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 Post subject: Re: DIY Spot Welder
PostPosted: Tue Feb 08, 2011 8:17 am 
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Joined: Thu Dec 02, 2010 12:25 pm
Posts: 75
Location: Minneapolis
FWIW, the spotwelder I had in my shop used a standard contactor, of the kind that's commonly used for stuff like heanting and air conditioning to switch the transformer primary. The controls and logic were run on 24VAC supplied by something that looked like a doorbell transformer. The timing was all done with electromechanical plug-in delay relays connected in a cascade. They were calibrated in cycles, 60 being one second.

For a low-power setup like this, you could likely switch the primary directly with a foot pedal, but something industrial grade would be preferred, because there can be some big transients when the field collapses in the transformer.

-Jim Hart


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 Post subject: Re: DIY Spot Welder
PostPosted: Mon Feb 14, 2011 1:29 pm 
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Joined: Fri Jan 21, 2011 12:04 pm
Posts: 39
Location: Uptown, MN
I was able to hook up an AC current probe on the primary and secondary side of the windings.
My thoughts are the thing is plenty powerful as a battery terminal spot welder -- probably too powerful. I'd agree - a footswitch on the primary side would be nice but should probably be pretty beefy to handle the EMF.
Good suggestions on the controls. I'm still not 100% on what to do -- I'd likely want to be able to dial down the output.

Here are some stats:

Primary coil (120VAC) Idle draw: 5.5 Amps @ 120VAC (0.707) = ~466 watts
Primary coil (120VAC) Welding draw: 15.4 Amps @ 120VAC (0.707) = ~1300 watts

Secondary coil (Weld side) Idle draw: 0 Amps (I would hope so!)
Secondary coil (Weld side) Welding draw: 525 Amps

I did not get a voltage read on the secondary coil.

A co-worker also got in on some of this action and had used slightly different copper contacts/materials and measured:
Primary coil (120VAC) Idle draw: 6.6 Amps @ 120VAC (0.707) = ~560 watts
Primary coil (120VAC) Welding draw: 14.5 Amps @ 120VAC (0.707) = ~1230 watts

Secondary coil (Weld side) Idle draw: 0 Amps (I would hope so!)
Secondary coil (Weld side) Welding draw: 490 Amps

Here's a photo of what was being spot welded:


Attachments:
File comment: Spot welds current test for v0.1
spot_welds_0_1.jpg
spot_welds_0_1.jpg [ 50.8 KiB | Viewed 1081 times ]

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 Post subject: Re: DIY Spot Welder
PostPosted: Mon Feb 14, 2011 2:56 pm 
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Joined: Thu Dec 02, 2010 12:25 pm
Posts: 75
Location: Minneapolis
I wonder if you could add a lower power tap to the secondary. Commercial units usually have about 5 different taps to roughly set the heat, then timers to fine tune it. You might just want to rip a few windings off the secondary.

You'll also get much nicer looking welds by holding the pressure on the electrodes until the metal cools enough to where it doesn't instantly oxidize when exposed to air. Then you'll get those nice shiny professional looking weld centers. An added benefit is the prevention of oxidation of the copper electrodes. If oxidation builds rapidly on the tips, then even with consistent weld timing the heat input to the weld will change correspondingly. You get more heat where you don't want (at the tip/workpiece interface), and less heat where you really want it (at the contact point between the 2 sheets being welded).

I found all this stuff out the hard way!

-Jim Hart


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