Schottky Diode

[makincoolstuff]

Hello,
I am trying to full wave rectify the output of a low voltage (around 12-19) high current (150+) transformer for a welder. I know that I could simply use a bridge rectifier of normal stud diode to do this, but I have come across some really cheap, 400amp 100v schottky diodes that I think might be able to do it. Is it possibly to use these? because the output will have a large inductor and capacitors to smooth it out and I'm not sure whether welding will cause voltage and current spikes which may be harsh on a schottky.

Thanks

 

[makincoolstuff]

Sorry, the ascii diodes messed up but i think you get the idea

 

[duke37]

You are likely to get much more than 100V spikes with a welder. The schottky diodes may not like this. They should run much cooler than standard diodes.

The first circuit you show only passes current for half the time. The arc could go out during the other half of the cycle. There is unidirectional current in the transformer which could lead to core saturation and uncontrolled primary current.

 

[makincoolstuff]

Thanks. What about a 200v schottky? Because thats the highest I can get. If not, could I somehow smooth the spikes? Or will I have to go for something like this? : http://www.ebay.com.au/itm/Black-Re...cal_Test_Equipment&hash=item2a2da8e43f&_uhb=1

Bit more in price, but should get the job done.

 

[duke37]

Now you are beginning to talk turkey. The reference says the diode bridge is suitable for welding so I would go for that.

Note that you will need considerable heat sinking, perhaps fan cooled.

 

[makincoolstuff]

Talk Turkey?? Haha. Its Just I'm a 15 year old with a very tight budget trying to build a welder and I can get 4 400amp, 200v schottky for $3 a peice or that 200amp 1600v rectifier for $32. Its not just price, I'd like the freedom of 400amp so I could add a few extra transformers if I need more power.

 

[shumifan50]

Wayyyyyyy back from my past I seem to remember that the quality of welds produced by a welder (and the ease of keeping the arc) is determined by the quality of the DC output. The more ripple it has the worse the welds produced. If you start with half wave rectification it will need a lot of work to smooth the output DC, so full wave rectification would seem to be the way to go.

 

[makincoolstuff]

Thanks Duke and Shumifan for the help. Sorry but I have another question, say I have 150amp output at 19v. It goes through the rectifier and becomes a choppy dc. If I want to smooth it using say 50v capacitors, is their a way to work out what capacitance I need. (I've already got my inductor sorted to smooth the current).

 

[duke37]

The voltage change across a capacitor is given by the equation
dV/dt = I/C
If my calcs are correct, then a 5V ripple at 150A and 50Hz will need 4.5F. This is impractical for an impecunious student.

I am not sure about the need for a stable voltage. Cheap welders use leakage inductance in the transformer to control current, the voltage will vary greatly. An arc has a negative dynamic resistance and needs a series impedance greater than this to get a stable arc.
My brother wanted a portable welder and he took a 120V DC generator driven by a tractor. The current was controlled with a very long thick steel rod wound on a wooden former.

Have you considered the power you need and where it comes from?