Homemade spot welder keeps blowing MOSFETs - Page 1
Forum section: electronics / projects

author: Refrigerator
author_rank: Super Contributor
timestamp: May 29, 2021, 02:56:23 pm
content: Seems i can't make a welder of any type that doesn't just immediately vaporize itself.Long story short: i got some 18650 cells from my internship and i want to make a pack but i don't have a welder. So i made one and it exploded over and over again.Long story long:Ok, so the first few failures were my fault because i was basically using whatever scrap MOSFETs i could salvage and was just trying my luck basically.But recently i was ordering parts from mouser and decided to throw in some proper high current MOSFETs and got 10 MDT4003SCT's.Made my MOSFET assembly with plenty of copper and added a 10 ohm resistor in series with each gate, more to protect my gate driver than anything really.Below included is a rough schematic for ease of understanding.And all was well until today. Previously i used an old battery from my car but it would discharge too quickly, just not enough juice left in it.So i took my 95Ah 850CCA 12V battery that had a shorted cell and cut that cell open to add a copper bar across the electrodes to bypass it so it's a 10V battery now.And it took only about three tries before my MOSFET assembly let out the smoke. But while it worked anything i tried to weld would turn into vapor, the peak current was massive.I actually didn't expect the battery to have this much oomph, like yeah, i know it's new, but it caught me completely offguard just how violent it is.I'm using a rather short 16mm2welding wire to tie everything together, which funnily enough was the thinnest welding wire i could find at the time.I still have 9 IRF1324PbF's that i got from aliexpress. Had 10 but decided to crack one open to see it it's genuine and it wasWell at least the silion die inside was massive and the bondwires were chunky as well, i'll include a pic.According to the datasheet IRF1324 has about half the RDSONof the MDT4003 so i have high hopes for it.So i still want to make a welder but i think it would be best to put some more effort forth before i blow it up again.Some thoughts in hindsight:1. This welder is kind of sort of a boost converter if you squint hard enough so there will be a high voltage spike once my MOSFET's turn off. I knew this already before but wishful thinking lead me to not use a snubber of any kind = big mistake. Now i'm thinking about using a MOSFET as a voltage clamp. I've been experimenting in LTspice already with some source follower MOSFET voltage clamps. I chose MOSFETs because BJTs seem to not handle that well with high peak current. Also i doubt that a zener or a transorb will do. The IRF1324 has a 24VDSSso i need the snubber/clamp to be fast and reliable.2. Were my wires too thick?Perhaps i should have used thinner wire to limit the current. Almost every DIY battery tab spot welder out there uses 6mm2wire, which is much thinner than what i use. I kind of went with the idea that more copper = more better but how good is too good?3. Is my gate driver not good enough? I tried to make it reasonably beefy and used transistors that are capable of 3A sustained current. Or perhaps is the slope too steep and is causing ringing or other unvanted behaviour? I've seen some DIY welder projects where MOSFETs were driven straight from an arduino pin and they worked fine. Surely my driver is not worse than that.I haven't spent that much money on this project (yet) but i'm already feeling like i should calm down before i bury myself in a pile of blown up MOSFETs.Also i took some cool looking macro shots of the blown up MDT4003 MOSFETs. Notice how small the silicon die is, also are those bondwires aluminum or silver?BTW does Dave still do the magic smoke pic thingy? I have plenty of thoseWhat do you guys think? I guess it's time to place bets whether it explodes again or not.Pic list:1. MDT4003 die shot from the side2. MDT4003 die shot from above3. MDT4003 blown right out4. IRF1324 from aliexpress cracked open5. Gate driver schematic, output stage x10 MOSFETs6. Gate drive signal with 10nF load cap. Red trace - arduino pin, yellow trace - gate driver output.7. MOSFET assembly, 10 MOSFETs in total, 5 per side. Double sided FR4 with copper bus bars soldered on. Under the black tape is a strip of protoboard that holds the gate resistors.

author: f4eru
author_rank: Super Contributor
timestamp: May 29, 2021, 03:27:20 pm
content: 1) shorting a battery is for sure an uncontrolled high current event, can give out over thousand amperes. Your mosfets are probably still too small for that.2) Yes, the inductive spike, with the same 1500 amperes has to be dealt with for a few 100ns.3) why not getting a working double pulse welder from Alibaba for 150-200$ ?

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: May 29, 2021, 03:51:59 pm
content: The current is uncontrolled and the stored energy of inductance in the circuit are bad for both the voltage and current rating of the FET. Consider using a limited current from the battery to charge a large capacitor instead ofr using the battery directly. This will limit the total energy of the pulse.Definitely add a properly sized diode from the drain of the FET to the battery. This will clamp the drain to way less than its 40V rating at turn-off.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 29, 2021, 04:14:42 pm
content: Quote from: f4eru on May 29, 2021, 03:27:20 pmNo fun in that

author: ledtester
author_rank: Super Contributor
timestamp: May 29, 2021, 04:28:55 pm
content: Quote from: Refrigerator on May 29, 2021, 04:14:42 pmYou can get workable spot welders from aliexpress/ebay now for < $20. They have flaws, but their designs are very simple and the flaws can be mitigated quite simply.This video goes over their schematics, problems and workarounds:All "Mini Spot Welder" versions: circuit Analysis, differences and faultshttps://youtu.be/RSrlXqFxhp8Even if you don't buy one of them it could serve as a good resource what the common problems are and how to build your own out of the parts you have on hand.

author: jmelson
author_rank: Super Contributor
timestamp: May 29, 2021, 04:40:30 pm
content: Well, you need to put in a little bit of resistance. Not a lot, we're talking about maybe a foot of #12 solid copper wire(sorry about US measurements) in series. How long is your weld pulse? It should only be 100 us to 1 ms long, I'd think.Also, check the rise and fall times of the gate drive. You want the voltage at the gate to transition in a 100 ns or less, never leaving the FET in the linear region very long. The Miller capacitance could make this hard to do, that's why commercial gate drivers can source/sink several amps.Jon

author: Refrigerator
author_rank: Super Contributor
timestamp: May 29, 2021, 04:54:20 pm
content: Quote from: jmelson on May 29, 2021, 04:40:30 pm100ns is pretty fast. I think my gate drive rise/fall time is well below 100µs right now, i'll have to whip out the scope to confirm.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 29, 2021, 04:57:33 pm
content: Perhaps it would make more sense to use a transformer so that you'll instead be generating higher voltage AC at a lower current? 48V would be a good primary voltage, reasonably safe to work with and large power supplies are fairly easy to find for cheap on the surplus market.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 29, 2021, 04:59:00 pm
content: Quote from: ledtester on May 29, 2021, 04:28:55 pmI've already seen these videos. It surprised me that tiny SMD transistors were enough to drive those MOSFETs.I didn't see any snubber on these but i'll have to check.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 29, 2021, 05:22:19 pm
content: Quote from: NiHaoMike on May 29, 2021, 04:57:33 pmI actually was thinking about making a switch mode power supply with a high current secondary. When you think about it i could build an inverter to run off the 10V battery with maybe a 1:5 ratio on the transformer, which would mean 5x less current on the primary. But finding a suitable core is hard and magnetic field coupling would be poor with few turns of thick wire. If i used copper strip maybe i could make it better, but finding copper strip is not that easy also. I can get some aluminum bar but it might be too thick and stiff to wind into a coil.I wanted to experiment with using regular laminated transformer cores for low frequency (sub 5kHz) SMPS so i might do just that with this.A simple ZVS circuit would be enough to drive the transformer at full tilt, since that's all you really need. With a more sophisticated controller you could add all kinds of control and PWM but i'm kind of getting carried away here.I have a ~15kg transformer with a primary wound sitting around ready to be used so if all else fails i'll throw a couple turns of thick wire on it and add an SSR with an arduino for zero crossing detection to control the thing.The transformer was meant to be a spot welder actually but for thick sheet metal and not battery tabs, hence why it's so big. IIRC the secondary was supposed to operate at a little over 2kA. But i can lower that with thinner wire and fewer turns.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 29, 2021, 05:39:51 pm
content: Played around with LTspice and came up with a way to make the MOSFETs eat their own back EMF.If i place a 10V zener from gate to drain the MOSFET will clamp the drain voltage at around 17-ish volts.Problem is that the peak current is the same as welding current so whatever snubber i use needs to be able to handle that.If i place a shottky on the drain and charge a cap with it a 4700µF cap charges up to over 20V in a single pulse, so that's a no-go.Also if i replaced my 16mm2wire with 6mm2wire the peak current should drop below 1kA.Problem is that the inductance rises as wire diameter decreases.I need about 10mto drop the current below 1kA, battery ISR is about 5-ish m.Bonus pic is my battery mod. I actually managed to un-short the cell but it was so badly sulphated that it would probably barely pass enough current to start a car.So since the bad cell was a massive chokepoint i bypassed itand cooked my MOSFETs shortly afterPS: it's kind of funny talking about kiloAmps, like oh yeah just add a couple milliOhms to drop it below 1k, she'll be right.

author: f4eru
author_rank: Super Contributor
timestamp: May 29, 2021, 07:25:55 pm
content: Quote from: Refrigerator on May 29, 2021, 04:54:20 pmToo slow. You have to speed it up. Put darlingtons, with final tranistor multiple in paralell, lower the resistors, etc...QuoteIt is used for some lower power stuff. Not a good idea here at all.2000A * 17V -> 34 kW instant power.Use big fat fast diodes over the load, and add also the biggest TVS you can find onto the mos, both are needed.QuoteNot nice. It my weld, but not well, because it's much too slow in rise/fall time for those small and fast spot welds, there's too much inductance in the transformer

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 29, 2021, 07:57:38 pm
content: Quote from: Refrigerator on May 29, 2021, 02:56:23 pmjust to emphasize what other have said that you seem to ignore... what you show in OP, without current limiting resistance at the drain and source, is a smoke guaranteed circuit, regardless of how fast the gate is. you cant respect only the gate pin, you need to respect all 3 pins, nuff said. hint: lower the voltage, or/and increase wire length, and mind the inductance spark (high voltage)... protection diodes?

author: jmelson
author_rank: Super Contributor
timestamp: May 29, 2021, 11:22:53 pm
content: Quote from: Refrigerator on May 29, 2021, 04:54:20 pm100 us??!!?? That will blow the FETs right there! Yes, 100 ns is a bit fast, you can probably survive full turn on and off (including Miller delay) of 500 ns or a little more. Anything over 1 us, however, in this extreme condition will guarantee failure of the FETs. Make sure to measure the gate charge and discharge with a voltage swing on the drain. As the drain voltage drops during turn-on, the gate-drain capacitance fights the driver trying to charge the gate. And, the opposite at turn-off. That is the Miller effect.You can put a 100 Ohm resistor in place of the welding electrodes to observe the Miller effect on the bench. You will be surprised how bad it gets with a weak gate driver.Jon

author: Refrigerator
author_rank: Super Contributor
timestamp: May 30, 2021, 09:20:11 am
content: Quote from: jmelson on May 29, 2021, 11:22:53 pmWell i did saywell below100µs. I haven't yet checked the rise/fall time with my scope.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 30, 2021, 09:44:53 am
content: Checked the rise and fall times with my scope using a 100nF load on the gate driver and might have found my problem.Rise time comes in at 79ns but fall time comes in at a much bigger 18µs.So it looks like my gate driver needs improving.Hopefully this slow fall time will increase the rise time on my learning curve, huh.Ps: i thought that maybe my low side transistor was blown but LTspice shows similar behaviour.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 30, 2021, 10:48:12 am
content: Played around with LTspice and managed lower the rise time down to about 280ns. I wonder if this is good enough.All it took was another shottky and an NPN under the low side PNP to form an SCR. The shottky it there to limit base current.There aren't any transistors equivalent to what i have so i think i'll just upgrade my gate driver and test it out to see what the new fall time is for real, brb.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 30, 2021, 11:27:20 am
content: Quote from: f4eru on May 29, 2021, 07:25:55 pmBut only a 30mJ increase in energy dissipation per MOSFET according to LTspice.I have some massive diodes that i'll put across the load to shunt as much of the overvoltage as possible back to the battery.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 30, 2021, 01:28:52 pm
content: So i modded the circuit and nothing seemed to change, weird right?But then i took a look at my breadboard and noticed that my load cap was inserted one pin over and was for the previous test as well so that one's completely false.I can't go back to the previous configuration but after the changes now i get around 1.1µs rise and 572ns fall times.Since the high side transistor wasn't changed in any way i can assume that the rise time was the same previously also.So it seems my gate drive circuit needs some improvement still.

author: jmelson
author_rank: Super Contributor
timestamp: May 30, 2021, 04:09:18 pm
content: Quote from: Refrigerator on May 30, 2021, 09:44:53 amWell, these are not real. You must connect the power FET and have representative voltage on the drain to see the Miller effect. On many FETs, the Miller effect is BIGGER than the plain gate capacitance. (Maybe not so with only 12 V supply, though.)Jon

author: jmelson
author_rank: Super Contributor
timestamp: May 30, 2021, 04:13:11 pm
content: Quote from: Refrigerator on May 30, 2021, 10:48:12 amIt looks like R5 is what is draining the capacitance, not your 2 transistors. You could try reducing R2, not sure it would help much.Jon

author: Refrigerator
author_rank: Super Contributor
timestamp: May 30, 2021, 04:51:56 pm
content: Quote from: jmelson on May 30, 2021, 04:13:11 pmR5 does barely anything actually, if i change it to 200 Ohms i can reduce the fall time by about 40-ish ns.It's purpose is to keep the gates discharged whenever the gate driver is unplugged from the MOSFET assembly.

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: May 30, 2021, 05:03:42 pm
content: Without a freewheel diode connected from the drains to the battery there is great risk of over stressing the FETs during turn-off.I prefer to use a gate driver chip for my designs (though discrete is fine if done right). My go-to parts are from IXYS/Littelfuse, e.g., IXD_604 or IXD_614. These are very rugged and inexpensive.https://ixapps.ixys.com/DataSheet/IXD_604.pdf

author: Refrigerator
author_rank: Super Contributor
timestamp: May 30, 2021, 05:21:51 pm
content: I don't really have that many components to choose from but i did recently receive some AP4506GEH that i had ordered from aliexpress.These are a PMOS + NMOS combo type thingies, joined at the drain.LTspice didn't have these but it's easy enough to just pop in some other MOSFETs that are close enough.And these MOSFETs also needed a gate driver each of their own so now my MOSFET gate driver MOSFETs needed a gate driver.But i couldn't drive the gate driver MOSFET gate drivers straight from the microcontroller pin.So i added a driver for the gate drivers of the gate driver MOSFETs.And after a bunch of tinkeringi managed to get rise and fall times below 4ns.But that's before the gate resistor. After the gate resistor the fall time alone increases up to almost 200ns.So i'd say it's pretty much impossible to get rise and fall times on the gate any lower than 100ns since it's limited by the gate resistors.I included the LTspice simulation in case anyone wants to check out.Quote from: Pulsepowerguy on May 30, 2021, 05:03:42 pmThat actually looks like a really nice part to have i'll have to check it out.

author: Manul
author_rank: Super Contributor
timestamp: May 30, 2021, 07:23:42 pm
content: Quote from: Refrigerator on May 30, 2021, 05:21:51 pmTo go faster, you need lower value resistor, more current. The problem is that most likely it will start ringing. If you want to push high current into gate, you can, but you need very low parasitic inductances to prevent ringing. That means a good layout, very short traces. Also, not all mosfets are equal, you might be able to find one with lower gate capacitance. Still, this is not the whole story, because there is miller effect, which can prolong the turn-on. Also there is parasitic source inductance, so high drain-source current spikes cause the actual source potential to rise above ground and can cause slow down of turn-on. That is why you can find mosfets in 4 pin packages with kelvin source pin used only by gate driver, but not carrying drain-source current. All in all, high speed, high current mosfet circuits can be a pain to do good.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 30, 2021, 08:58:30 pm
content: Quote from: Manul on May 30, 2021, 07:23:42 pmBut in the end i question whether it's even worth putting in the extra effort to get super fast switching.A 1µs rise/fall time only increases the dissipated energy by less than 5mJ.And a 10 Ohm gate resistor is already very low, in fact i don't have anything lower than that on hand, other than a couple 5W ceramic resistors of varying values.I think i might decrease the resistance of the top side PNP transistor base resistor to maybe 470 Ohms to help bring the rise time below 1µs and will see how things go from there.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: May 30, 2021, 08:59:41 pm
content: In this article the author provides a technique, without the use of the gate resistor, seeking maximum response speed.http://prosje.be/Projects/BelastingSchakelen.html

author: Refrigerator
author_rank: Super Contributor
timestamp: May 30, 2021, 09:33:00 pm
content: Quote from: Avelino Sampaio on May 30, 2021, 08:59:41 pmThe problem with my build is that i'm trying to drive 10 MOSFETs at once all in parallel.A single MOSFET would be no problem.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 30, 2021, 09:58:29 pm
content: While i'm sat here, sifting through my biggest diodes, it kind of makes me wonder how do these ebay kits last that long when they don't even have any visible protection for the MOSFETs.Well for starters (pun intended?), nobody uses massive overkill batteries and huge wires like i do but even then there should be significant voltage spikes.On a different note, i found three spare 80CPQ150 diodes that should work just fine i think.80A continuous and 1930A peak - only for 5µs pulses, but in parallel they should share the load just fine.

author: magic
author_rank: Super Contributor
timestamp: May 31, 2021, 07:43:14 am
content: A freewheeling diode in parallel with the electrodes will dissipate inductive energy stored in the electrode cables.The energy of power supply (and its cabling) inductance will still go into the FETs, I think.Wouldn't an RCD snubber closely in parallel with each FET be the right solution?

author: Refrigerator
author_rank: Super Contributor
timestamp: May 31, 2021, 08:46:07 am
content: Quote from: magic on May 31, 2021, 07:43:14 amI would need one massive snubber to handle a 1kA spike and not let it exceed 24V.And placing several on each MOSFET wouldn't be any better than placing a large one on a single MOSFET just because all MOSFETs are on a single copper bar.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 31, 2021, 08:53:49 am
content: Bought a 2m length of 6mm2wire and it's alot smaller than the 16mm2i used previously.This should give me about 5mOhms of resistance.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 31, 2021, 12:57:39 pm
content: Assembled the welder again and added the diodes.But while testing i noticed that the diodes do just about nothing.On the other hand my zener mod works perfectly.Currently i'm using a car headlight bulb on the high beam as my load.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 31, 2021, 02:00:36 pm
content: Previously i only had a zener on one MOSFET just for a quick test, now i've connected all gates to the zeners through a shottky diode on each MOSFET.Replaced the bulb with a 330mOhm ceramic resistor for load and everything seems to work fine so far.So i think i should be able to test the welder on some nickel now.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2021, 02:23:51 pm
content: I use a spot welder at work, which I've also repaired on a couple of occasions. It uses a DC:DC converter to charge a couple of large capacitors, which are discharged by a huge SCR, rather than a MOSFET. The amount of energy going into the weld is controlled, by changing the output voltage. The SCR is rated to 300A, at 400V, if I remember rightly and don't think it has any overvoltage clamping.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 31, 2021, 05:04:30 pm
content: Quote from: Zero999 on May 31, 2021, 02:23:51 pmSCR's don't need any protection because they only turn off when there's no longer current flowing.The biggest problem with CD welders is finding the right caps.Edit: and you need to wait for them to recharge with each weld.Chunky SCR's are easy enough to find, i have a couple big ones in my parts bin, no idea what their specs are.Made in USSR in '87, model number PM50 but internet doesn't seem to show any specs for it.

author: voltsandjolts
author_rank: Unknown
timestamp: May 31, 2021, 05:29:57 pm
content: Excuse me for slight off-topic, but just want to point out these capacitordischarge welders that pop up on flea-bay every so often. This one is a1998 vintage and its a well built work horse that makes perfect batterytab welds every time. The capacitor bank is 4 x 1500uF 250VDC whichdumps into a very nice custom output transformer. The output windingis 5 turns of 10mm x 5mm solid busbarThere are also various primarytaps, selected by the large front panel 'pulse width' knob.I think Hughes were the original manufacturer (guessing from the 'HCD'name) but there are HCD125's from Hughes, Micro-Join, Palomar. I thinkAmada-Miyachi may have the product line now and new equivalentwelders are in range of 4K to 10K GBP....gulp .The double-pulse output works very well, cleaning up the workpiece withthe first hit, then full weld with the second. Both pulses have controlled /programmable energy.I need to do a rough schematic of this unit some day.https://www.amadaweldtech.eu/products/resistance-welding/capacitive-discharge-welding-power-supplies

author: Refrigerator
author_rank: Super Contributor
timestamp: May 31, 2021, 05:56:26 pm
content: Quote from: voltsandjolts on May 31, 2021, 05:29:57 pmThat's pretty cool. I was also wondering if it's possible to dump a capacitor bank into a transformer because i have a bucket full of SMPS primary filtering capacitors.How are the primary taps selected? I assume there's an SCR going to each tap with a single common tap so that you can choose which SCR to fire to dump the capacitor bank into whichever tap.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 31, 2021, 06:18:20 pm
content: Received the nickel strips i had ordered so i could try my welder with fresh nickel instead of the stuff i pulled off the battery pack, which i took apart.At first i couldn't make any kind of decent weld no matter how long i set the pulse. Max pulse on my controller is 100ms.The nickel strip was visibly melted between the two spots but it just wouldn't stick to the battery.I even tried refiring it multiple times on the same spot with a 100ms pulse and that made no difference, other than making the 6mm2wire quite warm.So i started wondering whether i had actually introduced too much resistance into my loop and even thought about trimming my wire.Good thing i didn't because i found out why my welds weren't sticking. As i turns out i was applying too much pressure on my electrodes, go figureIf i apply only gentle pressure on my electrodes i get perfect welds. If i pull the nickel strip off i get very nice teardrop shaped nuggets left over on the battery.I read on a blog some years ago when researching battery tab spot welders, that a teardrop shaped nickel nugget left behind is the optimal weld. So i'm pretty stokedI guess it's time to finish up my battery analyzer so that i can re-test and cycle my batteries, so that i know for sure that their capacity is accurate.Pics:1 - Test using salvaged nickel. You can see how it melted but i pulled it right off with minimal effort. The strip was cleaned with acetone beforehand. Looks like a smiley face now that i've had a closer look at it.2, 3 and 4 - After i realized that less pressure makes for better welds you can see how much better those have turned out. Second pic with 0.2x8mm nickel strip, last two with 0.15x12mm strip.

author: parasole
author_rank: Contributor
timestamp: June 01, 2021, 05:21:02 am
content: I would recommend malectrics, bought his PCB time ago (I think it was V3.1) and added 1F capacitor bank which solves the power supply problem. I use 12V PSU and charge the bank via car light bulb as the current limiter, charging time is about 5-7 seconds which is just ok time interval you need to move for next welding position.Below is the picture of my setup, since that time I supplemented it with oled display, but that is more as a service function, you really need only a pulse generator for proper work.Now I may see there is PCB V4 with added capabilities, but again, most important is power electronic arrangement which is done in a proper way so you won't blow up your MOSFETs.https://malectrics.eu/product/diy-arduino-battery-spot-welder-pcb-set-v4/https://github.com/KaeptnBalu/Arduino_Spot_Welder_V4

author: Refrigerator
author_rank: Super Contributor
timestamp: June 01, 2021, 08:06:29 am
content: Quote from: parasole on June 01, 2021, 05:21:02 am5-7s between welds seems like a pretty long time to wait. With my welder i can fire as fast as i can get my electrodes on.That arduino welder sure looks cleaner than what i have

author: Refrigerator
author_rank: Super Contributor
timestamp: June 01, 2021, 08:27:28 am
content: Made myself a better foot switch yesterday and played around with the welder some more.Seems like i've got the hang of it and now my welds are super strong.Also it's fun welding nickel strips to anything metal. I even welded a strip to a steel bracket i had laying around.I didn't have any stiff switch for the foot switch so my pedal has a lever mechanism that presses on the stiffest microswitch i could find at home.It feels ok but i think there's still room for improvement. It's certainly better than what i had before that's for surePics:1 - old foot switch, was made in like 5 minutes.2 - new and improved foot switch.3 - new foot switch opened up, it kind of flops open like that4 - tried to pull of a nickel strip i welded and it tore right off, this was with a 50ms pulse.5 - leftover nickel nuggets can be seen on the metal bracket after i pulled the strip off.

author: parasole
author_rank: Contributor
timestamp: June 01, 2021, 10:59:36 am
content: Quote from: Refrigerator on June 01, 2021, 08:06:29 amwell, that is my personal setup which depends on PSU current capability (and it is massive as being server PSU) and ballast resistor (bulb), and I easily may shorten that time by simply replacing the bulb with more powerful (one or just a few in parallel, but I found that time to be just ok for me while building my bike battery.

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 02, 2021, 01:10:56 am
content: Quote from: Refrigerator on June 01, 2021, 08:27:28 amthanks for letting me know i'm not the worst builder here... btw, congratulation on your 6mm.sq wire purchase

author: Refrigerator
author_rank: Super Contributor
timestamp: June 02, 2021, 05:38:01 am
content: Quote from: Mechatrommer on June 02, 2021, 01:10:56 amYeah now i'll know not to go overboard with copper thickness.At least it finally works now!The new footswitch is on it's third revision already and i think i have it dialed in pretty good.Can't wait to get my battery analyzer up and running to test all the 18650 cells i have and assemble a nice big lithium pack.

author: vishaldotgupta
author_rank: Regular Contributor
timestamp: December 16, 2023, 06:32:51 am
content: hi, i might be late on this forum. Me too is working on home made welder using big lead acid battery as current source. my design as follows1. i will be using a miniature relay instead of gate driver. The timing pulse is from a low cost programmable timer from aliexpress2. Gate drive voltage will be sourced from 12V, but there will be diode and a capacitor. during the welding process the 12V will drop and may drive mosfet in linear region thus blowing them.3. just after welding pulse the relay will connect gate to ground thus discharging the gate capacitance asap (through a 10ohm resistance).5. i intend to use small filament bulb as snubber by placing it across mosfet drain and source. it has a very low cold resistance and should absorb the spike6. i also intend to use a led across welding probe. The advantage is, that in case a mosfet is shorted (from previous weld attempt), the led will glow up. so you can prevent an accident before actually welding is attempted with shorted mosfet.7. i also suggest adding MCB / delay type fuse in the battery circuit as a protection mode.i would like to have comments on this thought

author: DavidAlfa
author_rank: Super Contributor
timestamp: December 16, 2023, 07:02:39 am
content: 350A MOSFET, but crappy driver, you need a very fast driver or it'll just blow like that.Also a 3KW TVS would be help with inductive spikes.

author: Zero999
author_rank: Super Contributor
timestamp: December 16, 2023, 10:04:22 pm
content: The spot welder I use at work uses a large capacitor bank and a boost converter to charge it. The capacitor is discharged via a huge SCR, not a MOSFET. A complex arrangement involving floating the ground of the controller is used to trigger the SCR, which I think is to enable the battery 0V to be on the same ground as the clamp on the workpiece. It would have been much easier to have used a pulse transformer to fire the SCR.

author: shapirus
author_rank: Super Contributor
timestamp: December 16, 2023, 10:30:35 pm
content: Quote from: Refrigerator on May 31, 2021, 05:04:30 pmBetter late than never!It's "ТЧ50", quite searchable. High frequency thyristor, up to 25KHz, max current is 50A. Max 700V repeated reverse impulse voltage, and the rest of the marking isn't visible.

author: Refrigerator
author_rank: Super Contributor
timestamp: April 22, 2024, 03:35:14 pm
content: Quote from: Zero999 on December 16, 2023, 10:04:22 pmSCR's are easy to drive, because as long as you can provide a suitable trigger, the SCR will self sustain on a closed state. I agree, a transformer could do that quite easily.Quote from: vishaldotgupta on December 16, 2023, 06:32:51 am1> Won't work due to contact bounce. After i popped my controller (by connecting the power backwards) i just used an L9110 motor driver IC as a gate driver and it honestly works amazingly well. The rise and fall times are much sharper than with any other gate driver i had on hand and on par with my high current discrete solution.2> Fine, but relays will draw a lot of current so your storage caps will need to be large. Assuming you go for relays regardless of above point.3> Your relay will be too slow for that, the inductive spike will take nanoseconds to knock a hole in your FET's4> 404 point not found5> Well i mean some 600A is a walk in the park for a bulb to handle right? On a more serious note, it won't work. You need a very stiff snubber to not allow the voltage to rise above what your FET's can handle. My solution was to use the FET's themselves as snubbers through a diode arrangement, where the inductive spike would power the gate just enough for the FET's to keep it below VDSmax. It's a simple and cheap solution that i've found works well and has kept my FET's from blowing ever since i got my welder working.6> Sure, that sounds ok. Personally i find the sparks and burned holes give enough of an indication that my FET's are gone.7> The fuse will limit your max current by alot. Also most things become fuses at spot welding currents, including the thing you're welding.

author: costi_b
author_rank: Newbie
timestamp: April 22, 2024, 06:25:43 pm
content: I used 9 pieces P140LF4QL-5071, HousingLF (similar MO235B), 140A 40V, driven by a TC4422, 4R7 in each gate, mounted on an aluminum PCB. I have recovery diodes between drain and +.As a power source I use a 12V 100Ah gel battery connected by 16mm2 cables in parallel with a capacitor pack of approximately 0.6F.The soldering cables are 6 mm2, flexible.I've done several hundred welds in this setup, the only thing that gets hot are the welding leads. I use about 30ms welding time.

author: Refrigerator
author_rank: Super Contributor
timestamp: May 09, 2024, 11:37:38 am
content: Quote from: costi_b on April 22, 2024, 06:25:43 pmYeah i noticed 6mm2get a bit hot if you're welding fast enough.