Spot Welder 1-2-3 Arduino Printed Circuit Board

Some time ago I wrote an instructable where I explained how to control a spot welder in a sophisticated manner using Arduino and commonly available parts. Many people built the control circuit and I received quite some encouraging feedback.

This is a circuit operating at mains voltage and high currents, so the quality of the build is important to guarantee safe operations. While it is ok to prototype the sport welder using a non permanent setup, if you plan to build and really use this a good PCB will help a lot to achieve both a professional and a safe result.

Please read the first instructable, called Part I for short in the following, if you have not done so already. There is a lot of information there that will not be repeated here.

DISCLAIMER: I am providing some links and suggestions for third party sellers/manufactures purely for your convenience and to anticipate the questions I know will come. I have neither any relationship nor any interest in any of the third parties I mention. They purely worked well for me.

Step 1: Updated Schematic PCB

I have combined control and power circuit in a complete schematic, where several components are added to the simplified (albeit functional) one in Part I.

These include now fuse and line protection on board, and a buzzer to support producing sounds (a clicking sound when the encoder is rotated and a continuous tone when welding are quite nice). An additional MCU pin is exposed on a connector for firmware expansion or modification in order to add new features, e.g. a temperature or cooling fan control.

Step 2: Board Layout

The PCB is a standard 2-layer layout and in the picture you can see how components have been arranged using the EagleCAD layout software.

I have tried to keep things pretty compact using only one side of the board for components and have separated on board the hot and cold side (jargon for the mains AC and 5V DC voltage circuits). The board is about 60 x 80mm (less than 2.5 x 3.5 inches) so it will fit into a compact enclosure.

TRIAC mounting. Please read carefully the consideration on this in Step 6 of Part I. Concerning wire cross section for the connections to the TRIAC, I have used 1.5 mm2 (AWG 15-16) wire for the wires connecting the A1, A2, and G wire pads to the TRIAC terminals, and 2,5 mm2 (AWG 13) for the wires connecting the TRIAC terminals to the MOT (brown wires marked A in the picture in Step 6 of Part I). Keep these connections reasonably short, there should be no need for them to exceed a length of 20-30cm (8-12”).

Step 3: Getting the PCB

You can order the PCB from your preferred fab house, if you have one. I use JLCPCB (, and in my view they do an excellent job at a very reasonable price.

I am providing the needed Gerber files in an archive data, so you do not need to be able to use EagleCAD to order a PCB, just update the zip file on the fab house site and you are in business. Other suppliers will work in a similar fashion.

The components needed for this circuit are all pretty easy to obtain. I am anyway providing a BOM for your convenience with links to the sources I used to procure the less obvious ones.

Be careful when you order the Pro Mini. There are several layouts around, but the PCB is sized to fit the packaging of the Pro Mini version shown in the picture in the next step. Other geometries will not fit the PCB’s hole pattern.

Step 4: Downloads

The PCB data files can be downloaded from the SpotWelder 1-2-3 PCB repository. In the REAME file in the repository you will find a table of contents.

Have fun!



    • Epilog X Contest

      Epilog X Contest
    • Trash to Treasure

      Trash to Treasure
    • Pocket Sized Contest

      Pocket Sized Contest



    20 days ago

    WOW great job! THANK YOU very much for your work!. I already ordered some pcb from jlcpcb
    I 'll try to make a neat 3d printed case as soon I'll receive it.