Spot Welder Design Advice

Thread Starter

EDANN

Joined Oct 24, 2016
24
Hey folks,
I built a homemade spot welder using some parts i had around. The design intent was to use a push button to turn on a mosfet, and then quickly use two electrodes to spot weld a nickel strip. Unfortunately, my circuits is pretty rusty. I have built the circuit and attached schematic picture.

I am using:
Pushbutton SPST, 12 V 10 A rated: https://www.princessauto.com/en/detail/12v-10a-push-button-starter-switch/A-p8802654e
Mosfet as the power switch: https://cdn.sparkfun.com/datasheets/Components/General/FQP30N06L.pd
12V car battery

For the first test, the mosfet exploded and it seemed like the push button was staying on. Is this the wrong kind of push button to use, is my mosfet not rated/wired properly.

Any advice folks?

Dan
 

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MaxHeadRoom

Joined Jul 18, 2013
21,398
Hopefully you are also applying high pressure to the the electrodes and the strip before energizing!
A 12v automotive battery can deliver over 100amps in this situation!
It is also too high a voltage.
Max.
 

jpanhalt

Joined Jan 18, 2008
10,218
With a dead short at 12 V the max. RDS(on) is 35 mΩ. That's 34 A, which exceeds the specification for the device. It is likely the current is more, particularly if you had the source attached to the load. and not ground. Then it is mostly the forward biased diode.

No surprise it "blew."
 

scorbin1

Joined Dec 24, 2019
37
Most industrial welders use a water cooled SCR. The amperage these things pass is HUGE. Somewhere on the order of hundreds of amps or more at higher voltages before it is passed to a transformer to step the voltage down to between 10-30V also effectively stepping up the current. Now these are AC welders I'm referring to, but an SCR could be used to control this DC circuit as well.
 

jpanhalt

Joined Jan 18, 2008
10,218
For spot welding lithium batteries, you will probably find that timing with a push button is not consistent enough. I use capacitor discharge, adjustable voltage, and an SCR. It is sufficiently consistent for my needs. If I were to build another, I would probably include turn-off capability or better control of the current.
 

Janis59

Joined Aug 21, 2017
1,221
FIRSTLY, no-one is doing like that, push on and push off. Once I got such task to remake a large industrial spotwelder used russian genuine microcircuits (biggest on Planet) in size of half-brick each, the whole microcircuit cemetery had no any source for replacement thus they want to destroy all and create the "normal" system of driving it. I made it on basis of 555. Worked excellent.
Secondly, if timing will be hand-dependent, every sewed point will be different and most of them will be brake. The repeatedness will be catastrophic.
Thirdly, there are no need to use any igbt or Mosfet if the task is perfectly to the mighty of elementary triac. However use of half-period driving is necessity. Thus the triac must be driven via optron with half period sync functionality what stays after 555. Such circuit diagrams are plethora at internet.

Alternative for case of small spotwelders like for batteries is capacitor laden up to the chosen voltage (and capacitor battery capacitance may be switchable discrete, and then thyristor what lead it to MOT. Yet my experience with MOT says the two MOTs are better than one and three are better than two :)
So, now the last, about amperage. If industrial SW uses 10 000 to 100 000 Amps, then batteries rarely uses much over 1000-2000 Amps. Then putting SCR in secondary circ You are deep in the sh**, therefore it MUST stay in primary side. Then one MOT version You have 4 V out 220V in or k=220/4=55x what gives 1500A/55x=30 A in primary. So, the triac must be bulky and well cooled. Say the pulse will be 0,5 sec long every each 5 seconds new and new. Then radiator load will be 1,7V*30A*0.5 sec=25 Joules per cycle or 25W/5sec=5W. Such demands a minimum (at +40 C over air T with natural convection) 26 cm2 per W= 26*5= 125 cm2 area what means simple 6x10 cm alu platelet about 4 mm thick.
 
Last edited:

strantor

Joined Oct 3, 2010
5,329
Interesting video but he's using a 200 amp rated solenoid for switching.
And a 330CCA battery.

my experience with MOT says the two MOTs are better than one and three are better than two :)
So
My experience says 1 MOT is absolutely useless, 2 is mostly useless, and 3 is where you barely begin to see real results. However when i made my spot welder it was to join pieces of sheet steel a bit thicker than a battery tab.

Maybe only 2 two transformers would be enough for this. But please know that the pinterest/"life hack" 1-MOT spot welder 3-minute build videos are totally bogus.
 

Thread Starter

EDANN

Joined Oct 24, 2016
24
Great info folks, starting to learn up on my circuits some more. This project seems alot harder than i wanted :p
 

Janis59

Joined Aug 21, 2017
1,221
Strantor: thanks a lot for idea to use a welding clamp-tool instead of hydraulics. That is wonderful. My understanding the spot welder is thing for producing the boxes for electronics. Because the batteries are easy solderable by ortophosphoric acid. Only the negative side is need to wash the joint afterwards. Yet about the crap into one-core welders - I believe if the task is not the 1/2 mm but 1/12 mm like gillette, hen MAY BE one core is enough.
 
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