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I have a center-tapped transformer which outputs 12V-0V-12V. I was wondering what is the differences between using one full bridge rectifier and two to generate a positive and a negative DC rail.

Consider this schematic: One full bridge rectifier It uses one full bridge rectifier and uses the center tap directly as GND.

Consider the second schematic (smoothing caps omitted): Two full bridge rectifier

Besides from using more diodes, what are the differences between two approaches?

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  • \$\begingroup\$ That top diagram is a really confusing way to show a full wave bridge. \$\endgroup\$ – Olin Lathrop Jun 19 '17 at 20:03
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If you want to do approach #2, you need two fully isolated windings. The way you show it has two diode drops in one direction shorting out the windings as @WhatRoughBeast points out. You cannot use a center-tapped winding- the common must be made on the other side of the bridge rectifiers.

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The advantage of the second method (with two secondary windings isolated from each other) is that it uses the transformer more efficiently if the loads on the positive and negative rails are greatly imbalanced. You can get as much as 24% more DC current out of the transformer by using the two bridges.

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The biggest problem is that two of the "central" diodes will burn out. The ground connection is a disaster.

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You have a path from power terminal 1 to power terminal 3 through several forward biased diodes and the ground connections, forming a short during at least part of the A.C. cycle.

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Nothing wrong with the general approach, but it is not the best use of the transformer. The upper schematic shares the load between the two windings regardless of the imbalance, while the lower one would have all of the heavier load's reflected current in one winding. Also, the upper one has a single diode drop per side for losses, while the lower has two. You'd be better off using one-half of each bridge to copy the upper schematic.

I agree with @entrepreneur that as drawn you will short out, but I think I get the idea of what you are asking.

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One problem with the bottom design is you're not getting a DC value out of the PS, so the loads will not see a constant voltage. The second problem is the lower design is reference to ground on the full wave rectifier and you should use ground for return current.

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