[Review request] Spot Welder - Page 1
Forum section: electronics / projects

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: May 05, 2025, 12:19:49 pm
content: Hello all,I have finished the design for my spot welder. Its is my first serious attempt at power electronics, so I may have missed something major here.I spent a few weeks designing this but I now have realised that I have no use for such a spot welderWell, I'll build it anyway, why not.Page 1 - MCU section. Indicator LEDs, programming header, nothing too interestingPage 2 - MOSFET array driver. I am just using some SO-8 MOSFETs to drive the MOSFET array. Also the ESD diodes are across the MOSFET array (shown on page 4)Page 3 - Power input, LDOs to power MCU etc. I have designed the power input to be from 12V-30V, although I will be using it with 12V from a 7Ahr SLA battery. There's also an optional boost stage that I can use to boost the input voltage to a maximum of 28V to 30V to increase the energy in the capacitor bank.Page 4 - MOSFET array. These are IRL3803 TO-263 MOSFETs. 30V 140A 6mΩ, there are 12 of them in parallel. Although going by my calculations, I will be okay even if I just populate 7 of them, but to be safe I will populate 10.Page 5 (to 16) - These are just the individual MOSFET subcircuit copied, its the same for all 12 MOSFETs.Happy to answer any questions!

author: jwet
author_rank: Frequent Contributor
timestamp: May 05, 2025, 03:19:36 pm
content: This is a very ambitious project. Good luck. Here are a few things I noticed.1. Bypassing, in several places like at the first 7805 and following 1117, you have no high frequency bypassing with .1uF type caps in close. You do have large caps but these would be important also. It appears that the high side FET drive supply has no bypassing?2. It appears that the uP does a lot in the circuit. The combination of big current surges and micros often cause micro's to reset or misbehave. I would isolate the uP from the power electronics power wise. This could be accomplished with an LC filter up stream of the big power stuff that feeds the low voltage. Be especially careful of ground impedance and where your logic ground is taken. You have systems here like the boost converter that run on the micro- if the micro got off track, this little boost could run away and ruin your bank. The uP stuff is ok but consider some analog backup like a crowbar circuit or zener limiters. You should also consider fuses or circuit breakers at the input at least and perhaps some opto isolated drives.3. The drive required of all those parallel FET's will be quite a load. Your NPN/PNP follower is nominally ok but look at this issue. You might need peak currents for the gate drive of 30 amps if you want fast pulses. I don't know what's available for very high current FET drivers. You might consider some kind of opto-isolation or transformer drive in this circuit. The gates should be each have a resistor as decoupling. Watch grounds carefully here too.4. Paralleling big FET's can be done but requires some finesse. You need to watch avalanche currents and not exceed the avalanche rating of a single device since generally these currents don't share- they will all go to the strongest FET. A good app note on paralleling big MOSFET's- it covers the issues above too.https://www.onsemi.com/pub/collateral/and90108-d.pdfGood luck

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: May 06, 2025, 11:42:16 am
content: Hey jwet, many thanks for the comprehensive response. I'm fixing these issues.Also the document on paralleling MOSFETs was a good read, thanks

author: jwet
author_rank: Frequent Contributor
timestamp: May 07, 2025, 03:27:04 pm
content: You're very welcome- good luck. Its unusual to get a sincere thank you- much appreciated. It will serve you well in life.

author: nonius_
author_rank: Contributor
timestamp: May 07, 2025, 06:47:29 pm
content: QuoteThat's what I thought too when I built mine: that I might it use it a few times in my lifetime. In reality, it gets used several times a year. New applications come up rather often, it seems. If you have a hammer, you tend to see every problem as a nail....Mine uses a power-thyristor though, not MOSFETs. And a bank of capacitors (23 pcs, IIRC, of 68000 uF/25V each, for a total of 1.56F or 450J of energy). Enough to weld rather thick stainless steel (about 1-1.5mm max.). It certainly does well what it was designed to do, weld battery tabs. Mine's pretty unsophisticated: a modified PC AT(X) powersupply, with adjustable voltage (2.5-24V) and current-limited to 10A to recharge the capacitor bank. The thyristor does the heavy lifting (switching), triggered by a NE555 monostable-flop. Single-pulse welding only, but that hasn't been a problem for me so far.BTW, before welding, make sure your copper electrodes are oxide-free (use a bit of sandpaper or a file), as otherwise a shower of sparks may fly (put on safety glasses!) and blow a hole through your project. Learned that lesson the hard way....In my case, the thick cables (as thick as my thumbs, and heavy) jump about 20 cm in the air when a weld is made....Good luck with your welder, it's an interesting project!