Greetings fellow electronics buffs! I have a spot welder project that contains a 120V transformer with a primary from an 1100 watt microwave. I have a 1 gauge 1 m long cable double wound as my secondary which delivers 2.1 V. Also present is a 9 V AC transformer powering both a control board for the spot welder and a second board which delivers 5 V DC to a constant cooling fan. My question is what type of fusing should I use and where should it be placed within the circuits? My understanding is a slow blow fuse of approximately 16 A should be after the main power switch feeding the main transformer due to inrush current. My assumption is a second fuse should be installed before the 9 V breadboard type transformer but I’m not clear on the amperage and if it should be an AC or DC fuse?? Thanks CAB

 

Any overload is going to be reflected back to the primary, so I would not worry about the secondary side.
Normally a spot welder is a very momentary load.
Max.

 

Any overload is going to be reflected back to the primary, so I would not worry about the secondary side.
Normally a spot welder is a very momentary load.
Max.

Thanks Max, was hoping that was the case. Any thoughts on the
9v transformer? I was thinking of a 2A reg glass AC fuse on the hot incoming side ahead of the 9v. This way both boards would be on the same fuse. Does that sound appropriate?

 

Any low voltage control should be fused separate to the welder transformer.
A schematic or diagram would help!
Max.

 

Any low voltage control should be fused separate to the welder transformer.
A schematic or diagram would help!
Max.

Attached is my crude diagram of what I’m trying to accomplish. In Retrospect, I feel that both boards need to be fused AFTER the 9v transformer, since the main line coming in is already fused, correct? Note: The “rear panel” view shows 2 foot control sockets one for full power direct from transformer and the other goes through the control board circuit. There is also a switch to toggle between the two. I also have an additional pushbutton switch if I choose not to connect the foot controller. I’ve tested the spot welder with just the foot comptroller direct and all works great, I’m happy to report. Really appreciate all ideas & suggestions. Thanks, CAB

 

Just another fuse on the small transformer primary should do it.
Max.

 

Just another fuse on the small transformer primary should do it.
Max.

Thanks Max, I really appreciate you keeping me on the right path!