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For testing purposes, I need two circuits: 220Vac rectified and 48Vac rectified, both having the same reference, like this:

enter image description here

But I don't have an isolated transformer 220V:220V. I only have an isolated transformer 220V:48V and an autotransformer 127V:220V. If I connected like that, since autotransformer has no isolation, I'd say my circuit would be equivalent to the following:

enter image description here

I understand that working not isolated from the mains is dangerous but it would be a quick test of a simple circuit. I'd like to know, are the two circuits above equivalents in terms of references? Is there any problem connecting it like that?

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    \$\begingroup\$ Note that when you use the CircuitLab button on the editor toolbar and then "Save and Insert" that an editable schematic is saved in your post. This makes it handy for us to copy and edit in our answers without having to redraw the whole thing. No need for CircuitLab account, screengrab, image uploads or background grid. \$\endgroup\$
    – Transistor
    Jun 27 '20 at 8:41
  • \$\begingroup\$ Thanks for the tip! \$\endgroup\$
    – Rohde
    Jun 28 '20 at 21:29
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There is a severe problem with that.

Look at the 'live' output from the 220 V source, the one that swings +/- 320 V peak with respect to supply ground. That is connected on negative half cycles through a diode in your bridge to the common/reference terminal between your power supplies, a terminal I suspect you hope will be 'ground'.

The 48 V isolation will give you the same reference, but unfortunately that reference will be swinging between ground and -320 V, dangerous for any grounded test equipment you want to use, and potentially lethal for you.

If you have two 220:48 transformers, then you could run the second backwards from the first one, to get you an isolated 220 V, which would allow both rectified supplies to have a grounded reference.

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  • \$\begingroup\$ at 60Hz it's probably two halves of a center grounded phase. so 160 WRT ground. \$\endgroup\$
    – Jasen
    Jun 27 '20 at 8:19
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no part of the circuit will be safe to touch, but functionally it will be equivalent.

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