers 3

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KNX modules are mechanically just standard top hat 35mm DIN rail modules, so why not simply buy one of countless DIN rail power supply modules (which there are numerous varieties at various currents for both 12V, 24V, and other voltages) and connect its output through some standard KNX relays to your arbitrary relays' coils (or what have you). Switch a KNX relay, it will switch the 24V (or whatever) bus power through the arbitrary relay, and now you're up and running controlling arbitrary relays using a KNX relay module. Additionally, any arbitrary relays you don't need individual control over can be connected in parallel to one KNX relay, reducing the number of channels you have to use.

Plenty of KNX relay modules have ratings for lower voltage switching as well, and should handle the load just fine.

You can get 100W (4.2A) of 24V in a DIN module PSU for less than €40, which should be more than enough to control plenty of arbitrary relays. They will be installable in any panel that can house KMX modules since KMX modules are DIN rail modules, just ones that happen to speak the KMX protocol

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  • \$\begingroup\$ KNX relay modules are themselves packaged up bistable relays rated to, typically, 10A and 16A. I already have the relays, so controlling relays with relays is redundant. \$\endgroup\$ Feb 23 at 5:48
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There are a number of modules available which provide direct GPIO control via KNX KNX Input/output module 8DI+8DO, and other which are designed to allow MCUs such as Raspberry Pis to send and receive KNX messages KNX Raspberry Pi HAT.

Either could be interfaced with a relay bank to provide the kind of control you're seeking.

There are also a number of good pre-integrations with home automation platforms such as Home Assistant with KNX, which provide complete solutions rather than just building blocks.

Which route to choose depends on your ultimate goal.

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  • \$\begingroup\$ Thanks for the suggestions! To be honest I'm looking for a KNX solution to a KNX problem. Having RPi (or similar computer) would somewhat complicate things. (And, on a personal note, I find RPi to be very unreliable.) \$\endgroup\$ Feb 23 at 5:55
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The solution to this problem turned out to be a KNX output module, which is a module capable of taking low-voltage (e.g., 24V) input and having a large number of outputs. An example of such a module is presented below. This module is capable of outputting up to 100mA per channel, which is sufficient to power various relays (e.g., the CR-P intermediate relay requires about 80mA of power).

This solution was provided by ChatGPT :)

KNX output module

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