So i am working on a homemade plasma cutter i thought about taking the simple route of rewinding microwave transformers like i have done with the welder i build in the past but, i decided i want to learn how to design and build an inverter power supply(switch mode) and i need some help as i cannot find any good information on designing one for my specific need.

This power supply will be required to output at least 240v DC (open circuit voltage) and a short circuit current of at least 40 amps for a start, although i would like to implement variable current later on, and even higher current

My main questions are: how do i design and create the high frequency transformer? and how do i achieve the outputs required? what determines the output voltage and the output current? i understand that switch mode power supplies are PWM controlled, so does the transformer determine the output voltage and the PWM determine the output current?

Hopefully someone here will be able to help me

 

Anybody out there able to help me out?

 

I have worked with commercial welders and plasma cutters doing repair work on them for years and I can assure you they are not something you just toss together with bit's N pieces from old microwaves and inverter driven ones are definitely not something that anyone can just toss together either.

Unless you have had previous experience with designing high powered SMPS and their related control circuits making an inverter based plasma cutter is way over your head.

What would possibly be within your range would be to make a non inverter based unit that uses phase angle control to keep the output current stable.

Still that said to be honest if you need a 40 amp plasma cutter and are fair with power electronics just go out and buy a broken one. You will be way ahead on doing that.

For reference go here and download the two older style 40 amp plasma cutters service manuals for eht MAX 40 and the Max 42.

http://www.hypertherm.com/en-us/Service/Manuals/

I have worked on both over the years and design wise both are about as basic as they can get and still work well. The Max 40 is a transformer based unit and the Max 42 is an inverter based system.

 

Here is the stick welder i built from microwave transformers:
http://youtu.be/wSgOlPK_-q8

with an isolation transformer and simply rectifing and filtering the 240v mains i could build a power supply suitable for a plasma cutter, just by using a large heating element to limit the current, but I'm just looking for the experience of building a switch mode supply myself and learning what goes into designing one, i understand the basics of how the work, just dont understand all the details of what goes into them

 

Here is the stick welder i built from microwave transformers:
http://youtu.be/wSgOlPK_-q8

with an isolation transformer and simply rectifing and filtering the 240v mains i could build a power supply suitable for a plasma cutter, just by using a large heating element to limit the current, but I'm just looking for the experience of building a switch mode supply myself and learning what goes into designing one, i understand the basics of how the work, just dont understand all the details of what goes into them

Nice job with the welder ...
Is this for real work or just saying look what I can do? ???

 

I do have practical uses for the plasma cutter when its done, but more so I wanna do it for the learning experience, I have experience programming microcontrollers and want to combine that with a high power project, and I have a plasma cutter torch head laying around in need of a power supply, so why not build my own inverting supply?

 

I do have practical uses for the plasma cutter when its done, but more so I wanna do it for the learning experience, I have experience programming microcontrollers and want to combine that with a high power project, and I have a plasma cutter torch head laying around in need of a power supply, so why not build my own inverting supply?

What kind of electronics tools do you have? ?
Also are you using.transformer based or inverter based? ?

 

I would like to build an inverter power supply for it, that's what I need help with, I understand the basics, just need help with the more in depth parts of the design, like once I select the switching frequency, how do I design the transformer? And what aspects of the design contribute to the output voltage and current? I need at least 240v DC open circuit voltage, and I wanna shoot for 40 amps short circuit current for a start

 

For an inverter based system the transformer will have to be a large ferrite core unit which to be honest is not something you will find just lying around in common electronics devices.

To put out 40 amps at 240 VDC is going to take around 10 Kw of input power which is about 30 times the size and power handling capacity of the biggest common ferrite core transformers and switching devices will find in most any typical SMPS.

My point is making a functioning plasma cutter is not anywhere close to the same as making a little AC stick welder out of a old microwave oven power transformer.

To be honest the closest thing you will get to a HV AC source with an automatic current limiting function built in is HID lighting ballasts. Many of the large 1000 watt units can put out 200 VAC or more open circuit voltage and supply up to 6 - 8 amps of constant current into a dead short. Paralleling multiple units would be your best option to get the high power you will need.

 

I have an inverting tig welder that will put out 140 amps max at 80v DC that's similar to the 10,000 watts required, the transformer isn't all that big, about 3in by 3in around the outside

 

Yes and it probably works on a 40 -60 KHz switching frequency as well which again takes a lot of work to build a system around.

If you are thinking of scrapping a 140 amp TIG unit to build a 40 amp DIY plasma cutter you would be money and time ahead to just sell the TIG system and use the cash to buy a plasma cutter.

I am not trying to rain on your parade but honestly high power high frequency based power supply design is very difficult and not something you can just toss together with odd bits N pieces of other devices you have laying around.

Seriously just buying the switching devices and related HF rectifiers for the system will set you back a few hundred dollars and you will only have 10% of the items you will need.

 

I'm not planning on scrapping the tig, and I understand it won't be easy, I still would like to try it, I am sure I can do it, you seem to have a pretty good understanding of these types of power supplies, and well I'm willing to learn if you want to help me out

 

The most practical way I can help you would be to sell you one of my complete parts machines and let you fix it yourself.

I have a older Esab 80 amp big green suitcase type inverter plasma cutter that has a control board bug. The power supply and internals are complete and were functional when I took the torch off of it for parts.

$200 plus shipping from 58701 and it's yours. It weighs around 70 - 80 pounds and can be FedEx or UPS ground shipped.

At the worst you would have to design your own control board provided you don't blow up the switching devices while fixing it first and even then at least you would have all the critical HF magnetics parts for the primary power systems that you can't just go out and buy online anywhere.

Send me a PM is you are interested.

 

Thanks for the offer but I would still just like to try building one from scratch, although not the most practical route, but I know I can do it.

Taking a look inside my inverter TIG, it looks fairly simple, although I know its not as simple as it appears, I know I can do it

 

Okay. If you need big HF ferrite transformer cores, inductors, HF power capacitors and what not let me know.

I have a good collection of parts that came from other plasma cutters and I can part with some of them on the cheap.

 

Thank you, I'll let you know if I need anything

So do you know how I could go about figuring out how to design the transformer? Like calculating core size and number of turns on the primaries and secondaries? Where I need center taps, if required?

 

How many wraps did you put on that welder? Looks like 10 or more. Most of the ones I've seen use only two wraps.

 

The welder had 20 turns on each transformers secondary, placed in series had an open circuit voltage of about 40v, being so low arc starting was very difficult

 

http://weldingweb.com/showthread.php?752-DIY-plasma-cutter&
http://hackaday.com/2009/01/06/make-a-plasma-cutter/
http://www.pirate4x4.com/forum/shop-tools/1035524-120v-homemade-plasma-cutter.html

 

You can buy ferite E-cores and toroids to build your inverter welder from:

http://www.surplussales.com/index.html