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Capacitor Discharge Microspot Welder / Cutter

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radhoo

Thu Feb 09 2012, 05:30PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

As seen on Hack A Day:

Project Start, January 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.
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.
More details here:

update 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.

Update 3: February 22nd, 2012
I purchased a suitable capacitor. Finally! It's a Pyramid Royal Blue, CAP160DBL 1.6V . 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:

Update 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). 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):

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.

Update 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:
150x150

So far this proved to be an excellent choice! My device can now be powered from a 110V-220V outlet.
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.
150x150

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:
150x150

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.
150x150

Finally, here's the completed project:

Construction details here:

Inducktion

Thu Feb 09 2012, 07:18PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Wouldn't a 1 farad audio cap have too much ESR? Why not try using some of those Maxwell Boostcapacitors? Their ESR is .7 milliohms, which is insanely low, considering it's a whopping 2600 farads of capacitance.

m4ge123

Thu Feb 09 2012, 08:09PM

Registered Member #4118 Joined: Mon Oct 03 2011, 04:50PM
Location: MD
Posts: 140

The series resistance of the boostcap (0.7mOhm), the MOSFET array (0.9mOhm), and random wires (up to 100mOhm) will really limit the current when working with low voltage. I would aim for at least 12 volts in the caps. If you're not going to use ultracaps, I don't see a reason to use anything less than 30V though.

Patrick

Fri Feb 10 2012, 12:15AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i was thinking of putting some ultraltra caps in series, 2600F / 10 = 260 F at 20 or so volts.

Could a NC machine use this device as its quill? then we could NC the copper foil off circuit boards! no more acid.

Daedronus

Fri Feb 10 2012, 08:49AM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

If you want to use a CNC to make pcb boards just use a V or pyramid engraving bit.

This is what I use and it works very well, I can get some very fine and thin isolation traces, around 12mils.

As a bonus you only have to change the bit to drill the holes and cut out the board.

Just don't go for the ultra thin V bits (20 or 30 degrees) you will break them. 60 degrees bits are fine.

Turkey9

Fri Feb 10 2012, 09:21AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Wow that's pretty sweet! I never would have thought to do this, good job.

Dalus

Fri Feb 10 2012, 09:30AM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

You can get more info by going to this guys site

radhoo

Fri Feb 10 2012, 10:10AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Inducktion wrote ...

Wouldn't a 1 farad audio cap have too much ESR? Why not try using some of those Maxwell Boostcapacitors? Their ESR is .7 milliohms, which is insanely low, considering it's a whopping 2600 farads of capacitance.

I definitely need to bring an ESR-meter when going to the car audio shop looking for these caps.

m4ge123 wrote ...

The series resistance of the boostcap (0.7mOhm), the MOSFET array (0.9mOhm), and random wires (up to 100mOhm) will really limit the current when working with low voltage. I would aim for at least 12 volts in the caps. If you're not going to use ultracaps, I don't see a reason to use anything less than 30V though.

It 0.7mO is the only limitation of these boostcaps , it might be a good idea to get one for a few tests which I probably will.
I cannot use 30V, the discharges would be too extreme / the arc too extended. I need this to work as "micro" as possible: nice , clean cuts, with the minimum level of sparks possible.
However I am aware that ESR and wire resistance creates considerable problems. I already plan to implement the following, in the capacitor-mosfet array-discharge electrodes: shorter and ticker wires, MKT quality caps in parallel with the electrolytic I am using, for a lower ESR combined factor and faster discharge response, try to go up to 20V and see how it performs.

Patrick wrote ...

i was thinking of putting some ultraltra caps in series, 2600F / 10 = 260 F at 20 or so volts.
Could a NC machine use this device as its quill? then we could NC the copper foil off circuit boards! no more acid.

Yes, adding this device as a cutting head to a plotter/CNC is possible.

Dalus wrote ...

You can get more info by going to this guys site

Yes, but that design uses a PIC microcontroller. I'm more like an Atmel fan. Now that the hardware is done (more or less the capacitor bank part) I have total freedom to adjust the software. As a software developer, this will be fun and easy.

So guys, are there any suggestions on new and innovative operation modes?

Just think about the setup, we have this huge energy stored in a capacitor, and we can deliver it as precisely calculated pulses. What else to do with it?

Don't tell me to drive a flyback

LE reading about EDL caps, I found some interesting details:
Very low internal resistance allows extremely rapid discharge when shorted, resulting in a spark hazard similar to any other capacitor of similar voltage and capacitance (generally much higher than electrochemical cells).
Apparently the only limitation applying to my setup would be:
Low maximum voltage â€“ series connections are needed to obtain higher voltages, and voltage balancing may be required. As Patrick already indicated.

Proud Mary

Fri Feb 10 2012, 10:26AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Pulse electrolysis. There are dozens and dozens of papers about it.

Tonskulus

Fri Feb 10 2012, 02:21PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133

I made similar metal sheet cutter using welder transformer, rectifier, cap bank and heavy choke and carbon electrode.
For thin metals, I use just scissors as it is much faster way :)

Nice project anyway!

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