Dual-Purpose DIY Spot Welder Built with Safety in Mind

Ho-hum, another microwave oven transformer spot welder, right? Nope, not this one — [Kerry Wong]’s entry in the MOT spot welder arms race was built with safety in mind and has value-added features.

As [Kerry] points out, most MOT spot welder builds use a momentary switch of some sort to power the primary side of the transformer. Given that this means putting mains voltage dangerously close to your finger, [Kerry] chose to distance himself from the angry pixies and switch the primary with a triac. Not only that, he optically coupled the triac’s trigger to a small one-shot timer built around the venerable 555 chip. Pulse duration control results in the ability to weld different materials of varied thickness rather than burning out thin stock and getting weak welds on the thicker stuff. And a nice addition is a separate probe designed specifically for battery tab welding — bring on the 18650s.

Kudos to [Kerry] for building in some safety, but he may want to think about taking off or covering up that ring when working around high current sources. If you’re not quite so safety minded, this spot welder may or may not kill you.

11 thoughts on “Dual-Purpose DIY Spot Welder Built with Safety in Mind

  1. Actually it only puts 3V AC open circuit to the fingies. I used to use a big ol’ 50kW spot welder that was donated to my college after being used at a Ford plant, and used it with both my bare hands on the part. Big foot actuated clamp/trigger switch.

    I still have those hands

    1. The dangerous part is on the AC side though a good insulated case for the switch and good grounding would solve the angry pixie problem.
      Though the 555 one shot timer also would help make the welds more consistent.

  2. Safety in mind, but not really so much in practice. It appears that the mains terminals are insulated by a strip of duct tape from an ungrounded metal box. It would be way safer if this were built in an actual enclosure, with a grounded transformer.

    It’s not really that dangerous, but no safer than the conventional method.

  3. Don’t talk to me about safety and not have fusing or a switch on the mains before they get into the triac. Safety in devices like these are multi-staged. Plugged in? Switched on? Settings checked?

    I used a foot actuated spot welder for years and the extra bit you needed to press the pedal to engage the current was that final safety step. The timer is a nice touch.

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