AC to DC welding machine output voltage conversion - video

[Jon]

AC to DC welding machine output voltage conversion. By Murat Guney. 19:41 video:

[nova_robotics]

I'm not a huge fan of the rectifiers selected, but the way they were paralleled is very smart. Attaching wires to opposing ends like that helps ensure they load is shared equally between the rectifiers. The same arrangement can be found in self-balancing glycol heating systems.

[metric_taper]

I'm not a huge fan of the rectifiers selected, but the way they were paralleled is very smart. Attaching wires to opposing ends like that helps ensure they load is shared equally between the rectifiers. The same arrangement can be found in self-balancing glycol heating systems.

The problem with silicon rectifiers, they have a negative temperature coefficient (relative to resistance). So when you parallel the diodes, the hottest one has the lowest resistance, and hogs the current, making it hotter, and you end up with thermal runaway.
Circuits that parallel transistors to boost the current capability, put a small value resistor in series with each of their emitter current paths that inhibits the thermal runaway, as there is a voltage feedback from the resistors increased voltage drop, to the current injected into the base of the transistor, to regulate the collector current.
I do like the copper bus bars connecting the bridge rectifiers together.

[nova_robotics]

The problem with silicon rectifiers, they have a negative temperature coefficient (relative to resistance). So when you parallel the diodes, the hottest one has the lowest resistance, and hogs the current, making it hotter, and you end up with thermal runaway.
Circuits that parallel transistors to boost the current capability, put a small value resistor in series with each of their emitter current paths that inhibits the thermal runaway, as there is a voltage feedback from the resistors increased voltage drop, to the current injected into the base of the transistor, to regulate the collector current.
I do like the copper bus bars connecting the bridge rectifiers together.

This is true. A few months back I posted a comment on here about how to use the intrinsic diode in MOSFETs to make rectifiers that have positive temperature coefficients. I'd post it again but... that's pretty much the meat and potatoes of the comment. MOSFETs have positive temperature coefficients which distributes load when paralleling the things. Or you could just buy the proper component with an appropriate rating instead of bodging things together. One or the other.