Author Topic: MOT battery spot welder help  (Read 2203 times)

Offline shipto

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MOT battery spot welder help
« on: April 04, 2019, 04:27:32 PM »
I am trying without any success to make a battery spot welder using a microwave oven transformer and wondered if anyone had any input.
I built a standard spot welder before and it worked reasonably well so thought this would be easy.
I have it controlled by an Arduino Uno supplying output to a SSR and even though it kinda works and gets the strip hot it is not welding the strip to the batteries which I suspect is the power not going deep enough and staying in the strip.
I have tried with more windings to increase the voltage but still no joy.

Another thing I have wondered about is the what I thought was a thermal switch but maybe not, should this be removed or kept in place?
Turns out this life c**p is just one big distraction from death but a good one. For the love of god dont give yourself time to think.
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Offline S. Heslop

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Re: MOT battery spot welder help
« Reply #1 on: April 04, 2019, 10:41:15 PM »
I didn't have any success trying a similar thing. I burned a hole in a couple lipos before deciding i'm just going to cause a fire. My assumption, and I could definitely be wrong, is that this style of spot welder goes too slow compared to the capacitive discharge type - and lets the heat spread out alot before the weld is made. Which makes it too easy to burn a hole into the battery casing.

Offline shipto

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Re: MOT battery spot welder help
« Reply #2 on: April 05, 2019, 02:25:44 AM »
Yes I am thinking your right even though youtube has many who claim success.  :doh:
Turns out this life c**p is just one big distraction from death but a good one. For the love of god dont give yourself time to think.
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Offline ddmckee54

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Re: MOT battery spot welder help
« Reply #3 on: April 05, 2019, 06:06:09 PM »
How many turns did you have on your secondary side, and what size wire was it?  The less turns, the lower the voltage, but the higher the current you will get out of the winding.  Assuming that the transformer is 100% efficient, it isn't but assuming this will make it easy, then the VA (volts*amps) going into the primary side will equal the VA available on the secondary side.

When welding, you don't so much care about the voltage as you do the current.  It's the current that will do the welding, and the current will also determine how fast you can weld.  The faster you can weld, the less heat build up you will get.   The videos I've seen only use a few turns at most on the secondary side and those turns are done with some really BIG wire.

Don
Too many irons, not enough fire.

Offline Will_D

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Re: MOT battery spot welder help
« Reply #4 on: April 06, 2019, 05:37:19 AM »
I have 2 turns of 80mm2 It will just about spot weld 0.9 mm stainless. That is just manually squeezing the tips (I use big Mig welding tips).

Two improvements I think may be:

1. Lever system like on the pro jobs to squeeeze the metal more

2. I assume the pro jobs use a capacitor bank and a contactor and timer to up the weld current

Can someone confirm the capacitor theorey and recomend how many Farads would be needed

Another solution is to use more than one transformer connected in parallel - but get the phasing correct!!
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Offline awemawson

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Re: MOT battery spot welder help
« Reply #5 on: April 06, 2019, 05:46:50 AM »
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
Andrew Mawson
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Offline Will_D

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Re: MOT battery spot welder help
« Reply #6 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

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Offline shipto

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Re: MOT battery spot welder help
« Reply #7 on: April 06, 2019, 06:40:04 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
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.
Turns out this life c**p is just one big distraction from death but a good one. For the love of god dont give yourself time to think.
https://myshedblog.wordpress.com/

Offline Pete W.

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Re: MOT battery spot welder help
« Reply #8 on: April 07, 2019, 08:46:06 AM »
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


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. 
Best regards,

Pete W.

If you can keep your head when all about you are losing theirs, you haven't seen the latest design change-note!

Offline Will_D

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Re: MOT battery spot welder help
« Reply #9 on: April 07, 2019, 09:06:30 AM »
Thanks Pete, I think experiments best carried out outdoors!!
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