The ATmega328 based Spot Welder Microcontroller
Earlier when i did mini projects it was hard to solder the 18650 batteries that i had salvaged from old laptops. Not recommended by the manufacturers either. not all the cells in a laptop battery pack were bad, so why not take them out and make use of them to power some DIY stuffs like a power bank for example. They cost a lot when purchasing new ones from online stores. So the best way electrically connect them was to spot weld them using nickel strips.
One way to get it done is to salvage an old Microwave Oven Transformer (MOT) and modify it to get approximately 400 to 600amps (the turns ratio is something you’ll have to figure out). Here the current has to be kept low. Generally reducing the voltage will also reduce the current here. I’ve used 3 turns at secondary with 10mm width flexible cable. That would give enough amps to weld. I’ve set the weld time from the pot to 60ms.
- More turns with thicker cable would give more current
- Less turns would decrease current.
The components used here are cheapest those that are readily available at any local store.Then comes a Microcontroller built on the ATMEL ATmega328 chip. This controls how long the weld is done. The weld time are measured in milliseconds. When spot welding it is common as well as important that there be two welds, Why ? because the first weld clears the strip of all impurities that maybe stuck to it after production. The Second weld is when the strips melt and gets welded to the cell. The chip is powered by a regulated 5v supply from an on-board PCB transformer.
- Surge Protection
- Inbuilt Power Supply
- Dual Pulse Welding
- Dual Surge protection
- Thermal fuse at the secondary
- Zero Cross Peak detection
- Variable timed pulse (indicated by LED from 100ms to 450ms)
- Dual function weld
- Manual Weld
- Continuous Weld (upon holding weld button for more than 800ms)
- TVS protection diode at the dc side
- Snubber Circuit
- TRIAC to control AC load
- Audible Weld alert
The Spot Welder Microcontroller is powered by a PCB transformer rated 2 x 9v 177mA. The PCB is protected by a TVS diode D13. This protects all the underlying components from surge without which would burn the components during a surge such as lightning. here the TVS diode suppress the spike in voltage when the threshold voltage is reached.
Once switched on all the timing LEDs blinks once. The READY led indicates that it is ready for welding. The ATmega gets its clean power from the regulator IC LM7805 filtered by capacitors C9, C7 and C5. The ATmega continuously monitors the AC zero crossing signal from the Optoisolator H11AA1. The optoisolator sends out a pulse for every zero crossing of the AC signal and when the weld button is press, the ATmega sends out dual pulse to another optocoupler triggering the gate of the TRIAC BTA26.
The first Pulse duration is 500ms which prepares the tab for welding, the second pulse is when the actual weld happens. The TRIAC by nature switch’s off at every zero cross. Here a little late after zero crossing but that would not make a difference as it a few micro seconds. Since the load will be an MOT and thus highly inductive, it is very important that load is switched ON at the peak of the sine wave and not at zero. When the Weld button is pressed and held down for a period of 800ms, this helps when you have to weld several cells at a time to built a battery pack. The TRIAC conducts 6 times with a two second delay between each weld. The LEDs 50ms to 120ms reflects the value set by the potentiometer.
Working video can be viewed here.
The above scope shows the following, respective to the numbers
- AC reference voltage (YELLOW)
- Zero Crossing ( LIGHT BLUE)
- Second pulse send from the Arduino to trigger the TRIAC ( PURPLE)
- Secondary wave from the MOT (BLUE)
With a little modification or as a complete replacement, this pcb can used for the cheap Chinese Models like the SUNKO 788H
I have the PCB’s on sale here.
Sunko 708A Replacement:
One of the readers Giordano Cantori had been very nice to send me the pictures of his finished Sunko 708A Spot Welder after the stock board had failed. He had replaced it with the PCB described here with his own idea and design.
Here is the built in action