Transformer isolation and reference

Question

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

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:

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?

Answer 1

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.

Answer 2

no part of the circuit will be safe to touch, but functionally it will be equivalent.