There are hundreds of used transformers on ebay, $20-$40...but which one to get, what specs? (sometimes listed as MOT/Microwave Oven Transformer)
I have been watching the DIY spot-welding threads, and I am now very drawn to the "Spot Welding Copper Strips to 18650 Battery Cells" thread ( https://endless-sphere.com/forums/viewtopic.php?f=14&t=84680 ). Copper is cheap and available, nickel is expensive and will become worse over time. Nickel has been used because it is the common material in factory battery packs, but...those packs are usually low amp. I contend that copper is the future for high amp DIY battery packs.
That being said, due to it being uncommon, there are no mass-produced copper spot-welders, so the few models specifically made for that are expensive. Copper requires more energy than spot-welding nickel.
edit: the two magnetic shunts shown in these videos MUST be removed for what we want, they would limit the amount of current. We want to start out with "too much" current and then adjust it down to where the device works for copper or brass spot-welding.
I have seen videos of using a transformer that was scavenged from a trash-day microwave oven. The one in the pic above was a wall-mounted built-in unit, and it was large and 220V. I suspect that the more common smaller microwave ovens that use 120V might be very inadequate. The spot-welding of copper requires very high current, but doesn't need high voltage. Copper absorbs heat fast, and conducts current easily. Nickel has enough resistance that it welds easily, since the welding current causes localized heat, but...of course, we don't want resistance...Using thicker and wider nickel strips to help battery current only helps up to a point, resistance is bad...
Using the resistance of nickel to get enough localized heat to create a spot-weld makes life easier for the manufacturer, but its a bad design for performance. Since copper doesn't have that resistance to help it create localized heat to create a weld, we must use actual high currents (the high-temp tungsten probes even glow).
This video shows how to remove one of the two stock windings of the salvaged transformer, and he then uses only TWO wraps of very thick cable (The fattest that would fit) and doing that seems to work well for producing low voltage/high current at the probes...
modifying a salvaged transformer (5 minutes):
https://www.youtube.com/watch?v=d5pGN6pqkyY
Constructing a spot-welder (9 minutes):
https://www.youtube.com/watch?v=vrlvqib94xQ
I think it would be easy to make a simple timing switch on the 120V AC side of the input. That way, we would hit the foot-switch, and we would get a pulse. Once the pulse is timed properly, no further adjustment would be necessary, if we only weld the same thickness of copper strips.
I have been watching the DIY spot-welding threads, and I am now very drawn to the "Spot Welding Copper Strips to 18650 Battery Cells" thread ( https://endless-sphere.com/forums/viewtopic.php?f=14&t=84680 ). Copper is cheap and available, nickel is expensive and will become worse over time. Nickel has been used because it is the common material in factory battery packs, but...those packs are usually low amp. I contend that copper is the future for high amp DIY battery packs.
That being said, due to it being uncommon, there are no mass-produced copper spot-welders, so the few models specifically made for that are expensive. Copper requires more energy than spot-welding nickel.
edit: the two magnetic shunts shown in these videos MUST be removed for what we want, they would limit the amount of current. We want to start out with "too much" current and then adjust it down to where the device works for copper or brass spot-welding.
I have seen videos of using a transformer that was scavenged from a trash-day microwave oven. The one in the pic above was a wall-mounted built-in unit, and it was large and 220V. I suspect that the more common smaller microwave ovens that use 120V might be very inadequate. The spot-welding of copper requires very high current, but doesn't need high voltage. Copper absorbs heat fast, and conducts current easily. Nickel has enough resistance that it welds easily, since the welding current causes localized heat, but...of course, we don't want resistance...Using thicker and wider nickel strips to help battery current only helps up to a point, resistance is bad...
Using the resistance of nickel to get enough localized heat to create a spot-weld makes life easier for the manufacturer, but its a bad design for performance. Since copper doesn't have that resistance to help it create localized heat to create a weld, we must use actual high currents (the high-temp tungsten probes even glow).
This video shows how to remove one of the two stock windings of the salvaged transformer, and he then uses only TWO wraps of very thick cable (The fattest that would fit) and doing that seems to work well for producing low voltage/high current at the probes...
modifying a salvaged transformer (5 minutes):
https://www.youtube.com/watch?v=d5pGN6pqkyY
Constructing a spot-welder (9 minutes):
https://www.youtube.com/watch?v=vrlvqib94xQ
I think it would be easy to make a simple timing switch on the 120V AC side of the input. That way, we would hit the foot-switch, and we would get a pulse. Once the pulse is timed properly, no further adjustment would be necessary, if we only weld the same thickness of copper strips.