AC-DC conversion without Neutral

[JeremyB]

I am trying to determine if my present ac-dc design (attached image) can be tweaked to be used with a 208V input. This means NO NEUTRAL! The 208V input would be two 120V line voltages which are 120 degrees out of phase. So my questions are:

1) If it is possible, what would I need to do to achieve this?
2) Does operating without a neutral have any impact on the remaining board electronics (not shown) which currently use the neutral as a common ground?

Thanks in advance!
-Jeremy

 

[betwixt]

It should produce the correct DC voltage across it's output but there is no way you can connect either output to an existing neutral or earth point. It would have to be used as a floating supply. Please bear in mind that sitting it across two phases makes the whole circuit and the circuit it is powering potentially very dangerous to touch. It would also be sensible to fuse one or both input wires, at the moment, if the voltage rises above the MOV threshold and it's connected directly across the AC supply, it will turn to vapor followed shortly by the other components!

Brian.

 

[JeremyB]

It should produce the correct DC voltage across it's output but there is no way you can connect either output to an existing neutral or earth point. It would have to be used as a floating supply. Please bear in mind that sitting it across two phases makes the whole circuit and the circuit it is powering potentially very dangerous to touch.

Thank you, this is what I was thinking but needed to hear it from someone else. I agree that this is a dangerous design to troubleshoot. I think I could do most (if not all) of my testing when connected to a 120V & neutral input.

It would also be sensible to fuse one or both input wires, at the moment, if the voltage rises above the MOV threshold and it's connected directly across the AC supply, it will turn to vapor followed shortly by the other components!

Good call. R33 is currently a fusible resistor but it makes sense to fuse the other input as well for operating on 208.

Are there any additional concerns I should be aware of?