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I am not getting the statement "The load cannot be grounded without an input side transformer." Described as a disadvantage of bridge rectifiers in book on power electronics by M. H Rashid.

Why can't the load be grounded without an input side transformer?

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  • \$\begingroup\$ Transformers and rectifiers do different tasks. \$\endgroup\$
    – Solar Mike
    Feb 8, 2020 at 6:06
  • \$\begingroup\$ Yes, of course they are totally different things. \$\endgroup\$ Feb 8, 2020 at 6:18
  • \$\begingroup\$ Just a comment based on your title... \$\endgroup\$
    – Solar Mike
    Feb 8, 2020 at 6:23
  • \$\begingroup\$ Look at the voltages across time as AC input varies for a bridge rectifier with VinAC2 GROUNDED - which is what you effectively get with most mains supply arrangements. || at VinAC1, VinAC2 , Vout +ve and Vout -ve. \$\endgroup\$
    – Russell McMahon
    Feb 8, 2020 at 6:26

2 Answers 2

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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).

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Typical mains supply with neutral.

Now add in your rectifier and try grounding the output.

schematic

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.

schematic

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.

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  • \$\begingroup\$ Thank you very much for your time and response. 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? \$\endgroup\$ Feb 8, 2020 at 12:34
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    \$\begingroup\$ There will be no problem in the operation of the circuit on its own (in isolation) ... however if you touch it, or connect it to something else that IS grounded, you may get a nasty surprise. It's fine for a LED lightbulb, but not for a phone charger. \$\endgroup\$
    – user16324
    Feb 8, 2020 at 12:43
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    \$\begingroup\$ @Sohail: See the update. \$\endgroup\$
    – Transistor
    Feb 8, 2020 at 12:50
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Assuming you'd like to ground the negative output of the rectifier, let's think about what would happen.

A bridge rectifier alternately switches the hot and the neutral to its output terminals such that the negative half of the sine wave always lands at the negative terminal of the bridge output.

That means that half the time the hot is connected to the negative terminal, and half the time the neutral is connected to the negative terminal.

Now try to ground the negative terminal of the bridge rectifier.

Half the time, the negative terminal is connected to neutral. Neutral to ground is OK - they are connected together (in normal house wiring) anyway.

The other half of the time, hot is connected to the negative terminal. Since we've connected the negative terminal to ground, there will be a short circuit from hot to ground on each negative half of the wave on the hot side.

When the hot is negative, neutral is positive. That short will let nearly all the current your source can provide flow through the diodes.

Cue sparks, crash, bang, smoke, and unhappy people.

This is why you need an input transformer if you need to ground the output of the bridge rectifier.

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  • \$\begingroup\$ Thank you very much sir for your time and response. \$\endgroup\$ Feb 8, 2020 at 13:49

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