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I'm trying to understand the concept of a floated voltage, and why it is needed in some cases. For my own case, I am using a variac connected to a rectifier to provide a DC voltage for a circuit, (charging a large capacitor for discharge). The circuit needs a ground connection, but connecting a ground to complete the circuit also grounds the rectifier, which I am told will damage the rectifier. (I do not fully understand why this is the case, and would like to find out more.)

But it was suggested to me to use an isolation transformer before the variac and to float the voltage, and in this way rectifier would be kept safe. I would like to know understand how this works.

(I am aware another option could be to add a switch and just disconnect the rectifier from the capacitor, but I am more interested in understand the above).

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    \$\begingroup\$ It is needed also for safety. Isolation transformer will cut the link between the neutral and the earth(which normally exists) so you don't get electrocuted to death. \$\endgroup\$ – panic attack Nov 14 '18 at 14:02
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Usually a raw AC power feed is grounded on the neutral wire and, if tapping off a voltage via a variac, you are "keeping" one wire tied to earth. This is because variacs do not isolate. Adding a bridge rectifier to the output isn't a problem but earthing the DC output of the bridge rectifier is because you end up with a diode that is from the "hot" feed (as opposed to the neutral feed) that gets blasted with a lot of current 50/60 times per second and destroys.

Just draw out the circuit and see. Use an isolating transformer for safety, peace of mind and protection for the diodes.

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