# Can I use two bridge rectifiers to rectify 240 VAC split phase?

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.

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

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:

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

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:

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.

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:

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.

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.

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.