Greetings from the Northwest of England - I would like to present my version of the Dual Pulse Resistance Spot Welder.
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
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.
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.
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.
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.
excellent concise explanation of the circuit by flangefrog:
https://www.youtube.com/watch?v=mdKEcf8 ... e=youtu.be
Kicad project files for Power pcb:
Gerber files for PCB Fab house (power board):
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)
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]