6pence Resistance Spot Welder

[Valld]

The board didn't arrive yet. It takes at least a month from UK to Botswana usually. I will report when here

[pguk]

There is a new version of the control board. It now has a 1uf SMD film capacitor supplying the 1407 FET driver - so plenty of current available for a robust drive on the gates of the FETs since we changed from 6 to 8 of.

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Some shots of a fully built unit. It has the control board sat on top of the power board with a bracket fabricated from scrap aluminium. This way one doesn't need to find an enclosure.

One problem to note: I did not provide enough clearance for the mounting pilars. If you use plastic M3 pillars all will be fine, but if like here you use metal pillars, either use an insulating washer underneath, or just don't fit the one next to the Schottky as it will short it out to FET sources. aarrgh! Apart from this, all is good.
Edit: this is fixed on the 2nd version of the board.

[pguk]

Power board build
The idea here is to not rely at all on the pcb foil to carry any weld current.

1. Fabricate three copper plates to fit where the lugs clamp onto the board. I used some scrap copper pipe, hammered flat, drilled to M8 clear (M4 for schottky holes), cut to fit and tinned.

2. Next you want to mount those FETs. Position each one in place and tack a leg to hold it. This way they will not move as you solder them to the board in one sweep. Don't put too much solder down at this stage.

3. Next you want to build those busbars. I used 3 * 2.5mm sq copper wire. I formed two pieces and laid them and soldered these to the board together and directly onto the FET drains. Then I added another on top of these two. Solder the sources side busbar in the same way, taking care not to make a solder bridge to the gate pins.
I have not tested as yet but I hope this will be enough, although more copper is always going to be better. [Edit: tested good]

I used a professional soldering station for this job which made adding the busbars easy for me. The tip you see in the photos looks quite small, but it's the constant temp sensing station which makes it capable. It senses temperature drop of the tip and delivers more energy as needed. I expect you could do this with a cheap '100 watt' iron.

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[Valld]

So cool! It looks sexy and powerfull

[pguk]

So I finally got around to some testing today.
I ran the unit above which has the SMD film cap supplying the FET driver, and the new power pcb having 8 MOSFETs.
I have to say I'm pretty pleased with the results.
It went better than expected - last time I tested I had home built flyback diodes and a single TVS. Also I was using a leisure battery with my 45ah Bosch car battery. This time I had a decent schottky, 3 TVS diodes and two good quality 45ah car batteries connected in parallel.

I set P1 = 0.2ms, D = 1ms, P2 = 4ms, this was enough to weld a 0.1mm piece of nickel.
Yellow trace = gate drive
Blue (green?) trace = Drains
Zero volts is 2nd division from bottom for both traces.

zoom out shot of both pulses

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Zooming in to the pulse2 turn off

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a little more

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and a little more

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The difference between these shots and those on page one are clear. Last year I had managed to get the avalanche current (flat top of drains waveform at ~27V) down to 22us (from 70us without any diode in circuit). This is now down to barely over 1us. To quote my 9 year old, Sarah, "Booooomm!"

A big Thank You to all for your input and kind words of encouragement on this - I would have given up before now without your interest along the way Also to those whose ideas I built on for the power side - mainly crossbreak, okashira, riba and everyone on the other development thread.

I shot some video of a weld at P1=0 D=1 P2=15 for a 0.15mm weld. It'll take a while for me to edit on my slow machine.

[riba2233]

I'm glad you got it Nice work!

[dbaker]

Excellent work!

[pguk]

Thank you both
New video is in 1st post now.

[pguk]

I have a few of the schottky diodes which I bought from aliexpress.
I wanted to do some testing to see how they compare with the rather more expensive Vishay device.
I took a scope shot of gate and drains for turn off of a 10ms pulse. The gate shape looks different from the above shots because this time I put the scope probe right at the FET leg, whereas before I had it at the other side of the gate resistors.

First, the aliexpress device:

10ms_ali_schottky.png (41.7 KiB) Viewed 4509 times

..and here is the Vishay device:

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Well, there doesn't seem to be a great difference between the two.
The drains on the aliexpress device seem to peak higher at around 26V and then settle to just under 16V. The Vishay device clamps that peak voltage lower at just above 24V, then settles down to a little over 16V.
For a 24V device, its nice that the Vishay does clamp at this level. For a difference of ~2V, this must represent an amount of avalanche current pushing through the FETs with the aliexpress device, which is diverted with the Vishay device in place.
So, I guess its worthwhile fitting the Vishay.

[brumbrum]

I would love to get my hands on one of these, but i only have half the brain of a slug.
Question: Are you selling the whole unit as pictured above where i would only need to connect up a battery and the welding cables and copper welding tips? This would make it user friendly.

I think, if you were wanting to sell more of these on this forum and have many more people keep this thread alive rather than the select few that understand the underlying principles, why not sell plug n play units? I understand it would be more time consuming but maybe more profitable at the same time and even more worth while. There are many forum users looking for a better, more reliable, more powerful, and more user friendly battery welders than the chinese boxes. The china welder which are selling in the hundreds and are being bought for well over £100 with the uncertainty of whether they will work or not. Have tou noticed how many people are waiting on Riba spot welder kit?

[Quokka]

+1 for a tard version

[pguk]

Well, I reckon I'm proving what they say about the British not often capitalising on a good idea..

Seriously tho, I think that someone who is thinking of building their own battery has rather more than half the brain of a slug ! To put this together uses the same skills as a diy ebiker needs: it's all volts and amps and ohms law.

Actually I am shooting to be able to build these fully, but not anytime soon. I could buy a JBC station and away we go, but I've bought a JBC handle and a couple of tips and am hoping to be able to build an arduino based station to drive the JBC tips. This way it will be more fun building these with a tool I've built myself, and I won't get bored. Bret Beauregard has written a PID library for the arduino which looks promising, although from here the learning curve looks rather steep, and spare time is short.. (Excuses excuses)

So for now I'm continuing to offer the assembled control board with small parts, plus power pcb with smd gate resistors fitted only, plus supply the schottky and TVS diodes and FETs.

Thanks for your interest
Best, Paul

[Storapa]

Hi Folks,

I bought one of these kits from Paul. I am no electrical engineer, so for me this is a bit of an undertaking. I want to be able to make my own 18650 battery packs. This was the way forward for me. I thought I would post up my build, to hopefully inspire others.

The kit of parts arrived quickly. Paul has also been very helpful with my questions. The pic below is what arrived. All other parts have been sourced by using the links at the beginning of the thread.

I will post up the different stages of the build if people are interested. I know I have found such threads useful in the past.

[pguk]

Cool, thanks for posting Darren.

As someone without too much electrical experience, I'm sure it will be helpful for others to look over your shoulder as you go along with any questions, problems, observations you may have.

[Storapa]

Cool, thanks for posting Darren.

As someone without too much electrical experience, I'm sure it will be helpful for others to look over your shoulder as you go along with any questions, problems, observations you may have.

No problem Paul. Hopefully by posting the build, others will feel more confident and if I make any daft mistakes, I can be nudged in the right direction.

So onward. First job for me was soldering the MOSFETs onto the main board. To me, this looked daunting, due to the small individual pad for each MOSFET. It turned out to be easy using some 0.5mm solder and a 1mm double flat tip on my soldering iron. Once I had some solder on the individual pads, I positioned each MOSFET by hand and heated the single leg to secure it. Once all of the MOSFETs were secured, I went along and soldered the remaining legs. Job done. To solder the body to the ground I put a little weight on each one and ran my soldering iron along the run (with a 3.2mm double fat tip) feeding solder as I went. Again, it went smoothly. Pic below.

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Right, that's it for now.

[pguk]

So far so good.

The photo looks like you have a bit too much solder at the FET tab at this stage. You may want to take some of that away, so that when you add your copper wire runs next, you will be easily able to get the wire right up against those FET tabs.

For your copper wire runs, use solid 2.5mm sq house from house wiring cable. I put three runs down. Take a bit of time to make them as straight as possible, and form them where they come up onto your tinned copper plate.

[Storapa]

Hi Paul,

Thanks for the advice. I'll remove some of the solder.

Another question. I only had 1.5mm sq at home, so dug that out. I then used 4 strands and wound them into a single run, as per the photo. Is that an acceptable approach?

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[fechter]

Another question. I only had 1.5mm sq at home, so dug that out. I then used 4 strands and wound them into a single run, as per the photo. Is that an acceptable approach?

That should work fine.

[pguk]

^what he said.
Also bear in mind to come close but not over the holes for the 3 TVS diodes. You should be able to bridge the gap between TVS leg and busbar with solder. We don't rely on board foil to carry any heavy currents.

[Storapa]

Thank for your guidance gents......

I'll let you know how it progresses.

[Storapa]

I've ordered a larger tip for my soldering iron, so the busbars are on hold for the moment.

Whilst I am waiting for this to arrive I made the spot weld leads and tips.

I ordered welding cable and M8 crimp lugs as per the links at the beginning of the thread.

I turned up a couple of tip holders along with copper spot welding tips. I'll also make a pair of cranked / offset tips when I have time.

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By chance we had a crimp tool at work on loan, with the correct ends. The M8 crimp lugs and spot tip holders were crimped to the cables and covered in heat shrink.

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Hopefully the soldering iron tip will arrive this week so I can crack on.

[Valld]

I like this tip holders, where did you get them? I failed to find anything similar in both Aliexpress and Ebay, or other china online shops.....

[Storapa]

I like this tip holders, where did you get them? I failed to find anything similar in both Aliexpress and Ebay, or other china online shops.....

Hi,

I machined them up myself. Any small machine shop should be able to produce something similar.

If I made them again, I'd use copper as it would probably crimp a little better and everything else in the system is copper.

[Valld]

It's great to be able to machine yourself small parts, so useful! There is no machine shops here in Botswana, so I will stick to cable lugs with cut off eye for crimping the cable and electrode....

[Storapa]

It's great to be able to machine yourself small parts, so useful! There is no machine shops here in Botswana, so I will stick to cable lugs with cut off eye for crimping the cable and electrode....

Hi Valid,

I'm not that happy with the the brass tip holders. If I get this spot welder working (I'm stuggling at the moment) I'll look at machining copper versions of the brass bits. If I can machine them successfully, I'll make up a second pair.