KNX module current absorbtion question

[ypkdani]

Hello all,

i'm interesting to develop a new knx IO module for konnekting. I'm reading the KNX specification and the NCN5121 capability.
In the KNX specification file 03_03_02 document there is this table

Parameter
TP1-64/TP1-256
Bus current (at 20V - 32V) max. 12 mA
slope of input current max. 0,5 mA/ms
slope of input current for manually operated devices (e.g. push buttons) max. 2,5 mA/ms

so for the KNX standard a module can't absorb more than 20V*12mA = 240mW .
Now with a module as this https://www.mdt.de/download/MDT_DS_S...tor_AKI_03.pdf we have 8 relay to control. Every relays has an absorbtion of more or less 400mW for be activated (https://omronfs.omron.com/en_US/ecb/...df/en-g5le.pdf). In addon we have the microprocessor consumption (es 5V 10mA) and all the led (5V 10mA for every led). So my question is how this module that is powered by the bus(as written in the datasheet) can control these actuators respecting the knx specification?!

Thanks

[tuxedo]

Hey ypkdani,

we alread did this with KONNEKTING, see: https://wiki.konnekting.de/index.php/DFF4.1

Each relay draws more than it is "allowed" to be drawn from bus. And also more than the NCN5120 can deliver.

MDT and our solution is covering this issue by using enough capacitors resp. enough µF capacity to buffer the high current consumption.
For the LEDs: You can reduce the curren by using PWM, or just lower the LED current with a bigger resistor. My LEDs draw ~1mA per LED with still feasible brightness.

With enough capacity, you can - for a very short time - draw enough current.

Works like a charm.

Other devices, such as the IP router from MDT, use two KNX transceiver-chips to double the current that can be drawn.

You see, there are ways around this limitation, but with other limitations...

[jentz1986]

Simply by charging capacitors over time.

Regarding Relais: 400mW are at max drawn when the switch happens. The Relais are bistabil, so they draw no current when they stay in one the states. And LED can be low power modules with 1.2mA or less.

[Ing-Dom]

some relay modules need 230V AC to work they seem to have a seperate converter and do not draw current from the bus (resp. not much)

[ypkdani]

Hello,

thanks all. Ok so in this module you use a Latching relay for avoid to continue to drive current on this and optimize all the power consumption!! Now is more clear!!
I will see the module realized.

Thanks