Why is an input side transformer necessary with a bridge rectifier?
AC supplies are normally provided by live and neutral wires. The neutral wire is so-called because it has been "neutralised" by connection to earth through an earthing rod at the utility transformer or/and at the entrance to the building (depending on the country's regulations).







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Typical mains supply with neutral.

Now add in your rectifier and try grounding the output.







simulate this circuit

Figure 2. (a) The earth connection. (b) The fault current when the live goes negative.

Your earth connection will create a short-circuit on negative half-cycles. D2 will be destroyed.







simulate this circuit

Figure 3. Addition of an isolating transformer, XFMR2, breaks the earth loop.

XFMR2 provides mains isolation between the two ground-referenced circuits.



From the comments:


  If we do not ground the output then we can use the bridge rectifier without isolation transformer and there would be no problem of fault current when live goes negative. Is this true?


There will be no fault current but both DC+ and DC- lines will be live. The circuit would have to be fully insulated to prevent anyone touching any part of it. It would also not be possible to connect it to a circuit that has a ground connection.