jerome_speedy
100 mW
Your project looks excellent ! Smaller than the one I built (6pence resistance spot welder) and only asks for 1 source of energy !
kWeld - "Next level" DIY battery spot welder
- Thread starter tatus1969
- Start date
Thanks! Took the time to check your design... The chosen MOSFETs do not have enough avalanche rating yet, even with TVS diodes. These take away only a fraction of the stored inductive energy, there is still a lot of it being dumped into the MOSFET dies. And because their avalanche voltage may have a large production spread, you have to assume that one single of them will take it all. I have estimated 600nH of inductance for my setup, from that you can calculate the energy amount if the current is known.jerome_speedy said:
jerome_speedy
100 mW
When I said "built" I meant just that. I didn't CREATE the spot-welder, another ES member did (pguk)
The diode configuration is okay. The problem is a different one, and it is shown in these two pictures from the 6pence thread.jastill said:
Without diodes:
With diodes:
You can see in the second picture, that the MOSFETs are still being driven into avalanche (27V amplitude across D-S). Both the TVS and the diode together are not able to divert enough current such that this does not happen anymore. They only carry a fraction of the current, but they do help to reduce the duration of this avalanche situation.
I saw this picture from okashira, where he shows that he managed to completely prevent MOSFET avalanche:
Anyhow, I think that MOSFET avalanche is not generally a bad thing, because modern types are well capable of handling that. But you need to make sure that the dumped inductive energy finds a thermal mass that is large enough to limit the resulting temperature rise to an acceptable level. Therefore I chose MOSFETs with physically larger (and more expensive) silicon dies, in addition to a TVS used as a diode (again, because high surge rated TVS have large silicon dies). That reduces the avalanche duration from 13us to 5us (the voltage is 46.8V). If I assume that one transistor takes all the avalanche energy (uneven voltage thresholds), and all current is flowing through it all the time (which it isn't, because the diode takes some of it, and the current is of course dropping logarithmically during the pulse), then my calculation is for 2kA: E = 2000A * 46.8V * 5us = 0.47J. The MOSFETs have an avalanche rating of 1.1J each, so there is still plenty of room.
The best place to dump the energy is of course in the wire resistance, which is why I deliberately chose moderate 8AWG cross section.
I've been working on the assembly manual, and have just uploaded a first (incomplete) version to the kicksurfer website. I have made this a downloadable product in my store that is free of charge for all registered users: https://www.kicksurfer.de/index.php/product/assembly-manual/ That'll not help much against far east copiers, but maybe a bit...
And I have tested a different approach to manufacture the bus bars, the brass bars are just too expensive to finish. I have attached first pictures. These are laser-cut from 4mm x 12mm x 75mm aluminum bars. The first tests are very promising, the switch resistance goes up from 0.12 to 0.145 micro-ohms. That is just 25 micro-ohms more. They don't heat up noticeably during heavy firing.
Their price is quite good, so that for all people who have already purchased the brass bars, I can offer the following:
- if you have ordered them "raw" you can choose to receive the finished aluminum bars at no extra cost
- if you have ordered them "finished", I'll additionally refund the labor cost
Of course it'll also be okay to leave an order untouched.
I'll create a separate mail to all customers to ask for their wishes.
And I have tested a different approach to manufacture the bus bars, the brass bars are just too expensive to finish. I have attached first pictures. These are laser-cut from 4mm x 12mm x 75mm aluminum bars. The first tests are very promising, the switch resistance goes up from 0.12 to 0.145 micro-ohms. That is just 25 micro-ohms more. They don't heat up noticeably during heavy firing.
Their price is quite good, so that for all people who have already purchased the brass bars, I can offer the following:
- if you have ordered them "raw" you can choose to receive the finished aluminum bars at no extra cost
- if you have ordered them "finished", I'll additionally refund the labor cost
Of course it'll also be okay to leave an order untouched.
I'll create a separate mail to all customers to ask for their wishes.
parabellum
1 MW
What are you using to combat the oxide layer of Aluminium bars? ( I mean the flat side, pressed to circuit board)
I sanded them with 1200-grit paper, as I did before with the brass parts as well. It is necessary in both cases, and I expect that the screw clamping force will create a gas tight connection.parabellum said:
But good point, because I haven't had this vital step in the assembly manual. Noted!
Cheers Frank
Would thermal paste seal the aluminum bar surface from oxidation (after sanding and cleaning it, in preparation for mounting it?)
Anything that permanently keeps the air out will help.spinningmagnets said:
But I've got the same feedback on other places, and I have started investigating it. I found a shop that sells galvanic articles, and it would be possible to add chemical zinc, and then galvanize with gold. That would be a good match for the gold PCB traces.
On the other hand, I have built an audio amplifier decades ago, and I still have the bulk capacitors with screw terminals that are connected with aluminum bars. The capacitor's screw terminals were made from tinned brass.
That makes me think: would it prevent contact corrosion if I would order tinned PCB's instead of gold plated next time?
I have read that if the size of the conductor is not an issue, aluminum works well if it is about 50% larger (in cross-section) than a well-performing copper conductor. As to plating, there are many examples of nickel-plated copper, and now I am curious about the possibility of nickel-plated aluminum?
I have explored zinc-plating and also tin-plating...but Nickel remains the coating of choice. Using a thermal paste on raw aluminum/brass/copper seems cheaper and easier for the DIY builder (compared to ordering the bars to be plated), and aluminum square-bars are easily sourced locally, and very affordable (onlinemetals.com).
I have explored zinc-plating and also tin-plating...but Nickel remains the coating of choice. Using a thermal paste on raw aluminum/brass/copper seems cheaper and easier for the DIY builder (compared to ordering the bars to be plated), and aluminum square-bars are easily sourced locally, and very affordable (onlinemetals.com).
I just ordered some bottles of chemical nickel plating solvent, it is not very expensive and I will treat all sold aluminum bus bars with that. For aluminium, they sell an "activator" fluid.spinningmagnets said:
The 50% information is correct, which is because aluminum has roughly half the conductivity from that of copper. Brass is a bit lower, but close.
If there were no holes in an aluminum bus bar, its electrical resistance would be 2.65e-2 * 0.075m / ( 4mm * 12mm) = 41 micro ohms. The holes reduce the cross section to (12*75 - 6*15.9 - 1*33.2) / (12*75) = 86%, which increases the resistance to 48 micro ohms. At a current of 1kA, this results in a power dissipation per bar of 48W. The real value will be much lower though, because a fraction of the current will be diverted at each MOSFET, and the end of each bus bar will only see 1/6 of the total current. As I am too lazy to make the math, I tested it in real life, and it works well. I am waiting for some parts to arrive, and will then make another video with the new battery, new battery terminals (XT90 and XT150), and the new bus bars. Then there's the new firmware update feature to show as well. That is AES encrypted to keep far east spys out 8)
If you have a bolted connection with enough pressure, aluminum will hold up fine with no plating. I've seen nickel plated aluminum before. The plating peels off pretty easy.
Hillhater
100 TW
There are proprietry anti oxidising pastes for use with electrical conections.
Anti corrosion for aluminium is commonly done by "Anodising" the part..... A simple process that can also result in those pretty colors you see on many alloy parts ! (Think..Red and Black ?)
Anti corrosion for aluminium is commonly done by "Anodising" the part..... A simple process that can also result in those pretty colors you see on many alloy parts ! (Think..Red and Black ?)
Don't over-think/complicate it, raw aluminium will work just fine. This will be used indoors (warm, dry) so isn't going to corrode. The natural oxide film is very thin and won't hinder conductivity much. Anodising is an artificial thickening of the oxide layer. Making sure the surface is flat and smooth is likely to make a much greater difference to thermal conductivity.
Anodizing (hard anodizing, at least) is generally an electrically-insulating layer, though, so you'd have to pick a process that doesn't do that (if there is one) if you're anodizing to prevent oxidation of the electrical connections themselves.Hillhater said:
mxer
1 kW
I'd like to see the 3D printed case, has anyone printed one yet?
tatus, I bet you'd like to see your hard work refined in a neat casing. I'm trying to think who has one local to me would be great to see.
tatus, I bet you'd like to see your hard work refined in a neat casing. I'm trying to think who has one local to me would be great to see.
I am trying to learn FreeCAD for things just like this. If I can stumble through it, maybe I can post the file on thingiverse, so anyone can order one from a 3D printing service.
I am sure there are hobbyist project boxes that will work, but its nice to have one that is as small as possible, and fits to board perfectly.
Boxes seem to be the easiest starter project to learn.
https://www.youtube.com/watch?v=JhPxrHwLFuc
[youtube]JhPxrHwLFuc[/youtube]
I am sure there are hobbyist project boxes that will work, but its nice to have one that is as small as possible, and fits to board perfectly.
Boxes seem to be the easiest starter project to learn.
https://www.youtube.com/watch?v=JhPxrHwLFuc
[youtube]JhPxrHwLFuc[/youtube]
Just wanted to share this with you before going to sleep. Today the XT90 plugs arrived that I was waiting for. As the battery came with them, I wanted to test the suitability of these before cutting the wires and soldering XT150. First conclusion: they do not support insertion of AWG8.
What is more interesting is the first data that I captured from the new Turnigy nano-tech 3S / 5Ah / 130C (=650A) battery. The welder has measured an impressive current of 1230A. The oscilloscope diagram taken from (almost) the battery terminals shows a voltage drop of 6.56V at this current. This results in an internal resistance of these beasts of only 5.3mOhm. That is comparable to what is achievable with supercapacitors. Now the battery has to show how well it likes currents at this level (as specified, this is only 1.9 times of rated current). I will do a stress test in a new video this weekend.
View attachment 1
What is more interesting is the first data that I captured from the new Turnigy nano-tech 3S / 5Ah / 130C (=650A) battery. The welder has measured an impressive current of 1230A. The oscilloscope diagram taken from (almost) the battery terminals shows a voltage drop of 6.56V at this current. This results in an internal resistance of these beasts of only 5.3mOhm. That is comparable to what is achievable with supercapacitors. Now the battery has to show how well it likes currents at this level (as specified, this is only 1.9 times of rated current). I will do a stress test in a new video this weekend.
View attachment 1
macribs
10 MW
- Joined
- Jul 22, 2014
- Messages
- 3,702
Now the bottles are empty and I got the self esteem of a ox. So back up it goes.
Man what kind of pills are you taking? I need a few bags of those. :wink:
Oh man this is nice work. Really impressed.
I don't think I ever seen a more dedicated and rapid developer here on ES. The work you put in, the changes made to hardware as well as changing the firmware all in the span of a summer. And you kept this thread updated like I never seen before. All this while waiting for parts, getting up your website and a webshop.
If I'm ever in a position where i need to hire an EE you will be on my short list.
Man what kind of pills are you taking? I need a few bags of those. :wink:
Oh man this is nice work. Really impressed.
I don't think I ever seen a more dedicated and rapid developer here on ES. The work you put in, the changes made to hardware as well as changing the firmware all in the span of a summer. And you kept this thread updated like I never seen before. All this while waiting for parts, getting up your website and a webshop.
If I'm ever in a position where i need to hire an EE you will be on my short list.
