An easy-to-assemble, through-hole spot welder capable of switching high currents for welding nickel strip to lithium-ion cells. Controlled by an ATtiny13 microcontroller.

Features

• Eight-MOSFET switching bank Eight IRF1404 N-channel MOSFETs are connected in parallel to switch the welding current. Each MOSFET has an individual 10 Ω gate resistor to help control switching and reduce gate ringing.
• Dedicated MOSFET gate driver An MCP1407 drives all eight MOSFET gates simultaneously. A hardware pull-down resistor on the driver input keeps the MOSFET bank off while the microcontroller is starting, resetting, or unpowered.
• Automatic touch-to-weld triggering A PC817 optocoupler-based sensing circuit detects when both welding probes are in contact with a conductive workpiece. The isolated output signals the ATtiny13 to begin the welding sequence.
• Adjustable 1–10 ms main pulse A pushbutton connected to TP4 cycles the main weld-pulse duration from 1 to 10 ms.
• EEPROM setting storage The selected weld duration is saved in the ATtiny13 EEPROM and restored after the welder is disconnected from power.
• LED and buzzer feedback The onboard LED and passive piezo buzzer provide startup, welding, and setting-change feedback. When the duration is changed, the LED flashes and the buzzer beeps once for each selected millisecond.
• Optional double-pulse welding The firmware can produce a short pre-pulse followed by the adjustable main pulse. The pre-pulse can help settle the probe contact before the main weld.
• Separated control and power grounds The high-current welding ground (GND) and logic ground (CTRL_GND) are routed separately and joined at a single net-tie point to reduce switching noise at the microcontroller.
• Reverse-polarity protection A 1N5819 Schottky diode protects the 5 V control supply if the battery input is accidentally connected with reversed polarity.
• Transient suppression Four 5.0SMDJ14A TVS diodes help clamp voltage transients generated by the welding cables and other parasitic inductance when the MOSFET bank switches off.
• Large high-current connection pads The battery and welding connections use large exposed copper pads intended for soldered or mechanically secured high-current conductors.
• Four-layer PCB Copper zones on F.Cu, In1.Cu, In2.Cu, and B.Cu help distribute the short welding-current pulses through the MOSFET bank.

Enclosure

A two-piece enclosure is included and was designed in FreeCAD. 3xM4 Nuts should be installed while printing the enclosure.

Project status

The Rev A hardware has been assembled, powered, and successfully used for spot welding.

The following core functions have been tested:

• ATtiny13 startup
• MOSFET gate-driver control
• Automatic probe-contact triggering
• Timed welding pulses
• LED feedback
• Passive buzzer feedback
• Welding from a 12 V car battery

Full information available at GitHub Repo: https://github.com/sini6a/spotwelder