6pence Resistance Spot Welder

[pguk]

Greetings from the Northwest of England - I would like to present my version of the Dual Pulse Resistance Spot Welder.

a fully built unit. See P4 for how build the power board

IMG_2304.jpg (54.56 KiB) Viewed 12297 times

With this kind of welder there are no heavy transformers involved; the unit uses a bank of power mosfets to switch weld current sourced from a bank of capacitors, or a normal lead acid car battery, or RC Lipo battery. Lots of good info here

viewtopic.php?f=31&t=81400

I was inspired by crossbreak's thread viewtopic.php?f=2&t=59383 to begin to develop one of these spot welders. At the end of rather a steep learning curve with coding and kicad, I feel confident that what I have is robust enough to publish.

I offer a kit of essential parts for folks to build up their own unit. Kit from me:- Control board assembled and tested: supplied with LCD, pushbuttons, power supply connector; Power pcb with SMD gate resistors fitted. Buyer builds their own power board (using my pcb as a basis or your own from scratch) and sources the larger parts of the setup, ie weld cables, electrodes, enclosure if req'd, foot switch & connector, 12V wall adapter.

(footswitch pin in this shot has a pushbutton fitted for testing)

The following is an overview of the welder function.

The unit is of modular design. We have a power board, a control board with the MCU and FET driver IC (MCP1407), and an LCD.

User has control of the three parameters involved in a dual pulse weld. Weld process is as follows. The job of the 1st pulse is to clean impurities from the joint before the 2nd pulse comes which is actually the weld pulse. What we want is a strong enough 1st pulse with just enough umph to clean the joint but no more. So I provide user adjustment of duration, in millisecond steps, of each pulse and the time delay between them. For each workpiece the weld can now be dialled in by experiment.

For this functionality I use an LCD display with push buttons. The Hitachi HD44780 compatible 16*2 is used. The control board sits beneath the LCD via a 16 way header connector.

Unit operation:

A yellow 'ready' led comes on after initialisation. The four push buttons are Reset, Select, Up, Down. Upon power up the unit sits ready to weld with default parameter values. To change pulse 1, push Up or Down push button and the value goes up or down by 1ms. When you want to change the other parameters, then pressing Select cycles through to Delay, then Pulse 2, and back to Pulse 1.

The unit uses a foot switch for the weld fire signal and the code verifies a weld command thro a switch debounce routine - this way any spurious trigger is disregarded. A 2nd LED flashes indicating weld fire.

Power supply for the unit is via a common or garden 12Vdc mains adapter. I was initially using a 9V PP3, which worked but was soon drained. So a healthy 12V drops to 11.3v on the other side of a diode for reverse polarity protection, giving the FETS a more robust drive on the gate.

For my enclosure I recycled the case of a blown power adapter which, being plastic, was easy work to drill/file for mounting LCD & switches etc.

Electrodes can be cut from 5mm copper wire then soldered to your cable (25mm recommended) and covered with heatshrink sleeve. Tips are then rounded off with a file.


Pricing:
Essential kit of parts - assembled and tested control board plus LCD & pushbuttons & barrel connector for +12vdc PSU (incl programmed MCU) - 40 GBP
Power pcb with SMD gate resistors fitted - 6 GBP
Power pcb fully built - 90 GBP
Vishal Schottky diode - 4 GBP
TVS diodes, 3 of - 5 GBP

pm me if interested and for shipping cost. I accept paypal. All parcels will be well packed in ESD shielding materials.

Regards
----------------------------------------------
excellent concise explanation of the circuit by flangefrog:
viewtopic.php?f=14&t=81400&start=550#p1218325

demo video:
https://www.youtube.com/watch?v=mdKEcf8 ... e=youtu.be

Kicad project files for Power pcb:

6p8fv1_1.zip

add holes for schottky

(26.21 KiB) Downloaded 404 times

Gerber files for PCB Fab house (power board):

6p8fv1_1Gerb.zip

(28.03 KiB) Downloaded 276 times

BOM I carry stock of some of these

Weld cable - 25mmsq. Minimum, eg http://m.ebay.co.uk/itm/25mm-sq-Automot ... 1395792950

M8 crimp lugs (unless soldering weld cable directly to your own power board)
http://m.ebay.co.uk/itm/181407687544?_mwBanner=1

Electrodes - 5mm copper wire, 2 of 100mm in length. ('Normal' copper is fine for welding nickel)

Schottky power diode - I can supply - £4
http://www.mouser.co.uk/Search/ProductD ... -100BGQ015

TVS diode - I can supply, 3 of - £5
http://uk.farnell.com/vishay/5kp12a-e3- ... dp/2335201

12vdc wall wart adapter (I supply a 5.5mm female panel mount barrel jack for your enclosure)
http://m.ebay.co.uk/itm/322052316865?_t ... mwBanner=1

MOSFETs - AUIRF1324S-7P Do choose to use these FETs for best results. They are 24v rated, have high amps rating and excellent avalanche current tolerance. If not comfortable soldering SMD, there is a TO-220 version of the device. I carry stock of SMD version, 8 of - 27 GBP
https://octopart.com/auirf1324s-7p-inte ... r-18636041

Foot switch - Normally open. Replace connector with a female 5.5mm barrel jack as kit comes with a panel mounted male barrel jack for the foot switch input.
http://m.ebay.co.uk/itm/FP-Wholesale-AC ... 2462204825

Copper clad board single sided. (If fabricating your own power board). A board with 6 or 8 FETs can be fashioned using dremel type tool.
Tip: do not rely on copper foil of the board to carry weld current. Add copper bus bars soldered directly to FET drains and sources.

[moderator edit to fix link]

[jimwebo]

Hi.
If I understand you correctly, you sell the control module?
The bank mosfet I have to build me?
It is right?
I'm already using a JP spot welder with a capacitor bank about 4F.
But I like your display system to display parameters.
As shipping cost with Spain?
Regards
Jim

[pguk]

Hi, yes, you got it right!
I hope to get to work on the power module soon, but to get this far has taken a while...
I'll get back to you on shipping to Spain. Won't be so much tho - it's a small package.
Edit: 3.55GBP standard int'l delivery, 3-5 working days.

[riba2233]

Good luck with sales!

[pguk]

Thank you riba!
You were the first to run with this idea - I wish you well also, get well soon.

[pguk]

Wiring instructions

So there are five switches to wire to the control board. Four are pushbuttons, one is a foot switch, all of them are normally open and push to make to GND. There are four pins on the left hand side of the board. They are labelled on the board. The top one is 'Select' and is a pushbutton, under that is 'Foot' and goes to the foot switch, underneath that is 'Up' and is a pushbutton, and the bottom one is 'Down' and is a pushbutton. The fourth pushbutton is wired next to pin 1 of the microcontroller and is labelled 'Reset'. It's not strictly necessary to wire this - it is a hard reset for the chip and does the same as disconnecting board power.

Power pins are on the right hand side of board: +V is +12 to 15 Vdc. Next to it is GND. Input from your mains adapter goes onto these. Take a wire from the GND to your switches (daisy chain style). Take another wire from GND to the Sources of your MOSFETs.

The Gate pin is also on the rhs of the control board. Wire this through individual gate resistors (around 82 ohms) to the gates of your MOSFETs.

LEDs are wired as follows:
Y_A to anode of yellow led
Y_C to cathode of yellow led
R_A to anode of red led
R_C to cathode of red led

Edit: wiring diagram

6pence_wire.jpg (41.79 KiB) Viewed 13927 times

[pguk]

Testing

Edit 2/17 - see P4 for scope shots of new power board

On the oscilloscope I was able to observe what is actually going on during a weld fire. At the end of a day of testing I had a MOSFET explode. I am using 24V MOSFETs - you would think that 24V would be enough when using a 12V current supply (car battery) wouldn't you? Well, maybe for awhile... My power board made perhaps 50 or so good test welds with nickel onto cells. When it failed it had 2 12v batteries in parallel and the electrodes clamped together - dead shorted. A few single pulses of 2ms was enough to blow a FET. I have scope shots similar to the ones member Okashira has posted. At each turn off event, I saw a back emf of 27V. Anything over the 24V rating of those FETs is going to make for a short life.

[Edit: this is due to avalanche current. See below. Okashira managed to eliminate this large back emf to below the rating of the FETs as well as reducing ringing by installation of schottky flyback diode and TVS diodes]

After the blown FET event the microcontroller was still operational, only the 1402 driver chip had failed. So I'm happy that the control board is robust.

Power side diodes

The unit functions okay without these diodes, but if the user desires to look after the expensive power MOSFETs for long working life, or weld with high currents, then I recommend the addition on the power side of two diodes. These work together to divert the large avalanche current seen by the Drains at every turn off event. This transient varies according to the length and guage of weld cables used. For a thicker, longer cable, something over 30v may be seen; for my fairly light and short cables I see around 27 volts.

This effect is due to the inductance of the weld cables. The effect of the FETs turning off current flow in these large cables can be likened to instantaneously damming a stream. The flowing water (current) has an inertia which causes it to want to keep on flowing and so it backs up against the dam (Voltage surge at FETs). If we could find a way of diverting the water at the dam, giving the current somewhere to flow, we could attenuate that voltage spike. This is the job of a flyback or freewheeling diode. With a diode built from 2 IRF4368 FETs in parallel (connect gate to source and cut off Drain pin), I see my transient voltage of 27v decrease by 2 thirds from 69us to 22us. Use of a Schottky diode here will improve things even more. These have a faster switching time due to lower on voltage.

The heavy lifting is thus taken care of by the freewheeling diode but to get our voltage transient completely under control, we can also add a TVS (Transient Voltage Suppressor) diode. This is connected directly across our MOSFETs. It is chosen for system working voltage, breakdown voltage, and clamping voltage.

A shot of the Drains at turn off without freewheeling diode.

without freewheeling diode

2batt_noDiode.png (39.1 KiB) Viewed 14725 times

Same shot but with freewheeling diode in circuit.

with freewheeling diode

2batt+4368_2of.png (37.26 KiB) Viewed 14725 times

Lead acid setup showing power connections. This leisure battery does not supply enough current for a weld - hence jumper cables for paralleling my car battery. (please ignore mixing of red/black cables) Click for full resolution

Okashira on effect of Flyback and TVS diodes
viewtopic.php?f=14&t=81400&start=350#p1182616

Recommend Schottky for the flyback diode:
http://www.mouser.co.uk/Search/ProductD ... -100BGQ015

and a TVS diode:
http://uk.farnell.com/vishay/5kp12a-e3- ... dp/2335201

[gregh3269]

Thanks for the wiring info, newbe to electronics (not electrical!) and I want to make sure I understand this 100% (for the sake of your hard work and my £40):

On my board they are
'Foot' and goes to foot switch.
'Up' and is a pushbutton +
'Down' and is a pushbutton -
'Reset' next to pin #1

Is this correct? on the mosfets, the drain goes to +ve on the battery, and the sources go to the +ve electrode. The -ve electrode goes to the -ve on the battery. From your power board picture/video there two batteries in parallel, using a mixture of red/black wires on each side of the mosfet drain/source, ideally should they all be red on the +ve electrode all black on the -ve electrode? Or am I missing something?

Cheers Greg

[pguk]

Thanks for the wiring info, newbe to electronics (not electrical!) and I want to make sure I understand this 100% (for the sake of your hard work and my £40):

On my board they are
'Select' is a pushbutton
'Foot' and goes to foot switch.
'Up' and is a pushbutton +
'Down' and is a pushbutton -
'Reset' next to pin #1 is a pushbutton

Is this correct? on the mosfets, the drain goes to +ve on the battery, and the sources go to the +ve electrode. The -ve electrode goes to the -ve on the battery. From your power board picture/video there two batteries in parallel, using a mixture of red/black wires on each side of the mosfet drain/source, ideally should they all be red on the +ve electrode all black on the -ve electrode? Or am I missing something?

Cheers Greg

Power connections are not quite correct there. It would work but best practice is to have the FETs close to the battery negative:- batt +ve to +ve electrode, batt -ve to FET Sources, FET Drains to -ve electrode. Then you should install a flyback diode - anode as close to FET Drains as possible, cathode to batt +ve.
I have two batteries in parallel as the leisure battery I have there is not up to a fast discharge of current, so I added my car battery. My mixing of red and black cables on the video is not helpful - you're right, please ignore!

[gregh3269]

Just an idea, how about 2 or 3 of these Mosfet switches in parallel? < 100A should be enough.

http://www.aliexpress.com/item/Transist ... 82f710c51a

[pguk]

Ah, would that it were so. I fear these little modules would have a very short life trying to switch off 2000 ampere or so . Don't underestimate the effect of inductance of weld cables. I see them physically jump at each fire. Stick with the IRF1324S-7PPBF variants, RS stock No 688-6816.

[Matko Skutari]

Recieved and connected mine, the control board works, adjusting the pulses work, so that's great. I am planning on using this with headway 38120 10Ah cells. Has anyone tried using them? Also, 3 or 4 cells in series? Suggestions welcome

[pguk]

Code upgrade V2_0.
If those who have a unit would like to send your chip(s) to me I will upgrade them for free and post back to you.

1. Bug fix for a single pulse. If the user wants to weld with a single pulse rather than with a dual pulse, this is now possible. There was a problem with the code till now which meant that for a programmed delay of zero, the FETs were still briefly switched off/on between pulses. Obviously, if you have programmed for a zero delay, you are wanting to fire a single pulse. The chip now enters a separate routine for a single pulse when it sees that any of the 3 parameters are set to zero. With Delay set to zero it will fire a single pulse with duration of the sum of programmed values of Pulse1 and Pulse2.

This is useful for preserving the FETs if you are running without flyback diodes (very much recommended) and want to halve their exposure to avalanche currents. Or again you may consider that a dual pulse has little benefit over a single pulse for a good weld, and you want to give priority to longevity of the FETs.

2. Better resolution on the first pulse and delay. If you run a battery with low internal resistance and have heavy guage cables, there can be a need for more fine adjustment of these parameters. If metal is melted by your first pulse at 1ms, that is not so good. Better results are achieved when 2nd pulse does all of the weld. Pulse1 and Delay are now adjusted up/down in 0.1 second steps.

This of course means that it takes more button presses for larger adjustments. I did consider about a step change of .2 s. It's a trade off between resolution of adjustment and No of button pushes. It may be possible to make it so that the value rolls up/down for a long button press, but that is a whole lot more involved to implement in the code. On my test setup I found that these two parameters stay low and mostly equal in value.

6pence_display_v2.0.jpg (33.58 KiB) Viewed 13913 times

[mudflap5]

Recently finished a 20S12P battery using the 6pence welder. The welder worked very well, although it took a few attempts to get the pulse timing right. I think timing has a lot to do with the size and capacity 12 volt battery, using a very large deep cycle marine battery for this build. The nickel strips were .25mm thick. The power board was built with the FET's bolted to an old heat sink from a burned up controller, and using the flyback and avalanche diodes. Made the welding tips from 4 guage solid copper wire. Here is a picture of the pack and the welder is in the background.

[pguk]

Outstanding, looks very impressive. Thank you for the feedback mudflap.

I don't think this would fit on my kona stab...

[Builditgood]

How long did that beauty of a battery take you to weld Mudflap?

[mudflap5]

How long did that beauty of a battery take you to weld Mudflap?

About 3 weeks, working on it a few hours a day. A guess would be 25-30 hours, took my time, but it was still A LOT of work!

[Valld]

Hi PGUK,

How can I buy the board from you? I just joined the forum yesterday to be able to PM you, but the message is staying in the outbox....

[pguk]

pm sent.

[Valld]

Hi 6pence followers,

Below is my attempt to design a simple Power Board for the 6 pence spot welder working closely with the OP. Not having any education (and relatively little experience) in electronics, but wanting to make one myself I came up with this . All elements dimensions are taken from their data sheets and the four left mounting holes match the mounting holes of the controller board and the display board, if someone wants to combine everything in a compact single package/enclosure (I would install a small fan as well to keep everything cool in case I decide to put it in enclosure) . I'm posting it for the members like me who like the idea, but are not sure how to make the power board. Please don't hesitate to ask any questions if something is not clear, and even better, suggest improvements if you have better ideas.

[mxer]

If your planning making the power board side of things with the Op I'm in for one.

Only thing putting me off buying the 6pence welder is the power board, so if you plan on making them guve us a shout, cheers...

[pguk]

mxer: thanks for the feedback. There are a number of folks who would go with this if there was the power board included.

I've been working on it for some time now. its nearly there - over the christmas holiday I hope to get a good bloc of time to fine tune things before sending the gerbers off to the pcb fab house. "Right First Time" as they used to say at Ferranti.

It will look similar to how Valentin has it there, but I plan to get that Schottky completely on the board rather than hanging off the side. I have 3 TVS diodes to sit above the FETs. I have surface mount gate resistors on the underside of the board and will use both sides of the board to carry weld current (using a spattering of vias)
(Edit: Not good practice to rely on board foil to carry any weld current)

[Samd]

Whoa! Awesome


Sent from my iPhone using Tapatalk

[mxer]

Sounds great keeps us up to date..

Subscribed...

[spinningmagnets]

This looks like something that might work quite well, but I fear I do not know enough about electronics to make any useful suggestions. I am interested in figuring out how to spot-weld brass parallel strips onto the nickel-plated cell ends of 18650 cells.

Since brass has less resistance, that is good for performance (as opposed to the common pure nickel ribbon), but it makes it worse for achieving a good spot-weld. The latest theories suggest that much higher currents are required, and this brings me to my dilemma.

Does anyone who is watching this thread have any suggestions about possibly doubling the current on this design? Can I simply add six more FET's to make it a 12-FET welder?

If going to 12 FETs is possible, should they be mounted back-to-back, in a 6 + 6 pattern (like Lebowskis low-inductance controller), as opposed to 12 FETs "in-line"? I apologize in advance for how silly I must sound...