Is this the right mod for my spot welder? - Page 1
Forum section: electronics / beginners

author: metrologist
author_rank: Super Contributor
timestamp: March 14, 2023, 08:06:03 pm
content: I just got the spot welder shown in this video. The author says a problem with the design is that the mosfet gate can be pulled low because it's sourced from the shorted output, and he shows a mod to address the issue. I'm looking at 2min to 4min time marks for the before and after scope shots of the gate voltage. The mod produces only 6ms gate voltage buffer, but maybe that's all he had set the spot weld time?It looks like this mod will hold gate voltage for just over 100ms, is that correct? The device is settable for up to 99ms as I understand. Is there a better mod to consider?BTW, I would be using this device to weld battery strips with four SLA 7-10Ah batteries in parallel, or a FLA car battery. Thanks!

author: metrologist
author_rank: Super Contributor
timestamp: March 15, 2023, 02:41:49 pm
content: What is the first thing you do when you want to test your high current spot welder?

author: metrologist
author_rank: Super Contributor
timestamp: March 18, 2023, 08:12:20 pm
content: Just a question, why is Luca concerned about gate voltage dropping below 6V? The max gate voltage is 2.4.gate voltagebattery voltage

author: Buriedcode
author_rank: Super Contributor
timestamp: March 18, 2023, 09:44:13 pm
content: Quote from: metrologist on March 18, 2023, 08:12:20 pmDon't know much about spot welders but I'm guessing it's to reduce the Ron of the MOSFETs. I'm also going to assume by "max gate voltage" you mean "maximum gatethresholdvoltage" which is the threshold at which the FET starts to conduct. Higher gate voltage > lower Ron (generally). Look at the datasheet and check the Ron of the FETs in question and check at what gate voltage that is measured at.

author: metrologist
author_rank: Super Contributor
timestamp: March 20, 2023, 04:07:31 am
content: Yes, gate threshold voltage is what I meant.Seems anything above 4V should be fine by Fig. 1.https://www.infineon.com/dgdl/Infineon-IRL40SC209-DS-v02_00-EN.pdf?fileId=5546d462557e6e890155a1413329602d

author: Buriedcode
author_rank: Super Contributor
timestamp: March 20, 2023, 05:40:07 am
content: Yes, but according to the datasheet, the on resistance is 25% less with 10V Vgs, than with 4.5V. As loses are I^2R, thats 25% less power dissipation it has to cope with. Also bear in mind rise time of the gate voltage, driving the gate as hard as possible reduces conductive losses (at the expense of gate losses), but as you're dealing with huge currents, thats the idea.If you do the maths, pulse time, expected current etc.. you can then work out the energy of the pulse going though it, and check the maxium safe operating area (fig 10 pg 5).

author: metrologist
author_rank: Super Contributor
timestamp: March 20, 2023, 12:30:41 pm
content: Thank you. I had to look at figure 5 for a minute. I think the voltage I'd need to measure on my device is from battery (-) to output (-)?

author: Buriedcode
author_rank: Super Contributor
timestamp: March 20, 2023, 07:32:34 pm
content: Quote from: metrologist on March 20, 2023, 12:30:41 pmI'm not sure what output - is, I mean, if you're using the MOSFETs low side switches then its Vds (assuming source is battery -, and output - is drain) you'll need to measure. But also the current - that'll give you the Ron of the MOSFET(s) for a given gate voltage.I haven't designed anything with large current pulses through MOSFETs so I'm starting to get out of my depth- but I am assuming the idea is to keep the energy through the MOSFET small enough so it doesn't detonate off the board during a pulse. Given the low-duty of welders, I'm assuming the temperature of the MOSFET's isn't much of a concern since the pulses are very brief, and the time between pulses is large.

author: metrologist
author_rank: Super Contributor
timestamp: March 22, 2023, 12:30:20 am
content: That's exactly right.I guess I could do a 4-wire resistance measurement on the B+ through path, measure the voltage there with my scope during the pulse, and calculate current. My LCR meter is 0.001ohm resolution, +1% + 3lsd.I think I'll just do the mod before any more tests, just hope something with that doesn't go wrong later on, like the cap gets weak.I started out collecting SMD devices and fortunately realized they would not be able to handle the charge current for the proposed circuit, which I guess would have an initial current peak of around 400mA. Well, I tested it with a 1/8watt resistor and that seemed fine (and then I went mental numb and wanted to see the direct current through the resistor so I connected that to my 12V supply). I have a pack of these old diodes that I cannot find specs for 10166 in a little bit beefier black plastic case. Guess I could find the smoke point of one of those too