I modified a microwave trasformer and i got this measurements for the Voltage. I need around 140A at 5V (voltage isn't really important). I'm using a bridge rectifier to get DC and it can hold 150A. In this case the transformer has 10 loops of wire but the voltage goes from 8V to 40V and I'm worried that the A will go above 150 and my rectifier will get destroyed.

 

Did you remove the magnetic shunt, if it had one?

 

Why are you trying to measure an AC voltage with your meter set to a DC voltage range ?
Also the current is a function of the load. So for example a car battery is capable of supplying several hundred amps but whn the load is just the interior light in the car the current is probably only 0.5 of an amp.

Les.

 

With ~10 complete Turns around the Core, "~8-AC-Volts" is probably what You actually have.
AC-Volts is an "RMS"-Voltage.
The "Peak-Voltage" of the Sine-Wave-Output is
"RMS"-Voltage X ~1.4 = ~11.2-Peak-Volts.

If You run the Transformer-Output through a Bridge-Rectifier,
and then attach a Storage-Capacitor to the Bridge-Output,
You will get close to ~11-Volts-DC.

If You do not have a Capacitor,
and You are just "touching Wires together",
You can cause high-Voltage "Spikes" in the Output-Voltage.

These "Spikes" are extremely fast, and a normal Multi-Meter can't read them accurately.
These "Spikes" can easily smoke Semi-Conductors.
You are NOT getting ~40-Volts, either AC or DC.
The method You are using for measuring the Voltage is giving You a wrong indication.
( unless You have an extremely inductive-Load attached to the Output )

Be sure to remove the Magnetic-Shunts from the Transformer-Core.
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Ok so I need to remove the shunts and read the AC voltage? I will update here tomorrow at approx. this hour

 

What are You building ?
How much Voltage and Current will be required ?
What size Wire are You using for a Secondary-Winding ?
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I will use the transformer for mainly 2 things. First a spot welder (for that I don't need a precise voltage or amperage) and second I'm going to build a hydrogen generator thru electrolysis. I have two stainless steel electrodes that are both 1mm*100mm*100mm and I intend to use the bridge rectifier to get the DC required. For the electrolysis I aim to get as many A as I can while staying under the 150A max of the rectifier. I don't have the cable but I intend to make one myself by stripping old wires (that I already have) and combining the copper, I'm aiming to get around 8mm in diameter so that it can whitstand that many amps

 

I need around 140A at 5V (voltage isn't really important).

You may also want to look at the ampacity rating of the wire you are winding your secondary with. With a current of 140 amps max you want to consider a 1/0 wire gauge (AWG) or about 70mm^2.

 

For generating Hydrogen,
You need to stay at very low-Voltages,
and then increase the Current by having a MASSIVE amount of "Surface-Area",
and placing the Plates as close together as practical.
A pump to continuously circulate the Water helps a lot.

Higher than ~2-Volts will just generate a lot of other odd-Gases,
( depending on the particular Chemicals You use to make the Water conduct Electricity ),
and in addition to that, more Voltage will only make the Water get Hot and boil.
Some of the Gases produced may be Poisonous.
Be careful.
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I will use a solution of 80grams of sodium hydroxide for 5 gallons of water and for the electrodes I'll use 2 steel plates with a total surface area of 400cm² . For the current I want to use as low as a voltage as I can while having under 150amps because the bridge rectifier has 150A max and the transformer is a 700W MOT.

 

For a spot welder you do not need DC, and for 140 amps your transformer will not accept the wire size required. (See the wire size given in post #8)
And I am really wondering about what other gasses are produced when electrolyzing fairly clean water. I suppose that adding salt will possibly contribute chlorine gas, but I am not aware of it happening.

 

For a spot welder you do not need DC, and for 140 amps your transformer will not accept the wire size required. (See the wire size given in post #8)
And I am really wondering about what other gasses are produced when electrolyzing fairly clean water. I suppose that adding salt will possibly contribute chlorine gas, but I am not aware of it happening.

If I use sodium hydroxide and distilled water no gasses will be produced
edit: I meant the only gasses produced will be oxygen and hydrogen

 

Alright I removed the magenetic shunts and run some tests and for a turn of wire the transformer output 0.7V so all my problems are solved thanks

 

I'd say that your problems are just beginning !!!!
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Well if you say so I'll trust you but I'll try to fix'em

 

"" If I use sodium hydroxide and distilled water no gasses will be produced ""

This is not even close to accurate.
Things are going on that You are not aware of.
You are going to be creating all sorts of strange mixtures of Gases, and Compounds in the Water.
Some are Explosive, or Flammable, and some are Poisonous.

Have fun with it, but remember to be careful.

Plain-"Mild-Steel"-Electrodes will not last very long,
You need high-grade-Stainless-Steel if You expect it to last more than a Day.
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At a chemistry level I think no gasses will be produced except oxygen and hydrogen and that in the water ther will be sodium ions but I coul be wrong. For the steel I bought 304 stainless steel (how long will it last?)