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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 11th Jul 2018, 2:40 pm   #1
jonnybear
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Default Battery tag spot welder.

Having need to spot weld some batteries, I could not find anyone that could help, not wanting to purchase one on line for around 150 it would not viable for just a few batteries. I started off constructing one using a car battery and a starter relay, found this a bit Heath Robinson as some times the relay would stick and vaporise the nickel tag not good when using on lithium cells. So I decided to build one using a microwave transformer, after much experimentation I came up with a unit which spot welds perfectly, I started off stripping out an old PC power supply case, and removing the ht secondary winding on the transformer ( worst part) I finished up with this, did not cost much a little time and sweat, cost under 20 in parts.
John
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Old 11th Jul 2018, 3:21 pm   #2
Bobdger
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Default Re: Battery tag spot welder.

Hi John.

Nice unit, please can you post some internal photos and diagrams. Thanks.

Bob
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Old 11th Jul 2018, 5:17 pm   #3
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Default Re: Battery tag spot welder.

Hi Bob
Here are a couple of pictures of the inside, will draw out a circuit and post asap.
don't take any notice of the burnt singed part on the power supply as it is no longer in circuit the panel was removed from redundant emergency light, I just used it as it gave me 12vdc for the control circuit, the only parts I actually purchased was a solid state relay and a timer.
john
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Old 11th Jul 2018, 6:23 pm   #4
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Default Re: Battery tag spot welder.

What is the open circuit voltage on the welding pins?
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Old 11th Jul 2018, 6:26 pm   #5
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Default Re: Battery tag spot welder.

2 volts AC.
john
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Old 11th Jul 2018, 6:27 pm   #6
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Default Re: Battery tag spot welder.

Cheers.
I will remember that if I build one.
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Old 11th Jul 2018, 7:34 pm   #7
high_vacuum_house
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Smile Re: Battery tag spot welder.

Good evening,
That looks like rather a nice unit you have made there.
Do you have a circuit diagram of your device.

What are the details of the secondary winding on the old MOT transformer ?

Christopher Capener
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Old 11th Jul 2018, 8:03 pm   #8
merlinmaxwell
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Default Re: Battery tag spot welder.

How about an “instant heat” solder gun?
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Old 11th Jul 2018, 10:15 pm   #9
Herald1360
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Default Re: Battery tag spot welder.

MOTs are good for about a kilowatt for quite a while, an instant heat gun transformer only runs to about a hundred watts for not very long.....

Does it make a nice DUNNNNNGGGG noise when it's triggered like microwave ovens used to?
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Old 11th Jul 2018, 10:33 pm   #10
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Default Re: Battery tag spot welder.

I built a spot welder to use on battery tags using an old microwave transformer.
I removed the high voltage winding and using 5 mm welding cable wound 2 turns on the transformer, the O/C voltage was 2.6 VAC. The current running though a 3 mm bolt for a short circuit, measured more than 1000 amps on my clamp-on-Ammeter, perhaps not surprising as the Microwave was rated at 1200 Watts. Needless to say the bolt only lasted a few seconds as it became white hot and melted.

Since the experiment appeared to be much too aggressive for a simple spot welder, I abandoned it for the time being, but I have some 2.5 mm welding cable I might try one day
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Old 11th Jul 2018, 11:29 pm   #11
McMurdo
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Default Re: Battery tag spot welder.

I've repaired quite a few commercial spot welders over the years and for repeatable results, they generally use a phase-angle controlled triac on the primary of the transformer and a timer which is based on counting mains cycles. The ones we used to get were programmed with thumbwheel switches and used a postcard-sized CMOS board.

The sequence was Jaws Shut>Preheat Current On, time off. Fusing Current on, time off. Cooling period, release jaws.

Just in case anyone wants to make on with an arduino etc!
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Old 12th Jul 2018, 12:25 am   #12
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Default Re: Battery tag spot welder.

Just a quick follow up, the secondary windings of the microwave transformer will have to be cut out and drilled (pig of a job)removing every winding except the primary one, then I used 10mm2 tri rated cable as it is easier to use, double it up to give 2 turns in parallel = 20mm2 on the bobbin leaving plenty of length to connect to the choc connectors, try to avoid any joints as they can give you a hot spots, I have done a rough thumb nail of the circuit will do a better drawing later if required.
John
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Old 12th Jul 2018, 1:39 am   #13
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Default Re: Battery tag spot welder.

That is very impressive, well done. Very good use of resources and well constructed & finished too.
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Old 12th Jul 2018, 9:04 am   #14
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Default Re: Battery tag spot welder.

Quote:
Originally Posted by MrElectronicman View Post
I built a spot welder to use on battery tags using an old microwave transformer.
I removed the high voltage winding and using 5 mm welding cable wound 2 turns on the transformer, the O/C voltage was 2.6 VAC. The current running though a 3 mm bolt for a short circuit, measured more than 1000 amps on my clamp-on-Ammeter, perhaps not surprising as the Microwave was rated at 1200 Watts. Needless to say the bolt only lasted a few seconds as it became white hot and melted.

Since the experiment appeared to be much too aggressive for a simple spot welder, I abandoned it for the time being, but I have some 2.5 mm welding cable I might try one day
Hi David
I think your transformer with the 5mm welding cable will be fine for spot welding the secret is the short duration of the weld, using a momentary switch and a timer would sort it out, split seconds required not seconds.
John
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Old 12th Jul 2018, 4:33 pm   #15
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Default Re: Battery tag spot welder.

Thanks John, I had considered using a momentary switch, there are several examples around, but a timer maybe even better.

I read somewhere than the best way to weld thin metal was to have a two stage approach each with a short duration, so I may try that as well.
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Old 12th Jul 2018, 6:25 pm   #16
mark_in_manc
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Default Re: Battery tag spot welder.

I really like that - well done! Things made out of scrap really do it for me.

I wonder if I'm up to coming up with something like a flip-flop with variable mark-space to drive such a transformer. I can think of a use for such a device with a hot blade across the prongs, whose temperature is controllable - as well as liking the welder application.
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Old 31st Jul 2018, 2:27 pm   #17
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Default Re: Battery tag spot welder.

Thus guy is seen removing the "I" laminations, the lifting off the entire HT bobbin with relative ease.

https://www.youtube.com/watch?v=DRVlqwsIl6M

<edit>
Actually after watching it properly he safely removes the primary and filament windings and leaves the bit we want to know for another episode.

Looking at one of mine, the weld is quite superficial. More so in real life than this picture suggests, and I think a small grinder might be more suitable than a hacksaw.
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Old 31st Jul 2018, 2:36 pm   #18
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Default Re: Battery tag spot welder.

I’ve see someone hack saw off one side of the HT bobbin and knock the rest out with a hammer. Seems much easier than futzing with the laminations.
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Old 31st Jul 2018, 2:51 pm   #19
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Default Re: Battery tag spot welder.

I guess you're right. Can someone explain what the OP meant by "10mm2 tri rated cable"?
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Old 31st Jul 2018, 3:00 pm   #20
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Default Re: Battery tag spot welder.

This stuff
http://uk.farnell.com/pro-power/pp00...00m/dp/2501431
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