Dual-Pulse Capacitive Discharge Spot Welder

Introduction

The reason for the capacitive discharge spot welder is to aid in the process of building a battery pack for the UMD Bulldogbots combat robot team. By having a spot welder the team can weld nickel tabs to the multiple A123 battery cells that form the battery pack. This assures a quick and durable connection, that is more reliable than soldering from experience and will not degrade the performance of the battery cells from excessive heat.

 Specifications

An Arduino UNO and a MIC4451 MOSFET Driver is used to send the control signals to the IGBT, which delivers the pulse of electricity used to weld. The leads used to deliver the high current of roughly 400 amps pulse for 20 microseconds and 60 microseconds. A momentary footswitch with ¼” audio jack is used to send the control signal the microcontroller. It is powered by a triple output laboratory DC power supply that is capable to charge up the capacitor bank at 3amps at max voltage of 16.2v.  There is also a regulated 5V and 12V supply to provide logic power to the microcontroller and mosfet driver. Based off commercial spot welders, a dual pulse was used.  This allows for the circuit to first make a smaller discharge in order to clean the surface of the material of impurities such as oil and create a weak weld.  The second pulse with more energy makes the final bond. In order to have a stable pulse during discharge a large capacitor was employed.

Build Process

The welder had its circuitry designed  and tested before the design to make it user friendly.  Using Solidworks, a 3D CAD program we were able to design the case using ¼” lexan and because of the flexibility of CAD, I was able to added in mounting holes and user elements such as a handle.  Using the DXF files generated from the Solidworks model we then used the OMAX waterjet in the mechanical engineering department to cut out the case.  The custom red and black copper electrode hookups were design also in Solidworks and were then 3D printed out for use in the welder.

Concluding Remarks:

The welder performed as expected and was able to produce welds that were better than the factory welds that came with the A123 cells purchased.  Based on penetration of the weld.  On the left hand side of the image below you can see the factory welds and on the right hand side the welds produced by my personal spot welder.