Dk spot welder

Open Source Spot Welder

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.

The welder is powered by a 12 V car battery or another suitable high-current battery source. The PCB, schematic, firmware, enclosure, and fabrication files are included in this repository.

[!WARNING] This project switches extremely high current from a battery. Incorrect wiring, insufficient conductors, short circuits, or excessive pulse duration can cause fire, burns, damaged cells, destroyed MOSFETs, or battery failure.

Sponsor

PCB fabrication for this project was sponsored by PCBWay.

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.

[!IMPORTANT] PCB copper alone should not be treated as a complete high-current busbar. Thick copper cable, copper braid, copper strip, or another suitable reinforcement is recommended for the main battery and electrode current paths.

How it works

1. Power input A 12 V car battery or another suitable high-current battery source is connected to the IN/OUT+ and IN- pads.

2. Probe contact detection When both welding probes contact the conductive workpiece, the PC817 sensing circuit produces an isolated trigger signal for the ATtiny13.

3. Trigger validation The firmware verifies that the contact signal remains active for the configured debounce period.

4. Pre-pulse When double-pulse mode is enabled, the ATtiny13 briefly activates the MCP1407 and MOSFET bank for the configured pre-pulse duration.

5. Pulse gap The MOSFETs switch off for a short interval before the main pulse.

6. Main weld pulse The MOSFET bank switches on for the selected duration between 1 and 10 ms, passing a short burst of high current through the nickel strip and cell connection.

7. Feedback and cooldown The buzzer and LED confirm that the weld sequence completed. The firmware then enforces a cooldown period and waits for the probes to be lifted before another weld can begin.

Weld-time adjustment

TP4 is used to change the main weld-pulse duration.

Each complete button press advances the setting by 1 ms:

1 → 2 → 3 → 4 → 5 → 6 → 7 → 8 → 9 → 10 → 1 ms

After the value changes:

• The setting is saved to EEPROM.
• The LED flashes once per selected millisecond.
• The buzzer beeps at the same time as each flash.

For example, a 6 ms setting produces six LED flashes and six beeps.

The displayed value represents the main pulse only. When double-pulse mode is enabled, the fixed pre-pulse is added separately.

Firmware

The firmware is located at:

Firmware/main.c

It directly controls:

• The MCP1407 gate-driver input
• The PC817 automatic weld trigger
• The TP4 settings button
• The status LED
• The passive piezo buzzer
• EEPROM storage for the selected duration

The firmware forces the MOSFET-driver control pin low during startup. This works together with the hardware pull-down resistor on the MCP1407 input to reduce the risk of an unintended weld pulse during reset or power-up.

Default configuration

Review the values in Firmware/main.c before flashing, as development versions may use different pre-pulse and timing defaults.

Building and flashing

The firmware is built with AVR-GCC and optimized for the ATtiny13's limited 1 KB program memory.

cd Firmware

make
make flash
make fuses

The default Makefile targets are:

The default programmer configuration may need to be changed in the Makefile when using an Arduino as ISP instead of a USBasp.

Enclosure

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

Assembled	Top and bottom

The enclosure includes:

• Four mounting bosses
• Screw-together top and bottom sections
• Side openings for the battery and electrode cables
• Source files for modification and 3D printing

Included files:

Enclosure/SpotWelder.FCStd
Enclosure/Top Body.step
Enclosure/Bottom Body.step

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

License

This project is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license.

See LICENCE.md for the complete terms.

You may build, modify, and share the design for personal and non-commercial use, provided that attribution is retained and derivative works use the same license.

Commercial production or resale of boards based on this design requires permission from Datakonsulten Sverige AB.