Introduction: 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 (EXT connector), e.g. a temperature or cooling fan control. Activating buzzer and EXT connector will require future firmware enhancement.

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.

The PCB requires the 3W version of the Hi-Link power supply module (HLK-PM01 3W). The 5W variant will not fit.

The JP1 jumper marked PRG must be opened to flash the firmware without removing the Pro Mini from the PCB, and obviously kept closed for normal operation.

This is PCB version 1.1 and requires the 1.1 version firmware.

This PCB supports a buzzer to enrich the interface with sounds, however the 1.1 version firmware does not make use of the buzzer hardware (apologies for not making this clear).

Step 4: Downloads

The PCB data files can be downloaded from the SpotWelder 1-2-3 PCB repository (this is PCB version 1.1 and requires the 1.1 version firmware).

In the REAME file in the repository you will find a table of contents.

Have fun!