Gallery, Projects and General > How do I??

MOT battery spot welder help

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awemawson:
On a somewhat larger scale my stud welder does the same thing. Description in the thread linked below:

https://madmodder.net/index.php/topic,10358.0.html

Will_D:
Thanks Andrew for some brain stimuli:

Energy stored in a capacitor is 1/2 C V^2 (C in Farads , V in volts)

So pro spot welders use higher voltages and a capaciror bank to get the bang into the spot weld.

So as I have a very BIG 2F cap rated at 400v I could expect:

Charge at 5V = 25 Joules
10V   = 100J
100V = 10KJ

shipto:

--- Quote from: Will_D on April 06, 2019, 05:03:46 PM ---Thanks Andrew for some brain stimuli:

Energy stored in a capacitor is 1/2 C V^2 (C in Farads , V in volts)

So pro spot welders use higher voltages and a capaciror bank to get the bang into the spot weld.

So as I have a very BIG 2F cap rated at 400v I could expect:

Charge at 5V = 25 Joules
10V   = 100J
100V = 10KJ

--- End quote ---
Good to know and explains why my experiments today with some big 20000uf caps charging at 28v was blowing holes in the strip.
This battery is not for me and to be honest its doing my head so I am going to solder the strips on, tried it today with the largest tip I own and it worked pretty well but I am definatly going to do more experiments with the cap welder.

Pete W.:

--- Quote from: Will_D on April 06, 2019, 05:03:46 PM ---Thanks Andrew for some brain stimuli:

Energy stored in a capacitor is 1/2 C V^2 (C in Farads , V in volts)

So pro spot welders use higher voltages and a capaciror bank to get the bang into the spot weld.

So as I have a very BIG 2F cap rated at 400v I could expect:

Charge at 5V = 25 Joules
10V   = 100J
100V = 10KJ

--- End quote ---


Do bear in mind that the charge is not the whole story.

If you want to get all that charge out of the capacitor in a hurry, it needs to be designed and built accordingly.  I suggest that there are two (at least two) significant parameters:

1.)  The terminations, that is the internal connections between the actual storage metal/dielectric/metal sandwich and the capacitor terminals, need to be able to pass the high discharge current without fusing.

2.)  Those terminations need to present a low series resistance or else the CR time constant will be too big and will limit the available peak current. 

Will_D:
Thanks Pete, I think experiments best carried out outdoors!!

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