can i connect neutral to negative dc ?

[kookooli]

Hi,
i'm working on a switching power supply.
input 220 volt ac.
output 21 volt dc.
i want to measure input and output current simultanously,for this purpose,I use an ARM microcontroller.

to measure input current correctly i have to connect input neutral to ARM ground.
and to measure output current correctly i have to connect output negative dc to ARM ground.
So, now i have connected input neutral to output negative dc.
Is this OK ?

 

[FvM]

It's no problem for the SMPS. But it means that your processor and DC output circuit must have safe insulation as if it would be connected to hot terminal of AC supply, because you can't rely on neutral being earthed. All external circuit connections, e.g. UART, programmer need opto-isolation.

Using a current transformer for the AC input is probably the better way.

 

[kookooli]

It's no problem for the SMPS. But it means that your processor and DC output circuit must have safe insulation as if it would be connected to hot terminal of AC supply, because you can't rely on neutral being earthed. All external circuit connections, e.g. UART, programmer need opto-isolation.

Using a current transformer for the AC input is probably the better way.

but my problem is here that i want to read, not to command .
i have 2 microprocessors. micro number 1 measures input current via a small resistance and micro number 2 measures output current via small resistance. then micro number 2 send the measued data to micro number 1 by uart to devide the values.(input current/output current).
but now, I dont know how should i use optocoupler cuz as i know optocoupler and etc. are used for isolating when we want to command by a micro , i dont know how should i use optocoupler while i want measure values ?

 

[FvM]

I was talking about optocouplers for external digital interfaces in case you tie the microcontroller galvanically to the AC input side. This is the standard toplogy at least for energy meters.

Opto isolation of analog signals is also possible but involves some effort. As said, a current transformer with it's built-in isolation would be preferable in this case.

I'm missing somehow a clear problem specification. You didn't even mention if direct connection of SMPS output to AC power would be acceptable from an application viewpoint. As said, it's no problem for the power supply or the processor itself. But probably for other circuits connected to it.

 

[kookooli]

I was talking about optocouplers for external digital interfaces in case you tie the microcontroller galvanically to the AC input side. This is the standard toplogy at least for energy meters.

Opto isolation of analog signals is also possible but involves some effort. As said, a current transformer with it's built-in isolation would be preferable in this case.

I'm missing somehow a clear problem specification. You didn't even mention if direct connection of SMPS output to AC power would be acceptable from an application viewpoint. As said, it's no problem for the power supply or the processor itself. But probably for other circuits connected to it.

this is my circuit .

 

[chuckey]

Take your mains from the output of a isolation transformer.
Frank

 

[kookooli]

Take your mains from the output of a isolation transformer.
Frank

My input current is less than 0.4 ampere and i dont know if i can use isolation transformer or not ? and i dont know about isolation transformer size ? How big is an isolation transformer ?

 

[chuckey]

you can get isolation transformer from 100W to 3KW. If you can lay your hands on a pair of identical 100W mains transformers, you can go from mains -> 24V (say) then 24V -> 230V.
Frank