Eltako home control & galvanic seperation

Using Arduino General Electronics
1

I am currently working on a home control system with an Arduino Mega + ethernet shield.
My entire house makes use of Eltako modules to control the lights, dimmers, blinds,...
Now, since my wiring is made with the loads on the high side, I am forced to go with a pnp transistor to control the Eltako modules. The Arduino is effectively working in parallel with the other push buttons around the house to control the lights in different locations.
The current schematic is as shownbelow. However, the Eltako is not really a normal relay, so I don't think I'll have issues like flyback current (but I added a diode anyhow, just to be sure). The current when switching the Eltako's ranges from 0,1mA -> 0,3mA (depending on the type of module)

Now for the question:
I'm a bit afraid of connecting the 24V circuit ground to the Arduino ground. Should I add some kind of isolation between the power and arduino circuit, or am I being overprotective? How big is the risk of frying my Arduino with this setup?

To be clear, I tested this set up already for 4 outputs and it worked beautifully. In the final set up, I will have something around 35 of these circuits to control everything. On the Arduino side this shouldn't be a problem, as I am only sending pulses of ±150ms to one output at a time.

I am no electronics expert, so any suggestion on this or alternative solutions are very much apprectiated!

2

Replace the 2N3904 with an optocoupler.

3

Google did not find ELTAKO ESR127....

have you spelled wrong?

Perhaps there is an optocoupler on the input

Pelle

4

it's a Z i.s.o. a 7, so eltako esr12z-4dx :slight_smile:

Didn't find specifics on the eltako side myself.

Either way, would an optocoulpler on the eltako side matter? I would still have the 24V ground & Arduino ground connected?

5

Örtliche Universal-Steuerspannung 8..230 V UC. Mit zusätzlichen Universalspannungs-Steuerein- gängen zentral ein und aus für 8..230 V UC, von den örtlichen Steuereingängen galvanisch getrennt.

Looks like it's isolated from mains.

From what do you take the 8 to 230 volt?

Datenblatt:
http://www.eltako.com/fileadmin/downloads/de/Datenblatt/A_datenblatt_ESR12Z-4DX.pdf

Pelle

6

From a 24V eltako transformer.
All the push buttons are also powered with the same transformer, and also some other eltake devices which work on 24V DC (like the staircase switches & timers).

7

Now I have read it i Swedish and the UC are separated from mains.
Nothing about how many volts the insulation can withstand.

Pelle

8

PSS6:
Either way, would an optocoulpler on the eltako side matter? I would still have the 24V ground & Arduino ground connected?

You would be completely isolated. You would not have to share a ground between your device and the Arduino. The only thing that would be connected to the Arduino will be the LED in the optocoupler.

9

Got me some 4N37's and give them a try.

10

PSS6:
but I added a diode anyhow, just to be sure.

And you just added it in a totally random position?

Apologies for this less than straightforward explanation.

11

Thanks for the info.
I guess my reasoning was that in this way, no current would be able to flow backwards towards the circuit. But with the optocouplers I eliminate the risk of frying my Arduino altogether, so a diode won't be needed.

12

Would I even need the 2n3906? Or can't the 4n37 be used as a high side switch? If it can, it would really simplify my setup :slight_smile:

13

PSS6:
I guess my reasoning was that in this way, no current would be able to flow backwards towards the circuit. But with the optocouplers I eliminate the risk of frying my Arduino altogether, so a diode won't be needed.

You clearly have not understood at all what the diode is for.