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 Post subject: Successful Transformer Rewind ... Now I'm Worried !
PostPosted: Dec Tue 05, 2017 9:58 am 
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Joined: Jul Sun 17, 2011 1:11 am
Posts: 4328
Location: Los Angeles
I've got a tube project in the queue, that needs substantial amounts of filament current, at several voltages. 3 decent sized thyratrons, and the supporting vacuum tubes. Instead of buying a new transformer, or more likely transformers, I decided to try rewinding a large piece of iron, with relatively useless secondary voltages. Here are some photos, of the progression:

Xfmr_rewind_01s.jpg [ 127.63 KiB | Viewed 543 times ]

During (note, few turns, for a 6V winding):
Xfmr_rewind_02s.jpg [ 155.73 KiB | Viewed 543 times ]

Xfmr_rewind_03s.jpg [ 151.66 KiB | Viewed 543 times ]

It has two 5V windings, a 2.5V winding, and a 6.3V winding. All of them can output a significant amount of current, with the voltage dropping less than 10%. And that's with all secondaries attached to dummy loads drawing the intended current loads of the project. About 80W total. The transformer behaves well, and barely gets warm, even after running for some time. Dummy loads, on the other hand, aren't looking that good, but still in spec. 8)

It went too smoothly. Last time I rewound a transformer, was adding a new secondary to a microwave oven transformer. It was suppose to be an isolation transformer, and it worked pretty good, until I accidentally shorted the new secondary, and it promptly opened up. :(

On the plus side, filament windings don't require many turns. This new one, only needs ~3.5 turns per volt. That's 9 turns for the 2.5V winding, and 22 turns for 6.3V. That's at load. I've got a few more large pieces of iron that may also end up being converted. If for anything, just to thin out my junk collection.

Life is like a roll of toilet paper - The closer you get the end, the faster it goes - A.E. Neuman

 Post subject: Re: Successful Transformer Rewind ... Now I'm Worried !
PostPosted: Dec Tue 05, 2017 3:07 pm 

Joined: Jan Mon 28, 2013 9:35 pm
Posts: 946
Location: Santa Rosa, CA
Than ks for sharing this project with us. Might be useful
in the future. Transformers with the right outputs are getting
harder to find for a reasonable price.

 Post subject: Re: Successful Transformer Rewind ... Now I'm Worried !
PostPosted: Dec Wed 06, 2017 3:41 pm 

Joined: Jan Mon 28, 2008 3:19 pm
Posts: 1297
Location: Wilton,IA
Restacking the Is & Es is the biggest problem with rewinds.

I've rewound several heavy duty roll around automotive
battery chargers, but they had welded laminations.
Grind off the welds, & the windings are easily taken off
of the "E" section.

What do others do for restacking laminations on smaller

 Post subject: Re: Successful Transformer Rewind ... Now I'm Worried !
PostPosted: Dec Wed 06, 2017 3:52 pm 

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1592
Location: Lafayette, CO
Thanks for putting this up. I have re-wound fried B+ windings and just put the old heater windings back. I had the notion of changing filament windings but no point to it then. If you have patience to wind in the first place, the laminations become easy. Craig

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