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I have a bunch of bridge rectifiers rated at 25 Amps with a maximum input voltage of 140 VAC RMS. They work for fine for rectify the 120 VAC found from the wall in North America.

Is it possible to connect two units together to rectify the 240 VAC supply (really two out of phase 120 VAC supplies) found in North America? I think this means the DC side is going to above or below the neutral leg potential, but that seems inevitable in any case.

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  • \$\begingroup\$ The supply is 240V single phase. It's center tapped. It's neither "split" nor "out of" phase. \$\endgroup\$
    – Ecnerwal
    Commented Dec 17, 2016 at 4:08
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    \$\begingroup\$ en.wikipedia.org/wiki/Split-phase_electric_power \$\endgroup\$
    – Eric Urban
    Commented Dec 17, 2016 at 4:14
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    \$\begingroup\$ @Ecnerwal .. afraid your statement is contradictory. Centre tap= split single phase which creates opposite polarity wrt. centre tap. \$\endgroup\$ Commented Dec 17, 2016 at 5:01

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I'd strongly coach you to NEVER use a bridge rectifier direct on a 120 V or 240 V AC line. The voltage and available current are deadly and if you don't know exactly what you are doing you could kill or severely injure someone.

The main reason that rectifying 120 V AC is so dangerous is that the Neutral is connected to Earth in your house wiring and the DC output of the bridge rectifier cannot have one side of the output DC connected to Earth. It is effectively floating above Neutral/Earth. If you attempt to earth one side of your supply you short out diodes in the bridge (smoke and blown fuses will result).

The AC input for the US 120/240 lines looks like this: Line entry for split phase

If you use one of your bridge rectifiers it'll look like this:

Single 120 V rectifier

If you really do have a bunch of the bridge rectifiers and you absolutely need to do this, (safety considerations aside) then there are a couple of possibilities to use them to rectify 240 V AC. I'll cover what needs to be done for the bridge mods later.

You could consider the 240 V input as a center tapped transformer (which it is) and do this:

enter image description here

A single supply with a good ground reference, but the voltage out is the same as a bridge rectifier connected to L1 - N.

Or you could do this and get a positive and negative supply (twice the voltage combined)....and still in this configuration you have a continuous ground.

enter image description here

Now to deal with the fact that the bridge rectifiers are not able to withstand the reverse voltages at 240 V.

Each bridge looks like the image on the left but could be wired as the image on the right:

enter image description here

This makes an effective single diode with approximately twice the current rating and about twice the voltage rating. The resistors R1 and R2 would be required to balance the reverse leakage voltages. In an application for these voltages about 150k - 200k Ohms would suit. You could wire these up to replace each single diode in a bridge rectifier configuration.

Last advice.....don't do it.

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You may only use 1 full bridge across L1, L2 with 240Vac in, to make V+,V- with +/- 167Vpk with load and caps to N.

If the outside transformer was floating, it would work.

If you attempted to put 2 full bridges on each line to neutral and connect the -ve of one to the +-ve the other bridge out, you will find 1 diode from each bridge shorts out L1~ L2.

Keep in mind when you work directly offline with the grid it can supply thousands of amps at line voltage until the breaker trips, which is like hundreds of horsepower , so any fault without a fuse to protect the circuit and double insulation and a GFCI to protect the user may give distrubing results.

schematic

simulate this circuit – Schematic created using CircuitLab It is not obvious to some, but a double bridge with a common centre causes a line fault.

I hope this answers your question. enter image description here

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  • \$\begingroup\$ a diagram would be good Tony. I know what you're talking about, and it took a while squinting before I realised what you're saying, and why 2x 140v bridges is unsafe, even though Lx-N is only 120v AC. \$\endgroup\$
    – Neil_UK
    Commented Dec 17, 2016 at 7:31
  • \$\begingroup\$ there ya go.... \$\endgroup\$ Commented Dec 17, 2016 at 16:00
  • \$\begingroup\$ @Jack Creasey The question to use two bridges, may not be possible , not because it may be unsafe or the PIV is doubled but because the 2 bridges with a common DC connection cause a short circuit path thru 2 diodes on the L1 to L2. This is the fundamental reason. \$\endgroup\$ Commented Dec 17, 2016 at 17:58
  • \$\begingroup\$ @Tony Stewart. I thought I demonstrated that in the answer I gave? You are absolutely correct of course, but what I suggested was building a higher voltage bridge using 4 low voltage bridges (where each bridge is used to build a higher voltage diode) ...are you saying there is some problem with my solution? \$\endgroup\$ Commented Dec 17, 2016 at 23:05
  • \$\begingroup\$ The suggested configs were good as well as using a bridge as a single diode with twice the current and PIV rating is even better. I was directing my reason to his original question "Is it possible to connect two units together to rectify the 240 VAC", which is not possible but using 1 full or half bridge is ok. even if you use 4 bridges to use as 4 diodes , but again in 1 full bridge Max. for this bipolar supply. here is a low cost 2kW +/-130Vdc supply with a fixed load 3% ripple goo.gl/aG1oL4 . click on a trace and the component turns turqoise that I selected \$\endgroup\$ Commented Dec 18, 2016 at 0:18

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