Author Topic: kWeld - "Next level" DIY battery spot welder  (Read 47980 times)

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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #50 on: June 24, 2017, 10:31:28 PM »
Oops... too late now to implement one critical thing you omitted: a diode in between the LiPo battery and the PCB circuit with a decent amount of capacitance after the diode to provide "holdup" during the pulse firing.

The other suggestion is simply a matter of software: the best CD spot welders deliver two pulses of about 1-20ms width spaced about 1-20ms apart. The first pulse preheats and cleans the joint and the second pulse does the actual welding.
That makes it time to publish the finalized schematics  8) I also made provision for a freewheeling diode for the energy stored in the wiring inductance. But I expect that I don't need it. At 1kA, the stored magnetic energy is ~ 0.5 * L * I^2 = 0.5 * 200e-9 * 1000^2 = 0.1J. The MOSFETs should be able to handle this well, and they are repetitive avalance rated.

I plan to implement an energy based pulse control instead of a simple timer. i will try to see how that works with one single pulse, maybe that eliminates the need of a cleaning pulse. Constant energy delivered to the weld spot should result in more consistent welds. The circuit should also be able to detect a failed weld, in this case the requested energy cannot be delivered in a reasonable time.
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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #51 on: June 25, 2017, 12:53:38 AM »
Assembled module...



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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #52 on: July 02, 2017, 08:50:21 PM »
Most software parts are prepared, the weld control loop already works with 10us cycle time and ADC conversions running with DMA. Really nice microcontroller architecture, you set up everything, and then it will continuously update an array in memory with conversion results.

While waiting for the boards to arrive next week I have machined electrode holders from 6x12x35mm brass. What do you think about it (these are not the final screws yet)?

« Last Edit: July 02, 2017, 08:52:00 PM by tatus1969 »
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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #53 on: July 05, 2017, 08:07:00 AM »
The boards have arrived earlier than expected. That allowed me to use the evening to solder and assemble one. So far everything seems to work! High current tests to follow.
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Offline mk_

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #54 on: July 05, 2017, 04:54:52 PM »


 The MOSFETs should be able to handle this well, and they are repetitive avalance rated.


Take care of the Gate-resistors... the 220 Ohm are fine for high powerlosses during switching, even if the FETs are avalanche-rated... you should reduce them down to something like 33 or 47 Ohm.
Also the 220 Ohm let each FET switch at a minimal different time - so one FET has to handle all aviable current (for a very short time)... lowering the Gateresistors results in better load-sharing @ switchingtimes...

Michael
 

Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #55 on: July 05, 2017, 08:35:58 PM »


 The MOSFETs should be able to handle this well, and they are repetitive avalance rated.


Take care of the Gate-resistors... the 220 Ohm are fine for high powerlosses during switching, even if the FETs are avalanche-rated... you should reduce them down to something like 33 or 47 Ohm.
Also the 220 Ohm let each FET switch at a minimal different time - so one FET has to handle all aviable current (for a very short time)... lowering the Gateresistors results in better load-sharing @ switchingtimes...

Michael
thanks, I'll have to see how good my estimations are. I don't want to switch too fast to keep inductive ringing low, but fast enough to not exceed the max pulse energy. I expect anyway that the switching enwrgy is dissipated by one single transistor in worst case, when the gate threshold voltages differ. Switching time will be around 10us, at 1kA and 10V this would give a 0.1J ballpark figure. The MOSFETs are rated at 1.1J, so there should be plenty of room. Theoretically. But if I am wrong it will be a headache to change these resistors. ::)
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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #56 on: July 06, 2017, 07:27:20 PM »
Yesterday I learned three things:
1) a multichanel ADC has a source for offset errors that I didn't realize before. When you look into my circuit diagram, you'll see that the MOSFET voltage measurement (OUT-) has a 10k filter resistance. According to the STM32's datasheet, that would be enough to settle to <1LSB at full speed. But I measured ~50mV at the ADC input, even with the MOSFETs shorted. Where did that come from? The ADC appnotes from ST didn't give a hint either. It took me half of the evening to figure out that this is charge injection from the previous conversion. The ADC configuration is to continuously convert all five available channels one after another. Therefore it has a MUX that connects one of the sources to the ADC input. That input has a capacitance. If, as in my case, the channel that precedes my problematic one has a siginifcantly higher voltage level, then the ADC input capacitor is charged to that level when switching to that channel. This dumps the stored from energy that capacitor out of the channel's GPIO, into the source network. As this is done repeatedly, this causes a constant current flow. The ADC input capacitance is 8pF, and the conversion rate is approx 100kHz. Assuming the adjacent channel sees 3.3V, and my problematic one has 0V, the current calculates to 100k * 0.5 * 3.3^2 * 8p = 4.3uA. The voltage drop on the 10k resistor then calculates to 43mV.  Gotcha!

2) a buck converter with bootstrapped power switch driver can never live with an output capacitor (in my case 2200uF) that 'survives' zero input power longer than the bootstrap capacitor. When the input voltage comes back, then there is nothing that can charge the bootstrap capacitor (as the output almost equals the input). This situation is only resolved when the output voltage has dropped far enough for a restart, unluckily leaving the supplied circuit dead during that time... Solution: move the freewheeling diode from the input to the output of the regulator. Luckily I can do that because I do not need a precise voltage.

3) the ADC inputs of the STM32 are not 5V tolerant. If I would have noticed that earlier, then I would not have to replace a dead microcontroller now. As I need a low leakage input protection, and a zener diode just doesn't deliver on that, I ordered some TL431. They will protect the OUT- input from high voltages.

Luckily the microcontroller did not die before allowing me to make this measurement. (Yellow: voltage across ~700uOhm of AWG8 wire, blue: voltage across MOSFETs) I don't believe the readings yet (why does blue approach zero??), but it suggests that the pulse current (here, into a 100% short circuit) is in the order of 1.5kA. Scary.



« Last Edit: July 06, 2017, 07:31:08 PM by tatus1969 »
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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #57 on: July 08, 2017, 04:52:27 AM »
Interesting side note:

I have just finished completing the firmware code but cannot test it, because UPS didn't deliver the missing parts today as scheduled. This is the information that UPS gave me this morning:



Something is wrong with this information - who can tell me first?

Hints:
a) it has been sunny and calm for a few days now
b) I live near Hamburg, Germany
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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #58 on: July 09, 2017, 08:21:34 AM »
Yes! 8)

This is 0.1mm Hilumin.

A video showing the welder in action is in preparation!


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Online Fraser

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #59 on: July 09, 2017, 09:19:29 PM »
Nice project :)

I bought a Chinese battery tag welder a few years ago and it has served me well. It uses a flipping great transformer And the double output pulse energy is controlled by a micro-controller. The only problem I have had with it is that on the higher current settings, it trips the nouse power breaker !

Your design looks very neat. Are you considering selling the PCB's or kits of parts ?

All the Best for the project completion.

Fraser
« Last Edit: July 09, 2017, 09:30:45 PM by Fraser »
 

Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #60 on: July 10, 2017, 01:48:30 AM »
Nice project :)
Thanks  :D It is performing extremely well. Even when doing a full pull (1kA for 200ms) the MOSFETs don't get even slightly warm. I am considering doing a test row by removing one after another to see how much I overdid it. I already decided to delete the brass bus bars for the bottom layer and replace them by washers and nuts. The only things that get warm are the electrodes, the battery connection (I knew that a Deans connector would be problematic) and, a bit sadly, the battery. But the design should also work with other power sources like capacitors or a lead acid battery, ...

it trips the nouse power breaker !
Energy efficiency is probably extremely low there. I found that welding 0.1mm Hilumin onto a 18650 cell requires 50 to 100 joules of energy. A 230V/10A wall outlet only needs 0.02 seconds to deliver that.

Are you considering selling the PCB's or kits of parts ?
Originally I wanted to make a one-off as you can see from my first pictures. But my solution has some unique features (apart from being very robust - I will explain those in the upcoming video), so I am planning to sell it in form of a kit. It depends on you makers and builders how that kit should look like. My idea is to only sell assembled/tested/programmed PCBs and the mechanical parts in raw form. This means (apart from things like bolts, nuts, cable lugs, ...) brass blocks cut to length, copper rod, and enough AWG8 wire. You would need to drill and cut the brass pieces, cut and form the copper rod into electrodes, and cut / crimp the cables. The other extreme would be supplying complete systems, but this involves considerable machining work and would make everything much more expensive. I need some feedback from you folks... And if someone is interested in a kit, just send me a PM.
« Last Edit: July 10, 2017, 01:52:40 AM by tatus1969 »
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Online Fraser

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #61 on: July 10, 2017, 03:52:55 AM »
My feedback on this is that I would like the raw parts as I can do all the time consuming hardware bits that would cost a lot if it was a finished product. The raw kit would also encourage further hardware development in terms of probes, bus bars etc.

I am definitely interested in such a 'raw' kit. I will PM you.

Fraser
 

Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #62 on: July 10, 2017, 04:10:10 PM »
The video is online!

If you like it, you can help me by sharing the video via Facebook etc.


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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #63 on: July 11, 2017, 05:20:19 PM »
I made a video where I am doing robustnes testing:

« Last Edit: July 11, 2017, 06:54:16 PM by tatus1969 »
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Offline maukka

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #64 on: July 11, 2017, 07:53:48 PM »
That's an amazing project! Very well though out and I especially like the small form factor. Can't wait for the kit.
 

Offline Kjelt

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #65 on: July 11, 2017, 09:44:20 PM »
Originally I wanted to make a one-off as you can see from my first pictures. But my solution has some unique features (apart from being very robust - I will explain those in the upcoming video), so I am planning to sell it in form of a kit. It depends on you makers and builders how that kit should look like. My idea is to only sell assembled/tested/programmed PCBs and the mechanical parts in raw form. This means (apart from things like bolts, nuts, cable lugs, ...) brass blocks cut to length, copper rod, and enough AWG8 wire. You would need to drill and cut the brass pieces, cut and form the copper rod into electrodes, and cut / crimp the cables. The other extreme would be supplying complete systems, but this involves considerable machining work and would make everything much more expensive. I need some feedback from you folks... And if someone is interested in a kit, just send me a PM.

Great work there  :-+ For me the described kit without the machining would be great. Just a small description of the steps and taps and other tools involved would be enough.
 

Online Fraser

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #66 on: July 11, 2017, 10:29:16 PM »
I am impressed with the performance of this excellent design. The control over the energy presented to the probe tips is excellent. As others have stated, commercial tag welders tend to use a double pulse system as that is said to improve welding performance by 'tacking' the metal together and then applying the true welding pulse to the joint. Your welder seems to cope fine as it is though.

As already stated, I would love the 'raw' kit to build. I think I would make probes that suit my specific needs.

One other comment, commercial tag welders tend to use cold drawn (hard) copper probes and these last well.

Fraser
« Last Edit: July 12, 2017, 03:03:51 AM by Fraser »
 

Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #67 on: July 12, 2017, 02:56:21 AM »
I will go ahead and make these items available:
1) PCB with all SMT components soldered and microcontroller programmed
2) set of through-hole components like capacitor, connectors, LCD, potentiometer, etc
3) fully assembled and programmed PCB
4) cable assembly for those who don't have a suitable crimp tool
5) set of mechanical components like bolts and nuts, excluding the parts that need machining
5a) the foot switch
6) set of raw brass and copper parts, cut to length
7) set of finished brass and copper parts
8 ) programmer for firmware updates that plugs into USB (the Segger tool is a full debugger and too costly), plus an update tool (Windows)
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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #68 on: July 12, 2017, 03:01:18 AM »
I am impressed with the performance of this e cellent design.
Great work there  :-+
That's an amazing project! Very well though out and I especially like the small form factor.
Thanks  8)

The control over the energy presented to the probe tips is excellent. As others have stated, commercial tag welders tend to use a double pulse system as that is said to improve welding performance by 'tacking' the metal together and then applying the true welding pulse to the joint. Your welder seems to cope fine as it is though.
That could still be added easily, but I don't think that it is necessary because of my new approach. But I want to leave it up to the community to decide once the first kits are shipped.

One other comment, commercial tag welders tend to use cold drawn (hard) copper probes and these last well.
Thanks, noted!

Just a small description of the steps and taps and other tools involved would be enough.
for sure!
« Last Edit: July 12, 2017, 03:03:14 AM by tatus1969 »
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Online Fraser

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #69 on: July 12, 2017, 03:09:53 AM »
Seeing your list of what will be made available in due course has me getting excited. Definitely something for everyone there.

Very much looking forward to further kit details and pricing when available. If I can afford it, I will be at the head of the queue for one :)

Many thanks again for shating your design.

Fraser
 

Offline SvanGool

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #70 on: July 12, 2017, 05:10:33 AM »
I am also interested in a kit.

I do think your circuit would also work for a (5 or 6x 2.7V or 1x14V) ultra-capacitor version, what do you think?
I still have less faith, then you do, of getting 1000 A out of a Li-Po that is rated for 250 A, on the long term.
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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #71 on: July 12, 2017, 05:46:35 AM »
I am also interested in a kit.
I've put you on my list!

I do think your circuit would also work for a (5 or 6x 2.7V or 1x14V) ultra-capacitor version, what do you think?
The system will work well with any type of low impedance power source with a voltage range of 12V to ~35V. The weld algorithm continuously monitors output voltage and current, and by this it takes input voltage/current changes into account when calculating the accumulated weld energy. It also simplifies a CD welder because you don't need two capacitor banks for two-pulse welding. It still has to be confirmed by you guys, but I am confident that my system does not need a cleaning pulse.

For a capacitor based solution, you could also skip the fuse. I will add instructions on how to set it up without it. I added the fuse for pure safety reasons when using batteries. A shorted Lipo can quickly cause a fire.

As the impedance of the power source is a main contributor to the achieved current level, and the welder switch contributes only 140 micro-Ohms to that, you need to control the total loop resistance such that a desirable current is achieved. I did this by deliberately choosing "thin" AWG8 wires, which account for roughly 2mOhms. To help you controlling this, I added a current level readout after each shot (while keeping the trigger pressed), and the software immediately aborts when the current exceeds the switch limit of 2000A.

I still have less faith, then you do, of getting 1000 A out of a Li-Po that is rated for 250 A, on the long term.
For a good reason. The batteries definitely don't like how I treated them the last days, and I consequently keep them in an ammunition box when not in use. I think lead acid batteries are much more forgiving here, but I found the size and cost of these Lipos tempting. But I already think of paralleling four of them, because I don't want to have a car battery in my lab  8)
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Offline tatus1969

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #72 on: July 12, 2017, 05:53:35 AM »
Okay, I'll continue finishing the release design and send an RFQ to my PCB assembly company, but I wanted to share this one with you first. It's rather crazy (and educational?), please don't take it too seriously - I am a quality minded professional EE despite of what you see there  :o


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Online Fraser

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #73 on: July 12, 2017, 06:25:24 AM »
Tatus1969,

At last I have found a good use for the pile of brand new sealed Pb 12V 36Ah batteries that are sitting in my garage. I have looked after them for a couple of years hoping to find a use for them beyond jump starting my Mini Moke !  :-+

Fraser

 

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Re: low cost DIY battery spot welder [guesses on ... - solved]
« Reply #74 on: July 12, 2017, 08:03:31 AM »
 :-+
I'm tempted :)
 


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