Welding Inverter Schematic

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Microwave oven transformer welder (inverter)?

I’ve been collecting some microwaves to modify the MOT’s and create a home welder when I cam across a Panasonic inverter microwave. It would be a lot more convenient to modify it so that I can have a very lightweight welder, but is that even possible?
I know that I could easily change the coils on the secondary to produce the low welding voltage.

My concern is will i blow up the control circuit as I think it tries to regulate the power by regulating the current draw. Is my concern valid?

Here is the schematic drawing:

http://www.vk3hz.net/amps/Microwave_Oven_Inverter_HV_Power_Supply.pdf

WARNING! Trying to weld with high-voltage (in the kV range) is extremely dangerous and foolhardy. I’d urge you not to even consider it.

The key is low voltage, HIGH current.

For proper welding you need about around 70 volts, and 80-150 amps output current! You will not achieve that kind of current except perhaps with a string of about half a dozen microwave power X-formers in parallel. You would also need to buy or else design a very high current potentiometer. or rheostat. Something capable of 200 amps continuous at 50 volts.

Welding machines are typically of the “constant current” type, having a very steep “V-I curve.” This is due to the high internal resistance of rheostat used to control the current. Typically the max open-circuit voltage is around 80 volts. While the on-arc voltage will fluctuate quite a bit, from 20- 50 volts. at, for example, 120 amps. That’s 2.4 – 6.0 kilowatts, at only 20-50 volts BTW.

The load voltage fluctuations are due to changes in the arc length, changes in the geometry and chemistry of the puddle, but mostly due to the unstable volatile nature of the arc plasma itself.

Besides the danger a high-voltage low-current arc is quite inefficient at heating the base metal and the end of the filler electrode. In practice the most important heating mechanism in arc welding is due to free electrons in the plasma impinging on the end of the electrode. For heating in this manner to be efficient it requires a very high current density. Otherwise the heat is simply carried away through thermal conduction of the metal.

Welding typically uses DC electrode positive. That means the electrons travel from the base metal to the molten metal droplet on the end of the electrode. This heats the droplet to near the boiling point of iron. Eventually the droplet grows large enough that it pinches off from the end of the electrode and into the weld puddle below. This transfers heat to the base metal like pouring boiling water onto ice. (Here’s a cool slow-motion video of this process:

A high voltage, low-current arc produces a high-density, high resistance, low temperature plasma that is very tenuous, and also has very poor current density. Almost all of the power is dissipated by heating the dense, poorly ionized plasma, not in heating the base metal. Also due to the inherently high resistance of the arc and high closed-circuit voltage, you’re going to be worrying about stray currents through your body if you get to close to the base metal, which is grounded.

I’ve worked as a welder for a while. Most welders can tell you about “one summer” when they were covered with sweat and their gloves were fairly damp. They leaned against the grounded metal table while adjusting their electrode, and got a mild jolt. Not terribly serious since the OCV is only 80 volts. If the OCV was several thousand volts on the other hand such casual contact would have probably killed them.