DIY Spotwelder v4.0

by Jon Nuljon January 2018

A resistance welder with these features

• Dual Pulse Welds- user configurable timing for pre-weld pulse, pause, main weld pulse

• 4 selectable modes display values and allow user to make changes via front panel interface

    mode 0:  Thermal Control threshold and cut-off
    mode 1:  dual weld pulse pattern
    mode 2:  dual weld pulse pattern
    mode 3:  user can set for continuous weld pattern
  

• Thermal protection via temperature controlled cooling fan and weld current cutoff at user defined temperatures

• LCD display - changes color based on mode and state

• user programmable settings - saved in eeprom memory - now working!

• Powered by MAINS

Hardware details included in the build:

• Rewinding the secondary core(s) of a microwave oven transformer with a single turn of the heaviest wire that will fit, which yields 2 to 4v at up to 1200 amperes.

• A smaller transformer is used having an output of 12v (rectified) for the fan. Further regulation reduces that to 5v DC for the control circuitry.

• various switches for mode select and programmable interface

• a thermistor to detect the temperature

• a transistor to switch a DC fan motor for cooling

• a Grove RGB backlight LCD display to give visual signals (resistance welders should not arc)

• a few resistors, LED, capacitors, diodes, etc.

• notable is a 1 meg resistor and capacitor4 used to relay AC signal to MCU for sensing zero-crossing

• the software runs on an Arduino microcontroller mounted on a van Dalen board that I altered.

• the board now supports thermal sensing and controls cooling, as well as an i2c connection to the LCD display.

The software works with the circuit: DIYspotwelder4.pdf which is a derivitive of Albert van Dalen's http://www.avdweb.nl/arduino/hardware-interfacing/spot-welder-controller.html

As to calculating weld pulse start time, it is preferred to start weld pulses at peak power. For inductive loads, the amperage will follow the voltage phase by 90 degrees. So weld start time is calculated based on mains voltage zero crossing point (from negative crossing to positive voltage).

MAINS POWER where I live is delivered by 60Hz sinus cycle. Each cycle duration is 6.667ms (1 sec divided by 60). Divide this by 4 to calculate 90 degrees lag time, which equals 4167 microseconds (from zero cross until peak amps).

The rotary switch I use has 4 positions but one is NC - which I code as mode0 and use to display the transformer core temperature and fan speed. If program toggle is switched, this mode is also used to configure the thermal control activation threshold and weld current cutoff temperature. The activation threshold is the temperature whereby the fan motor starts to spin. As the sensed temperature increases, the speed is ramped up using a PWM signal from the Arduino.

For each of the 3 weldPattern instances, modes 1, 2, and 3, the toggle switch will also trigger a method call that allows the user to set weld pulse timing. Custom values can be configured for the preWeldPulse, pause between pulses, and the weldPulse. All changes are visible from the front panel display. Setting weldPulse to 0 is interpreted as continuous weld, and the weld current will stop either upon releasing the weld button, pedal, or whatever trigger device. The weld current will also stop if sensor indicates temperature of the core has reached or exceeded the cutoff setting.