Spot Welder Design - Sanity Check - Page 1
Forum section: electronics / projects

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: April 13, 2025, 11:32:10 am
content: tl;dr Designing a capacitor bank based spot welder, do I really need a sub millisecond pulse control using FETs or should I just change the voltage of the cap bank and short it directly to the nickel strip I'm trying to weld (aka no control).Hey guys, hope everyone is well. I needed a bit of sanity of check.I am designing and making a small spot welder for 18650 cells. The commercial ones seem quite expensive for what they are. My plan is simple, take 20x 2200uF 63V electrolytic caps, charge them up to 50V (55J energy max), use 5x-10x IRF3205 NFETs (55V 110A Rds=8mΩ) in parallel, then use an STM32G0 MCU to generate the adjustable pulses to drive the FET gates (using a decently fast gate driver of course, probably discrete power BJTs) to discharge the cap bank against the electrodes. And also a nice 7 segment display to show the energy level and pulse time.I told an acquaintance of my plan, and they told me that its not worth using expensive MOSFETs for this purpose, in fact no switching or MCU is required, only the capacitor bank. The pulse timing can be controlled by changing the voltage on the capacitor bank instead of an MCU and active switching. And when ready, the electrodes just have to be shorted against the nickel strip.So here are some things that I am wondering.I'm sure their setup will work just fine, but if commercial solutions have the pulse timing as a feature, it must surely solve some kind of problem? Or can I live without this feature?Any idea what kind of circuit protection I should be including in the design? 100s or 1000s of amps worth of current must surely upset some stray inductance somewhere in the circuit right? The last thing I want is a inductive kickback of 100s of volts to blow up my circuit. For whatever reason, its difficult for me to mentally model what such a high current must behave like. Even if its for 1ms

author: oz2cpu
author_rank: Frequent Contributor
timestamp: April 15, 2025, 07:52:08 am
content: to be able to design a spotwelder, you need to know how to opperate one, and how it works,in short : you want to be able to press the electrodes firmly to the place you want to weld,this must be done cold. then when you are ready, you hit the activate, foot switch is the best,this way hands dont move or release the pressure, when pulse is done, you want zero power fast again,now your electrodes dont stick or cause other problems.normally you want a way to adjust the pulse energy, many ways work almost the same way,variable charge voltage, change in capacitor value, variable pulse time,i prefer a rather low fixed voltage, but very high current, and then variable time,this way the sparks dont fly all over the place, too high voltage tend to make pulse timing sensitive,and you can easier blow thru the thinn sheet, and sparks fly all over your hands.protect hands, and face ! and no easy to burn materials nearby, no carpet,think safety all the way

author: voltsandjolts
author_rank: Unknown
timestamp: April 15, 2025, 08:14:19 am
content: To get consistent results you absolutely need digital control. Like all good commercial battery terminal resistance welders, you need to apply two pulses. The first is lower energy and cleans the workpiece, making better conduction path for the higher energy second pulse which completes the weld. Timing and energy control are critical. The materials involved in the process also have a large effect, both the materials on the workpiece and in the weld head. You should read these links;https://www.eevblog.com/forum/manufacture/hughes-hcd125-capacitor-discharge-welder/https://www.eevblog.com/forum/chat/battery-welding/msg5724289/#msg5724289

author: mikerj
author_rank: Super Contributor
timestamp: April 15, 2025, 11:23:14 am
content: Quote from: BadeBhaiya on April 13, 2025, 11:32:10 amWith all due respect your acquaintance has clearly never tried spot welding cells before. You absolutely need some method of triggering the currentafteryou have applied the probes and pushed the nickel strip onto the cell. His method will result in holes blown in the nickel strip and no welding. Some systems use a foot switch or a switch on the electrode assembly, but I one I use detects when you apply the electrodes to the strip (i.e. when the electrodes are shorted) and then applies current after a user settable delay.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: April 16, 2025, 08:37:46 am
content: >And when ready, the electrodes just have to be shorted against the nickel strip.EXACTLY Mikerj, this advice must be the worst idea we have heard in a long time,so it is a good thing BadeBhaiya come here to ask for a bit more info..

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 18, 2025, 11:26:17 am
content: Have you looked at your own BOM cost?22mF 63V capacitors start at around USD 6 at Digikey. So that alone is over USD 100 worth of capacitors.For the rest, a good gate driver for the FET's is essential.A common flaw of the cheapest Chinese spot welders is a missing diode, which lets the gate drive voltage sag during spotwelding, which results in releasing all the magic smoke from the MOS fets.For the rest. Accurate timing is trivial to make these days.And like the others here also already mentioned: You really need to switch the power with a reliable switch, after careful placement of the electrodes, and making sure enough pressure is applied.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: April 19, 2025, 11:56:56 am
content: Hello all, thanks for your responses. I was busy with designing the circuit, so sorry for the late reply. Also attaching the schematics so far, feel free to let me know if something looks bad.The input voltage of the circuit is 12V to 30V, the capacitor bank voltage can actually go up to 45V to 50V using a boost converter part of the circuit. In all likelihood, I will be driving this at 12V.Quote from: oz2cpu on April 15, 2025, 07:52:08 amunderstood, I am always very careful when dealing with power electronics. I too am leaning towards having a fixed low-ish voltage and a variable time but I am unsure if longer times would be possible with the FETs that I am planning on using.Quote from: voltsandjolts on April 15, 2025, 08:14:19 amThe links were quite educational, and the two pulse concept (to first clean the work piece and then weld it) is quite new to me, nevertheless makes sense. I also read through the K-weld eevblog thread, that too was very enlightening.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: April 19, 2025, 12:40:11 pm
content: Quote from: mikerj on April 15, 2025, 11:23:14 amYou are right on that, my acquaintance has never tried spot welding cells before (although they do have some rod welding experience iirc).It does makes sense to include some kind of control to trigger the weld, from a safety and usability view. It probably wouldn't end well if I were to touch the rods accidentallyThe on-contact auto trigger sounds like a cool (and relatively simple) feature to implement. I'll probably add it soon.Quote from: Doctorandus_P on April 18, 2025, 11:26:17 amI haven't looked at the BoM costs yet but its probably going the be ~75USD to 100USD for everything. I am using a few local websites so the component costs are a lot lower than from digikey (2.2mF 50V caps are ~0.25USD/each). Of course, these are all components from China, but they are inexpensive enough to buy and then validate myself. Or not. The FETs are similarly 0.35USD/each. Its a hobby project so I don't mind getting dubious components. More experience for me!Quote from: Doctorandus_P on April 18, 2025, 11:26:17 amYes, I want to have a good gate driver but I'm not too keen with the idea of buying an IC for this purpose. For now the "gate driver" is simply a FET totem pole which switches 12V. I also can increase the drive voltage (by increasing the input voltage) and pulse the FET gate so that the turn on time is even shorter, while the turn on pulse is just short enough to let the zener on the gate shunt the voltage to something non lethal and still survive. I am optimistic since the load is pulsed in the end, and not continuous.Could you elaborate what you mean by the missing diode thing? Do you mean the heavy load (the nickel strip) being switched causes a voltage dip in the power source which would actually suck out charge from the gates, lowering the voltage and increasing the Rds and killing the MOSFET? If that is indeed what you are referring to, in my circuit the capacitor bank can be isolated from the rest of the circuit, once its charged adequately using a FET. I don't think it will be an issue here.