Homemade Inverter Plasma Cutter

This does *not* look like a good 'beginners' project to me...

--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm

Another non-PFC RFI-blasting flame-throwing 5kW killer power supply...

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 Thanks,
    - Win

IF i not mistake the transforma is a " step-down transforma ". but are you sure is 150V ? and at The torch is a AC or DC voltage ?. what i know about the transforma is --- primary power input WATT ( I X V ) = secondary power output WATT ( I X V )

Hello Jeremy, as a fellow hobbyist, I do not want to dampen your enthusiasm but there are no good books that tell you how to build this sort of gear. You will just have to keep your eyes and ears open picking up snippets of information here and there.

You can find out how other people have fared by joining this Do It Yourself Welder group.

Only a couple of members have had some success but costly in burnt up expensive semiconductor parts and time.

If you are really keen, see if you can buy damaged goods at a rock bottom price and use your skills repairing them. Much cheaper that way. Somewhere something must have fallen off a fork lift and got bashed. Just keep asking and looking.

Since you say it is an educational project you might be able to make improvements that are two luxurious to be fitted to a normal commercial item. For example I mistakenly bought a cheap used SIP brand MIG welder. It was rubbish in comparison to regular welders but I have modified it to work only on sheet metal car body thickness material and it does a nice job. It has an extra inductor, bank of large electrolytic caps, extra switch to allow shielding gas to flow before welding commenses. A better motor drive with control box near operator so I can adjust the wire speed while welding.

I learned a lot from that project. Be warned, this hobby is addictive, one welder or one cutter is never enough. :-)

Have Fun, John Crighton Sydney

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Why not a PWM IC with overcurrent protection, controlled by the pic? Use a DAC from the pic into the PWM. Control the solenoid with pre/post flow, temperature fuse, PWM, LED's, and possibly current feedback using the pic.

I've found a "hyperfast" rectifier used in a Lincoln plasma cutter inverter. It's part RHRG30120, 30 amps 1200 volts, 65 ns recovery time. Pretty cheap too, $3.

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I can solder fairly well, not surface mount but I've made a few perf board circuits (ignition coil 555 drivers, etc.)

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The first would be to lashup the BIG POWER stuff, to see if it will cut what needs to be cut when at maximum uncontrolled power.

The second might be to add the control components to accomplish the level of control you want....

If you can't get the FIRST STEP working, there is no hope for the second, and it will save you most of your development time....

If the FIRST STEP blasts a hole thru to another dimension, or creates a wormhole, you are sure your idea can be made to work. Then it is simply a matter a few six-packs and some well-thought-out circuits.....

Good luck, if you get it going, please tell us about it....

Andy

I worked on one of these, long time ago tho, they were amasing - fun to play about with too, signed my name in thick sheet steel with the prototype lol.

It was very large - with power factor corection, had several stages of conversion and at the time when mosfets/igbts up to the job wernt that easy to get hold of.

I would sugest looking at phase controled, bridge driving smps ICs, rather than trying to control with a pic. every time your software failed or you pic crashed you would end up with lots of burnt bits of expensive plastic. they will have curent control wich would mean you would not have to rely so much on the design of the transformer to limit the current.

I sugest you get your soldering up to speed before you attempt something so unforgiving.

Colin =^.^=

This has been done, google plasma cutter on the web & rec.crafts.metalworking. Perhaps some welding news groups. The Teslaphiles at pupman.com could also help. I've seen schematics out there. You can e-mail me if you like, I'm somewhat interested.

Scott

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DIY Piezo-Gyro, PCB Drill Bot & More Soon!

POLITICS, n. A strife of interests masquerading as a contest of principles. The conduct of public affairs for private advantage. - Ambrose Bierce

There is no giant behind the devastation of the world?only a shriveled creature with the wizened face of a child who is out to blow up the kitchen because he cannot steal his cookies and eat them, too. - Ayn Rand

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The phase shifting controler is efectivly PWM control, but instead of varying the mark space ratio of each half cycle, it drives each side of the bridge with 50% squarewaves and the PWM is efected by varying the phase, the actual drive to the transformer sees a PWM squarewave, the advantage is that simple drive transformers can be used to drive the upper devices wich is not feasable with widely varing MS ratio, also tuning can be used to drasticaly reduce switching transients as it alows switching at low volt/current.

probably stil best to let the controler manage the curent control loop, just tell it what curent to limit it to either from a simple potentiometer of from a DAC.

You wil probably go through quite a lot of power devices before you can get it to work reliably.

Colin =^.^=

Jeremy, How much do you know about H-bridges? Use courier or another fixed-width font for the following ASCII art schematic:

V+ | +-------+---------+ | | \\ S1 \\ S3 \\ \\ | +----+ | +------|load|-----+ | +----+ | \\ S2 \\ S4 \\ \\ | | +-------+---------+ | GND

I'm not a power electronics guru, but here are some things to keep in mind:

S1-S4 must be in a well-defined state any time the rails are energized. This may have implications for power sequencing.

I think there are only three states you will want: all switches off, S1 and S4 only on, and S2 and S3 only on.

If S1 and S2 are ever on simultaneously, or S3 and S4, you will experience rapid catastrophic failure of some sort.

Since switches don't turn off instantly, you may need to wait a while (10's or 100's of nanoseconds? I'm not sure) during transitions after you turn off a set of switches but before you turn on the other set.

You cannot directly drive IGBT's with a pic.

The only state which can be safely maintained for a long time is all switches off.

Whatever circuit drives the upper part of the H-bridge will probably have to be referenced to the emitter of the upper-side IGBT's. You may need to use an optocoupler or something.

Neither side of the load is ground. Neither side is safe to touch. Neither side can have the ground probe of an oscilloscope connected to it, unless the oscilloscope ground is floating, in which case, the entire oscilloscope should be regarded as unsafe to touch.

HTH.

--Mac

I agree with who ever said "not a good project for a beginner." You are in for no end of nightmares trying to get a high frequency high power H-bridge to work. The fact that you are asking about the transformer illustrates the point. Transformer design is critical and H-bridges are notorius for causing problems like "flux walking" that are guaranteed to destroy your switches until you get a handle on the design and thoroughly understand it's nuances. Even experienced engineers have trouble with these things until they learn the art. I suggest you visit Harbor Freight and save your self a lot of headaches. It will certainly be cheaper in the long run. I'm all for education but you need to thoroughly study switching power supply design and work with low power, easy to understand projects first. Secondly you will need first rate test equipment like a digital strorage scope to capture transient events that will plague any design. Good luck. Bob

Getting a project like this to work is harder than it looks.

I'll say right now:

• I AM NOT AN EXPERT. Don't take my advice very seriously - I'm just hoping to point you in the direction of reasonable looking documentation.

• SAFETY FIRST. Be very careful with high voltage supplies. Keep one hand in your pocket :-). Be particularly careful with charged capacitors. It's possible to get a nasty shock even with the power off! Ensure that you are not the only one around while you work on the thing.

I attempted to build a high current H-bridge as part of my undergrad thesis. My report (and some photos) are available here:

You might consider reading the application notes I have referenced in the bibligraphy - particularly those from International Rectifier.

The inductance of the leads holding the bridge together can result in some nasty transients when you turn the H-bridge switches on or off fast. These transients can destroy the switching devices and/or the gate drivers.

You will need to be extremely careful with layout. Use high power modules instead of paralleled discretes where possible.

Use google to search sci.electronics.design for some good posts on snubber networks by Win Hill. Read them all twice.

You will need a fast storage oscilloscope and a variable DC supply to drive the bridge with while you test it.

When something fails, there will be a big bang and it will be very hard to determine what failed first :-). The best approach I know of is to test the bridge VERY cautiously. I'd use a procedure something like the one outlined below.

1. Use a light resistive load to test the bridge for the first time.

1. Keep the bridge supply voltage at zero, and look at the gate drives to the IGBTs. Check that bridge phasing is correct, that the gates are not being overdriven. Look for overshoot at the edges of the pulses (due to wiring inductance oscillating with the gate capacitance). FETs and IGBTs can be easily damaged by exceeding Vgs(max) or Vge(max). Be aware that your oscilloscope probe has capacitance of its own.

2. Slowly increase the bridge supply voltage. Look at the drain-source/emitter-collector voltages at the edges of the square pulses. Zoom right in to the rising and falling edges. Look carefully for very short spikes which would exceed the maximum Vds / Vce. Exceeding Vds even very briefly can write off FETs. Dunno what the situation is with IGBTs.

1. Look out for capacitive coupling from the drain/emitter to the gate. It is possible for this capacitance to turn on the switching devices briefly at inappropriate times (possibly with catastrophic results). For this reason, do not to use gate resistors which are too large.

There was an interesting discussion on this group about the purpose of gate resistors for fast switching of power FETs. There seems to be some disagreement about why we put them there. Maybe they appease the Gods :>

I'm sure people more knowledgeable than I will post good advice on this topic. I think the best advice I ever read on line-powered switching PSUs was in the Art of Electronics (by Win Hill, Paul Horowitz)- "Buy it."

Best of luck - and let us know how it goes!

Alan

--
Alan R. Turner | Live never to be ashamed of anything you do or say.
To reply by email, remove Mr Blobby.

Thought about doing that. I would use a smps to convert the mains voltage. I would use a ferrite transformer of the ETD 49 type or a bigger design of the etd cores.

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With the exception of using MOSFET's, your H bridge is nearly exactly what I had in mind. I have a half bridge IGBT module, but obtaining another would be difficult. I think using the same IGBT would be ideal, so I'll either have to buy some IGBT modules or use a half bridge.

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I can see issues with wiring distance vs. cooling being an issue. Would you recomend using insulated or bare wire? Insulated wire might add some capacitance, which might increase transients as well as affect the maximum frequency.

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Use straight wires whenever possible? Straight wires=less inductance?

snubber

DC supply can be a PC power supply. An addition power supply can be added in series/parallel, if I insulated the output voltage from ground (but keep the case grounded). A combination of two power supplies with

-12,-5,5, and 12 volts would be precise enought.

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With my understanding, IGBT's are also sensitive to gate over voltage.

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Thank you for the advice, very informative.