Kerry D. Wong Takes a Pocket-Size Spot Welder Apart to See Just What Makes It Tick — and Weld
Designed to be portable, this pocket-size welder is clever — but as Wong found out, you should handle it with care.
Engineer Kerry D. Wong is no stranger to the operation of spot welders, having converted parts from an old microwave oven into one — but was curious as to how a more professional design worked, so set about dismantling a popular pocket-sized model.
"I built a spot welder with an old microwave oven transformer (MOT)," Wong explains. "Since then, I have used it in many of my projects including using it to make a battery bank. While that spot welder does an excellent job, it is very bulky, so this time around I got a commercially available battery powered spot welder and wanted to see how well it works."
The chosen welder: The BIFRC DH20, a compact battery-powered unit that comes with interchangeable welding tips. As well as being particularly portable, the DH20 has a feature many of its rivals lack: Rather than manually triggering the welding process, it detects when the tips are in contact with the material and automatically powers up.
"The DH20 has nine selectable welding durations, and the waveform consists of two separate pulses: A pilot pulse that is roughly 11ms in duration which is used to pre-weld the materials together," Wong explains, having verified its operation with an oscilloscope. "The secondary pulse which is delayed by 3ms , it is used to complete the weld. The width of the secondary pulse is selectable and can chosen from 30 ms to 150 ms at an interval of roughly 10 to 15 milliseconds per step."
From there, Wong took the unit apart — discovering four MOSFETs wired in parallel, thick copper wires to handle the high current, an STC 15W404AS 8051-based microcontroller with voltage measuring analog to digital converters, and a handy 5V USB output — letting the unit double as an emergency USB battery bank.
"The Lipo battery is mounted on the reverse side of the PCB. You can see that the battery is soldered directly onto the PCB and the protection circuitry is not on the battery terminals," Wong notes. "This is due to the large pulse current required during normal operation. So extreme caution needs to be taken when the circuit board is removed from the chassis."
The full tear-down is available on Wong's blog, while a companion video has been published to YouTube.
