# Inverter Technology - How DC Converted Back to AC

I have a little electronics background. I was informed that Inverter Welders convert wall-socket AC to DC and then back to high frequency
AC. How is the second conversion (DC to high frequency AC) done? TIA
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Basically an oscillator (variable frequency) then high current low voltage output transistors driving a step-up transformer.
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The input voltage is rectified into relatively high-voltage DC. The DC is changed to high-frequency AC power by semiconductors and then the high-voltage, high-frequency AC is stepped-down to high-frequency low voltage AC. This relatively low voltage AC is then rectified.
Most inverter circuits utilize various feedback loops to maintain the desired output voltage or current settings. Then you factor in other features found in high-end machines and you end up with a pretty complex circuit.
Simplified explanation here: http://content.lincolnelectric.com/pdfs/products/literature/nx130.pdf
Billh

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Line voltage is rectified to DC high voltage (several hundred volts), switched by high voltage transistors (IGBTs or MOSFETs) at a high frequency (about 20 KHz), stepped down in a high currrent ferrite core transformer, then rectified back to DC low voltage for welding. The main advantage is it eliminates the large heavy iron core transformer needed for 60 Hz.
Tom
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This is correct. I first noticed inverter technology while repairing computer mainframe power supplies. They use only 5 volts DC output but at 200 amps. The reason for using inverter technology was to provide a much cleaner DC output at higher current loads. Anything higher than 50 millivolts of AC ripple on your 5 volt DC output creates serious computer instabilities. I initially had trouble understanding how the DC got turned back into AC as transformers will only pass AC. The method is by turning the DC on and off with the transistor, IGBT, SCR or mosfet. A transformer is only concerned about expanding and collapsing magnetic fields, so it sees the DC switching on and off as a square wave AC source which is much easier to filter than the pulsating DC created thru normal rectification. You may think of inverter technology as double rectification. (sinusoidal AC) to DC to (squarewave AC) to DC = very clean DC without the use of massive transformers. This essentially makes for a very smooth DC welding source in a very lightweight package. PS: Has anyone ever tried welding with non-spillable UPS batteries? talk about a smooth welding process, they make almost no noise and provide a very superior weld to say a buzz box.

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"... A transformer is only concerned about expanding and collapsing magnetic fields, so it sees the DC switching on and off as a square wave AC source..."
Is switching the DC off/on enough to generate 'true' AC? Isn't it necessary to somehow produce the negative component of the AC?
Thanks. John
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Transformer coupling is by the change in magnetic fields and the secondary side has no idea what the primary side voltage is referenced to. On the secondary side, when the field builds secondary wire A goes positive with respect to secondary wire B, when the field collapses secondary wire A then goes negative with respect to wire B. Billh

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The final goal is to achieve the purest DC possible, without any AC riding on top of it. When the sinusoidal AC goes thru a bridge rectifier, the output will be pulsating DC. Usually capacitors are placed inline to attempt to smooth out the gaps between the pulses. By then taking this crude DC and using a thyristor ( scr,transistor,mosfet,igbt, etc.) to switch this crude DC on and off, you will create a square wave AC. The square wave AC is then sent thru another bridge rectifier and the output is a much purer DC because there are no gaps between the pulses. Pure DC is very desirable when welding. It is very smooth and makes almost no noise. If you ever get the chance to take sealed batteries and weld with them, you will see what I mean. Please do not attempt to use car batteries as the hydrogen fumes could possibly ignite and become a very effective explosive device. You will need 3 to 4 sealed 12 volt batteries to make a very smooth welder. I have a very large resistor inline to control the current so that I am able to weld smaller items without any burn thru. I made a set of stainless steel headers for my car using this method and I was amazed at the quality of the welds. They are absolutely beautiful. These days, I use a Lincoln portable inverter with Tig attachment to do this type of work but I am constantly amazed at what good old batteries will do.

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