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The Capacitor discharge microspot welder has been featured on HaD, see it here . Thank you guys!

The circuit diagram and complete source code is available at the bottom of this article. This project is based on Jiri Pittner's welder for the Atmega16, and so the source code (Below) is released under GPL.
welder sparks 7 welder sparks 6 welder sparks 5 welder sparks 3
This project took a lot of time to complete. This article is structured on several parts, the bottom containing the final results. The microcontroller source code for the Dual Pulse Welder/Cutter is included at the bottom part of this article.

Progress summary:

2012-02-08, First tests
2012-02-09, Choosing the LCD
2012-02-22, 1,000,000uF Capacitor update
2012-03-01, Project box
2012-03-03, Power supply and some innovative electrodes
2012-03-18, Tungsten electrodes
2012-07-15, Source code and circuit diagram



Part 1: February 8th, 2012

I've just completed the first tests of a new challenging project, a pulsed discharge micro spot welder and cutter tool. It stores energy into a huge capacitor bank, and discharges it via two electrodes in the given target, regularly metal foils/sheets. The logic and the precise timing (of the order of micro-seconds) is controlled by an AtMEGA16-16PU microcontroller running at 16MHz. It can be used for spot welding and for plasma cutting.

The power switch is handled by 5 power mosfets, IRFP2907, set in parallel and driven by a darlington array IC with further transistor amplification. A single mosfet is rated for 75V 209Amps and an extremely low Rds. Do the math for the 5 working together!

Here is one of the construction tests, where I cut a beer can:

The molten material sticks to the discharge electrodes, making the operation difficult. I plan to try manufacturing them our of Graphite or Tungsten. Let's see which one works best!

This project is work in progress. My capacitors bank is aprox. 60000uFarad, but I need to get close to 1 Farad to have enough energy for correct operation. I will probably use a Car Audio capacitor.

Schematics, Microcontroller software, PCB layout to be added. Video showing spot welding will follow soon.

Some construction pictures:


The tool currently operates in two modes
1) Micro spot soldering
A predefined (uC+LCD+controls) high current pulse is delivered to the two electrodes, when the trigger button is pressed. Metal wires or sheets pressed between the electrodes will be soldered.

2) Cutting
A burst of short pulses are sent to the electrodes to generate plasma and cut thin metal foils.

The pulse duration, repetition rates are configurable. I got best results with pairs of pulses. Duration in the range of tens - a few hundreds of microseconds. For welding, the first pulse melts the material, and the second consolidates the soldering. For cutting the first pulse does the damage, and the second pulse helps to clean the electrode off the molten micro spot.

More operating modes can be added, so please give me ideas!

TODOs:
- I need to find better electrodes, that do not stick to the material , I'm considering tungsten or graphite. Suggestions?
- bigger capacitor bank: I need more power, and will probably go for a 1Farad Car Audio capacitor . Those Sprague I'm currently using are not enough for ticker sheets.
- finish and publish the software
- publish schematics and PCB layout.



Part 2: February 9th, 2012

I need to decide what type of LCD to use: the 2 lines HD44780 LCD, or the bigger one, with 4 lines. The code I wrote works with both. The larger one would have the advantage of allowing more text to be displayed. The smaller one is on the other hand more compact, better suited for my small case.
2012-02-09 14.02.32 2012-02-09 17.06.06



Part 3: February 22nd, 2012

I purchased a suitable capacitor. Finally! It's a Pyramid Royal Blue, CAP160DBL 1.6 Farads . The first thing I did was to dismantle the useless electronic part and the display. By doing so, the new size fits perfectly to my project box. For those complaining that this type of capacitor, when used for car audio systems, doesn't switch off and depletes the car battery, simply unscrew the connectors to take off the display part. You won't be needing that anyway:
pyramid 16farad 1 pyramid 16farad 5 pyramid 16farad 6
The price for this one was aprox. 55USD from a local shop and this makes it the most expensive component of this entire project, as expected.



Part 4: March 1st, 2012

The project box, in the final shape is ready. I had to cut some more wood, but that's an easy task if using the proper tools (I got a nice pendular saw as a gift, last Christmas).
For a quick test I tried using a halogen 12V electronic transformer. That was a waste of time, as those things are unstable, and not working properly (they are barely oscillating):
capacitor dual pulse welder 1 capacitor dual pulse welder 2 capacitor dual pulse welder 3
The box exposes the heatsink, a standard PC power connector, and a fuse holder. The big capacitor is mounted on the top, as it looks nice and is too big to fit inside anyway.



Part 5: March 3rd, 2012

I resisted the temptation to throw that halogen electronic transformer to the trash. It was such a waste of time! Instead I found a charger, from an old laptop. It is a 90W, 19.5V SMPS supply, with the perfect size for my box. Initial tests shown it is capable of charging the capacitor (with a short initial blackout), and even hold the intense Arc cutting mode power consumption. As this was another Chinese cheapo, some optional components such as filter, capacitors, etc where missing from the PCB, and I added those first. Next I did a heatsink upgrade, to make sure it will run continuously, for as long as I need to use the tool:
smps 2 smps 3 smps 4
So far this proved to be an excellent choice! My device can now be powered from a 110V-220V outlet. Alternatively, you can choose one of the SMPS power supplies presented here.

For the control panel, I needed a few potentiometers and some push buttons. I used a few NOS Russian 47K pots, I got from Gintaras Ebay show, KWTUBES. You can get some high quality components there at fair prices.
kwtubes 1 kwtubes 3 kwtubes 4
Some pins are used to interface with the floating cables coming from the PCB modules. By doing so, I can easily dismantle and re-mantle everything together.

Here's how the setup looks like now:
setup 1 setup 2 setup 3

For the working electrodes I ended up using copper. But not just copper, also a new innovative design: the thick wires are connected to the electrodes using a screw, a screw nut and a washer. The electrode itself is a 10cm metal hollow tube. Inside, copper wire can be inserted, and while working, it can be pulled outside when needed. Exactly like a rotring pencil : the cooper electrodes is eaten while in use, but conveniently can be extracted from within the electrode, or replaced.
electrodes 2 electrodes 3 electrodes 4
Here's a video showing the working principle:

The positive electrode also holds the discharge command push button that can be pressed with one of the fingers.

Finally, here's the completed project:
welder_ready 1 welder_ready 2 welder_ready 4

A few sparks, as recorded in some interesting photos, timing was crucial to get them, but I managed to do that eventually:

The power components are interconnected using thick copper wires. The wires to the electrodes are Copper Litz wire with section are of 16mm^2.
The 1.6F Capacitor that I used, has an ESR of 0.0016 Ohm.



Part 6: March 18th, 2012

I recently received a few Tungsten Carbide rods. This excellent material has a high melting point (2,870 °C), it is extremely hard (8.5–9.0 Mohs scale) with low electrical resistivity (~2×10−7 Ohm·m) making it an good electrode material for the Capacitor Welder. I hope to see it solves the molten sticky metal issue I had with copper electrodes.
Results show excellent performance:

Here is another video:

After a few minutes of use, some signs of damage are observed on the electrode, but they are minor at least when compared to copper:
tungsten_carbide-2 tungsten_carbide-1

Components used:

There were many modules involved in this project, but I will try to mention the most important:
1] 5x IRFP 2907 21USD . Source . These were used in parallel, on the large heatsink, to switch the capacitor discharge on and off
2] 2x IRF2807 3USD . Source . One handles the charging and the other the discharging of the capacitor, to keep it at the predefined voltage level.
3] 1x ULN2803 5USD (10 pieces pack). Source . TRANSISTOR ARRAY-8 NPN DARLINGTONS used in the transistor driver board.
4] 1x Atmel ATMega 16-16PU 12USD. Source .
5] 1x 16x2 HD44780 Character LCD 3USD . Source .
6] 1x Power Supply 19.5V 4.5A 13USD . Source . I had a few of these laptop adapters and it seems they are well fitted for my Capacitor Pulse discharge tool, for both welding and cutting.
7] 1x 1.6F 20V Capacitor 50USD . Source: local car audio shop.
8] 4x 47K Potentiometers 9USD (50 pieces pack). Source .
9] Other 20USD (16MHz Crystal, connectors, switches, wires, etc)



Part 7: July 15th, 2012

Schematics:

The eagle PCH can be downloaded here.
PCB Layout:

If you need to print the PCB for your own board, please use this PDF file.
Software, released under GPL License:
Here is my own software implementation, featuring the C++ LCB library that I described in a previous article:
Hex file and complete source code

Other resources:
You might also want to see Jiri Pittner's welder, presented here.

Looking forward to see your own variants of this cool project!

Here are some variants created by my readers

David's capacitor discharge spot welder
David did an excellent job at building this tool, with a very nice case and a pedal as a trigger
david_capacitor_discharge_welder_1 david_capacitor_discharge_welder_2 david_capacitor_discharge_welder_3 david_capacitor_discharge_welder_4
david_capacitor_discharge_welder_5 david_capacitor_discharge_welder_6 david_capacitor_discharge_welder_7 david_capacitor_discharge_welder_8

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