Even though the seller says that I needed a bigger battery with at least 400A Short-Circuit current - I tried it out yesterday.
I made one weld - now 7 out of 8 Mosfets are dead - only one survived it.
They can switch 250A each - that means the power must have been much much higher than 1Ah * 130C = 130A ?
So - is the high C-Rating not the short-circuit-rating on these cells?
I found nothing about this on the internet, so I’m asking you guys.
C rating is the max sustainable discharge current rating; short circuits shouldn’t be done for long periods of time (a couple of seconds) and therefore are rated higher.
Since I can’t measure Ri - the spot welder can switch up to 1000A - that’s ~10,000W out of this tiny little battery.
That’s needed to blow those FETs. I can’t really believe that that’s possible
The seller/manufacturer sends me replacement-FETs - if they blow up as well, I need a new battery.
So… the seller says you need a bigger battery, you tried anyway and the welder broke, and now you are going to try again? Perhaps you should listen to his advice and get a bigger battery?
That little thing is made to supply 65A, and imagine for how long it can do that when its 1Ah… not very long. Playing around with 18650 is a breeze compared to lipos I’d say, there are a lot of people that have learned the hard way that lipos are dangerous, please be careful.
The Seller uses a 40Ah Car-Battery, but I wanted to try LiPo because he didn’t and I don’t want to have a bigass battery standing around here.
I will keep track on the temperature of that little LiPo, that’s for sure.
The New FETs will be tested to see if anything was wrong with the old ones and if that is what caused the failure - if not, I will buy a different battery.
Take a look at the chip on the board with reference designator “P1”—what is the part number on the chip? Is it a 1410 or 1407?
That is the FET driver chip and it requires 6 Amps at 12 volts from the power supply in order to drive all the fets hard enough to carry the high welding current.
If your pack wasn’t stiff enough to hold the voltage up for the control board chip and the fet driver chip AND supply the current for welding all at the same time, then it may have let the fets switch into the linear region with high internal resistance and power dissipation. That’s why you need a big honking battery for the power supply.
The fets may have come on momentarily, then the drive current was pinched off by the huge load current. The residual charge on the gate kept it open but at a non-saturated level—then they went AMF, adios my fets…
Mmm, kennybobby said it. The problem with that puny LiPo is its voltage crumbles. Current does nothing by itself, fellows. Power is voltage drop times current, and the amount of voltage drop in a given load is proportional to the magnitude of its resistance versus total circuit resistance. I can short out a battery and measure a crazy high amps useless figure, as nearly all of the voltage and power will drop in the battery.
You can do with that little LiPo for powering up the thing, but something waay more powerful is required to provide the welding current. When that tiny LiPo gets the load, voltage crumbles, thus gate voltage crumbles, thus MOSFET RDS skyrockets while the current flows and the MOSFETs gets fried.
You'd need at least 6 of those Turnigy Graphene 65C 1800mAh 3S LiPo to consistently provide the required ≈440A of welding current without too much voltage drop, while keeping the Turnigy Bolt for powering the thing and driving the gates, of course.
Maybe this could be achieved via a combination of 4 of these good LiPos plus a honking super capacitor setup.
This looks to be the same board as the ones that aarone bought direct from OSHPark and has populated. He had a problem with Chineseium FET’s that he hoped to save some cash on which he posted. I doubt that the guy who designed this product tried to use cut rate parts but use the pictures aarone provided to check.
I can understand why you don’t want a honking car battery sitting on your bench in the house, but unless you are planning on doing a LOT of spot welding, can’t you use the battery in your car? A short ‘jumper cable’ type attachment to the car, zap zap a few spot welds and Bob’s your Aunty. On the other hand, if you plan on using the spot welder a lot, it probably DOES make sense to invest in a good car battery that will hold up that you can use at your pack building station. Don’t forget to keep a battery minder on it. The above discussion is almost certainly what killed the FET’s if they were the good ones.
I am aware that car ownership is not as universal around the world as it is here in the States, so perhaps I am out of line assuming you have access to one.
Indeed it could have just been a batch of fake FETs, but just to be clear you had a separate power supply for the top board—what sort of supply was it, and what power rating did it have?
The reason i ask is that Aaron has mailed me one of his spare boards, should be here tomorrow, and i’m trying to get up to speed on what works and what to avoid…thanks.
The FETs are the “legit” ones yay
The Power-supply for the upper board might have been the problem - I used a tiny one that I had laying around <1A.
Next time I will use a bigger one or a 18650 3S pack.
The new FETs should come today and I will repair the welder this Weekend. @Lazy-R-Us: I have a car (and a motorcycle - I am in germany, so I need something fast :D) but it would be a pain in the a*s to go to my car for every weld, because I only live in a flat and not my own house, so the car stands usually in a public parking lot
