DIY Spot Welder with Discrete Components

Today we take access to inexpensive microcontrollers for granted – even if, with supply chain issues, you might not get your first choice – so we tend to use them, even if they’re not strictly necessary. A spot welder is one such device where computing power might be useful, but that functions in a straightforward enough manner that it could be controlled sans microcontroller.

In this write-up, Wojciech "Adalbert" J. outlines how he built such an MCU-less spot welder. The device is powered by a bank of electrolytic capacitors which add up to 142500 µF at 35 volts. This is reportedly enough energy to make a spot weld, but not much more than that, minimizing danger in the case of a short circuit. It's is built on a perfboard, and uses a recycled T90RIA120 SCR (silicone controlled relay) that can handle over 1000 amps of peak current and doesn’t require sophisticated drivers.

The device employs a dual 12V relay module for triggering, modified with a series of capacitors and resistors to work out the timing and guard against unwanted sparks/relay malfunction. In order to carry sufficient current, Wojciech soldered copper solder wick to his board, held in place with the help of the capacitor’s legs. 10 AWG wires form the current-carrying leads of the spot welder, with a trigger button connected to the control circuitry via much smaller wires to start the weld process. Charging power can be supplied through USB-C PD.

Overall, it looks like a fantastic build. Like all projects seen here, especially involving high voltage and/or current, be safe if you attempt to do something similar, and be sure to get help if you need it!