[a small introduction, I am working on a solid state advanced square wave inverter to add to my DC tig welding machine]
I want to say a big THANKS and a BIG HUG to all those who helped me so far. I would not be where I am, without you. I am not quite at my destination yet, but the light at the end of the tunnel becomes more and more apparent.
To readers of s.e.d., I would appreciate if you could please scroll down and answer my question about the need for a fast recovery diode for my RCD snubber that I made today. I think that it is needed to prevent voltage spikes that are still seen.
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Today was a big day for me. I changed my snubber and plugged my prototype into my actual TIG welder.
I was very afraid and wore protective goggles all the time, but nothing terrible happened.
Here you see pictures of my DC -> AC inverter working with my actual Hobart Cyber Tig welding machine.
On the pictures you see the welder running the bridge at 65 amps. The outputs are simply shorted with a 1 gauge cable. No actual welding was taking place. HF arc starting was disabled.
It is quite apparent that whatever I have, in terms of bridge shorting time, as well as the snubber, is working adequately (more on this below). The 150V varistor that I placed across outputs, did not even get warm at 135 amps.
I went all the way to about 135 amperes. The only problem that I has was that the snubber diode got quite warm soon and I had to discontinue the test (reduce amperage to 65 amps).
The bridge seems to be functioning perfectly in a stable manner at 65 amps.
RCD Snubber
After much experimentation, I realized that the resistor on a simple RC snubber was overheating very quickly and that the capacitor was dumping too much energy when the bridge shorted. I replaced it with an RCD snubber, which you see on the picture 4.
(I may have gotten the schematic wrong, it is late) DC + -------------------------------+-------| \\ --- Diode R / /\\ BRIDGE \\------| --- --- Cap DC - ---------------------------------|
The diode that I used in the RCD is NOT fast recovery. I just do not have a fast recovery diode. That, I think, explains spikes that I still observe. I will probably plop down $25 and buy a 240A diode:
I want to buy this diode: Digikey information:
Technical/Catalog Information 497-4408-5-ND Standard Package 100 Category Discrete Semiconductor Products Family Diodes/Rectifiers Vendor STMicroelectronics Diode/Rectifier Type Fast Recovery Voltage-Rated 600V Current Rating 240A Package / Case ISOTOP Packaging Tube Lead Free Status Lead Free RoHS Status RoHS Compliant Other Names 497-4408-5
Let me know if you that that it is wrong for my application.
(Click on the thumbnail images to enlarge)
Front panel of my cybertig. Shows 65 amps of current.
Prototype of inverter. Screwed down to a wood board so that nothing falls off.
Top -- timing signal going into the Semikron 23 drivers Bottom -- voltage across the DC rail. You see spikes. at 65 amps, they are about 90 volts.
Snubber Circuit -- a big diode, capacitor, and 20 ohm resistor.