kWeld - "Next level" DIY battery spot welder

[tatus1969]

 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.

[tatus1969]

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.

[tatus1969]

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

[tatus1969]

Yes!

This is 0.1mm Hilumin.

A video showing the welder in action is in preparation!

[Fraser]

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

[tatus1969]

Nice project

Thanks  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.

[Fraser]

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

[tatus1969]

The video is online!

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

https://youtu.be/Ceos88VO6p4

[tatus1969]

I made a video where I am doing robustnes testing:

https://youtu.be/fWXphoDE_H8

[maukka]

That's an amazing project! Very well though out and I especially like the small form factor. Can't wait for the kit.

[Kjelt]

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.

[Fraser]

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

[tatus1969]

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)

[tatus1969]

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 

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!

[Fraser]

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

[SvanGool]

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.

[tatus1969]

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 

[tatus1969]

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 

https://youtu.be/JR3GJYScquc

[Fraser]

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

[casinada]

 
I'm tempted

[SvanGool]

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.

Yes you could omit the fuse, but I wouldn't for the 6x 3000F/2.7V I have here, which is the equivalent of your Li-Po pack at 0.5 Ah (16.2V -> 12V) but capable of 1900 A (within spec), you still might get a fire
That's why I am interested to try the direction of a CD spotwelder with your circuit.

[tatus1969]

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.

For sure! But wouldn't the ESR of those not be a problem? The last time I worked with those it was still a few mOhm per cell.

Yes you could omit the fuse, but I wouldn't for the 6x 3000F/2.7V I have here, which is the equivalent of your Li-Po pack at 0.5 Ah (16.2V -> 12V) but capable of 1900 A (within spec), you still might get a fire
That's why I am interested to try the direction of a CD spotwelder with your circuit.

I've finished the redesign yesterday - I think that I didn't mention yet that I added an aux DC input that can be fed with 10~12V when the system is used as a CD welder. The problem here is that the internal holdup capacitor is only good for 250ms. The power caps will probably not be recharged quick enough to keep it from power cycling otherwise.

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.

[tatus1969]

There goes the next video, showing some improvements:
https://youtu.be/9-CQd02EDIs

[Kjelt]

It gets better and better, can't wait. 
BTW I already have a Segger Jlink and STLink , probably many users here have one so can you make ordering the programmer for future fw updates optional ?

[tatus1969]

can you make ordering the programmer for future fw updates optional ?

That's the plan!