Hi,.
I want to measure the voltage of a rectified 3 phase(3*400V) bus with a microcontroller. The 3 phase is just rectified, not isolated with transformer.

The microcontroller will be suplied from a one phase transfomer(~12V).

Can I connect the two negative line after the rectifier? I need it because the microcontroller compares the voltage to its negative.

Any help is much appreciated!

 

Can I connect the two negative line after the rectifier?

What negative lines?
Post a schematic of the 3-phase rectifier circuit and how you want to measure the voltage.

 

Its just a common rectifier like http://hu.mouser.com/ProductDetail/IXYS/VUO62-14NO7/?qs=t7yjd2JO/gS5go51uXvBaQ==

 

No you can't connect the common of two of them together from different phases.
What, exactly are you trying to measure?

 

The voltage and the current on the three pahse rectifier.

 

That's one Heck of a rectifier just to monitor three phases!
As long as sufficient galvanic isolation between both supplies
you should be able to common up the negatives, I assume the 12v is rectified (DC)?
Max..

 

Yes. That is a rectified voltage of a small transformer.

We need that big rectifier fo the 3 phase because of the load.