Re: An update on my TIG inverter project

Ignoramus30105 wrote:

>[...]
>>> I will check the IGBT application notes and look for that.
>> I think it is more a characteristic of the welder that makes you need the
>> varistors, so the IGBT manufacturer may not have foreseen this need.
>> Anyway I would just look in your preferred component catalogue for a
>> varistor rated at maybe 1.5 or 2 times the open circuit voltage of the
>> welder, and which looks stout enough that it won't melt on the first
>> little
>> spike. I have used Siemens varistors in the past but they are called
>> Epcos
>> now. I think Harris used to make them too.
>
> I will definitely check on that. I would hate to burn up these IGBTs.
>
> That said, open circuit voltage of my welder is 85 volts, and IGBTs
> are rated for 1,200 volts, or 14 times more than OCV.
>
> So, when you say get a varistor rated for 2 times OCV, I have to
> wonder what purpose would it serve.
Well, if you ever played with old telephone relays when you were in school,
you will know that if you connect one to a 6 Volt battery, and then
disconnect it from the battery, you can get a spike of hundreds if not
thousands of volts across the coil at the moment when you disconnect it
from the battery. The sensation is unmistakable. The high voltage is
developed in the inductance of the relay coil.

Similarly, 85V from your welder can, where inductors are involved, lead to
voltages high enough to blow your IGBTs. The caps might prevent this, but
the varistors will be extra insurance, and cheap compared to IGBTs.

You could choose the varistor voltage anywhere from 85V up to 1200V, but I
would not go lower than 100-150V in case the varistors get hot, and I would
not go above 400-500V in case there is some more inductance after the
varistors. If you could guarantee that the varistor will be directly
across the terminals of the IGBT module, (closer than 2 inches) then you
could go for 1000V varistors, but this would bring no advantage over 150V
varistors. It will probably be easiest to get varistors rated for the AC
mains voltage in your location. These should work fine.

Chris

.

Quantcast