KNX/EIB Advice with power supply

[yoseikan]

Hello dear forum!

Sorry for the English and not German!

Thirst of all, congratulations for your forum activity. It is really helpful, even for those who, like me, don't understand a word of German (thanks to chrome automatic translation!)

I am an automation enthusiast finishing the KNX project design for my home.
I was going to use a regular UPS system (such as from a computer) to backup the system bus in case of electrical mains failure, but I came across this power supply from meanwell:
Meanwell Power Supply Products

It has 27,8V, adjustable to 29v, is enough for two bus lines (with two couplers and 640mA limiters in place, off course), can be connected to a Lead battery and even has relays to signal status, which may be used to connect to binary inputs in the system.

The price is also nice, about 30€, and meanwell I think its quite reliable

Am I missing something or this is an excellent solution for my house?

Thank you

[Hennessy]

You need one KNX-Choke per line, if you do not use a KNX power supply with an integrated choke.

[yoseikan]

Yes, thank you, I understand that. Still, even having in account 2 chokes and two limiters, it is a very versatile setup, with much redundancy possible and very cheap, don't you think?

The Gira power supply with battery backups surpasses 400 euro, and even then, only one choke. That is why I am asking if I am missing something, as it seems too good... the full setup for two lines woud be likely less 200 euros (two chokes, two limiters, power supply, battery)

[yoseikan]

Hello again

I am troubled with the question of whether I need a current limiter for each line.
Of course I will use the choke/drossel for each line, but if my device can supply more than 640mA to each line, even if not used nominally by the devices (will have about 40 devices in each line), will I be in trouble?

I can get the limiters from EIBmarkt, but is it justifiable?

Thank you, and, again, sorry for the english

Ricardo

[walterweber]

Ricardo,
I will approach it from a different point of view. According KNX specification one line addresses 64 devices.
Let´s keep it simple for this example and disregard the option for repeaters and additional power supply with additional choke (then you could reach the "software maximum" of 255)
Again simply, one line feeds 64 devices. And I think this is the reason for the need for 640 mA (for an average certified device within limits 10mA each)
So if you use a certified power supply it includes the choke needed and will make sure your voltage stays within limits for both full load and empty load case and in addition it is short circuit proof, so you won´t get damage.
(And it possibly offers in addition the extra "off-bus" supply voltage needed for some devices)
If you choose an "own" PS solution (uncertified by KNX) and you guarantee
to stay within these limits, I think: then, own risk, feel free.
If your PS can even more than 640 mA, so what, then it is possibly oversized, you won´t need the extra current unless you use extremely power consuming devices.
I know the discussion how can a simple PS be so expensive is old, is it the KNX certification what makes it costly, I don´t know.
Just my 5 cents, maybe I am wrong....
Regards Walter

[yoseikan]

Thank you very much for your insight!

The problem is that:

1. I would like to keep line segments bellow 64 devices (I have like 81) because of the possible bus/current overload

2. I would like to have the option to quickly exchange a power supply in case of failure

3. It would be nice to have battery backup power


Given these constraints, I would need at least 3 power supplies (1 spare) and if with backup with battery we would be talking north of 1600 € for the three.

With the Meanwell PSC 100 I could, at least theoretically, power the 2 lines with 1 PSU and 2 chokes, with backup power and alarm outputs for power or psu failure.
2 PSU (1 spare), 3 chokes (1 spare, 24v batteries, all just under 200-250€.

If there is no unanticipated (newbie related ) issues, that is what I intend to try, and would very much appreciate other opinion.

Ricardo