Yet another spot welder circuit - Page 1
Forum section: electronics / projects

author: Virus
author_rank: Newbie
timestamp: April 04, 2019, 02:05:54 am
content: I'm making my own attempt at a welding machine, as importing alternatives such as KWeld are really expensive because of customs and shipping, I have some supercaps lying around and I feel like taking this as an instance to learn about power electronics.I made an iteration of the circuit on a perfboard and it came out to be a really good MOSFET destroyer. So I decided to tidy up my circuit and make a PCB out of it.I used four IRFB7430s as MOSFETs and MCP1407 as drivers.After redesigning the circuit, the power electronics portion of it looks like this.Which measures do I have to take to make sure my MOSFETs are not going to blow again?

author: texaspyro
author_rank: Super Contributor
timestamp: April 04, 2019, 03:36:31 am
content: Use only one gate driver for all the MOSFETs. Match the FETs for gate threshold voltage. Beware of too fast of a gate driver... high dI/dT can be a problem. 5V weld voltage is too low for most usable welds.Also, no matter what you do, expect to kill a lot of FETs getting this thing to work.

author: Virus
author_rank: Newbie
timestamp: April 04, 2019, 04:22:53 am
content: >5V weld voltage is too low for most usable weldsHowever, the ESR of the capacitors is a factor and if I switch the arrangement from a 5S2P, ESR will make the maximum current to be something less than 600A rather than the 1300A that the current arrangement is capable to deliver for welding surface resistance equal to zero. Is it still a good option to change the arrangement of the capacitors?As for the MOSFET drivers, if I use only one MCP there should not be much trouble as it's a monostable and not another demanding kind of application that requires frequent charge and discharge such as a waveform amplifier. Still, the input capacitance of each of these MOSFETs is quite high (14,000pF) and I'm afraid MOSFETs get hot in that active period of time, but if slowing things down helps to avoid the effect of layout hidden inductances, I'll gladly do that.Is there a thumb rule to check inductive transients in such circuits? Sorry for the dumb questions.

author: texaspyro
author_rank: Super Contributor
timestamp: April 04, 2019, 05:34:19 am
content: My welder has 18 IRFP2907 MOSFETs in parallel driven by 1 obsolete DS0026 driver chip. The capacitor bank is 20V, 3.6F, <0.6 milliohm ESR. It can spank around 20,000 amps.

author: beduino
author_rank: Regular Contributor
timestamp: April 04, 2019, 08:54:00 pm
content: I've made my spot welding machine based on old bulky transformer 230VAC instead of playing with tricky condensator based ones.In the case of transformer in my case two thyristors in anti-parallel configuration did the job on transformer primary, so no need to deal with huge current like in the case of condensator based spot welders.Quite interesting way to controll transformer based spot welders is triggering by pulse train synchronization by the mains voltage described in detail here:http://www.datelec.fr/secteur/ST%20AN308.pdfbut I didn't had time to test this so far.Anyway, I will make a try soon, since I need more powerfull and reliable spot welder.

author: beduino
author_rank: Regular Contributor
timestamp: April 04, 2019, 09:31:01 pm
content: Quote from: texaspyro on April 04, 2019, 03:36:31 amI have yet another tricky idea to drive transformer primary in my spot welder by using pulse train described above,but instead of triac or thyristors in anti-parallel I used so far without any problem-my idea is to use AC mosfet switch driven directly from MPU by gate drive transformersI used this concept of GDT with 100kHz PWM on two MPU pins in my current roof LED lights project, where AC mosfet switch block 230VAC and GDT with 100k resitor on AC mosfets gate does the job, but it is low current needed.In the case of spot welder 230VAC mains can draw much higher currents on transformer primary, but still oders of magnitude lower than in capacitor based spot welder, so I do not expect any fireworks in my new spot welder project but will see soon

author: Virus
author_rank: Newbie
timestamp: April 05, 2019, 04:48:32 am
content: Yesterday I found KWeld schematic and I shamelessly ripped off some ideas, specially what's concerning about measuring the voltages in both terminals of the welder and the positive terminal of the capacitor in order to see if I can replicate that pulse length to joules calibrations method by myself or see if I can get something worth of processing out of measuring these voltages.Any comments on this?

author: Virus
author_rank: Newbie
timestamp: April 07, 2019, 02:03:40 am
content: I am making the PCB, will update when it's ready.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 07, 2019, 09:16:01 am
content: The simplest would be to use a modified microwave transformer and combine that with a EUR 10 Spotweld timer from Ebay / Ali / China / Whatever.

author: beduino
author_rank: Regular Contributor
timestamp: April 07, 2019, 09:44:02 pm
content: Quote from: Doctorandus_P on April 07, 2019, 09:16:01 amNope, in many cases those microwave transformer have aluminium primary windings, so forget about decent quality