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Winding Transformers on Laminated Iron Cores

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LAN_403

Firefox

Thu May 08 2008, 02:29AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I recently got an old 12V 10A battery charger from a friend (broken, so it was free) and removed the transformer from the setup. I then separated the E-I core, and took the windings off of the core. Now I would like to rewind it into a high voltage transformer, perhaps to provide 12KV @ 5-10mA, or more if it is possible. I thought about reversing the 12V and 120V windings, but thought better of it when I realized that the primaries were designed for 120V not 1200V, not to mention that the layers weren't separated by more than paper.

The core is 3.5" tall and 4.125" wide. The middle of the E is 1.625"x1.375" and the outsides are 1.625"x.625".

J. Aaron Holmes

Thu May 08 2008, 02:50AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Hey, Firefox.

There are a number of good threads on this, both here on 4hv and on TCML. I'd recommend doing some poking around. Perhaps Finn Hammer will chime in, too. He wound a fine-looking HV transformer on a surplus core, albeit one bought with HV transformer winding in mind. Not all cores are created equal; you'll need some good-sized windows in your E-I core to do a proper job of insulating between layers and all that.

Finn's homemade HV transformer:
http://home5.inet.tele.dk/f-hammer/tesla/museum/pig/porkchop.htm

Cheers,
Aaron, N7OE

Dr. Dark Current

Thu May 08 2008, 06:31AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Remove the secondary and leave just your primary. Now wind 10 turns of any wire on it and power on. Measure the voltage on your 10 turns and divide by 10, this will give you how much volts per turn you can use. Then divide your desired voltage by your volt per turn, this will tell you how much turns you will need.
I think here is where you'll stop because you'll get a figure well into tens of thousands turns with your transofmer

Reaching

Thu May 08 2008, 08:42AM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

Yep. Winding transformers is not as easy as you might think. theres a lot of work and calculations behind it.
not every core is suitable, especially these E I cores might lead you into trouble. not to mention to get many tenthousands of turns with heavy isolation on a small window like an E I Core for 120VA..

i know how difficult it is cause i build a high voltage transformer on my own some time ago.

its easy to wind transformers for vacuum tubes on new EI cores with voltages well above 1kV. ive build many small high oltage transformers with voltages up to 4kV. But 12kV is difficult to reach for a hobbyist. insulation will be the first problem, saturating the core is your second. burning the windings by short circuit them to draw an arc is your third problem etc and so on.
after i build several hv transformers and spending a lot of money for suitable materials and cores, i dont think its worth it. buy a small nst with 12kv and youve done well.

Steve Conner

Thu May 08 2008, 09:39AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If you're going to make a HV transformer yourself, you should make it as big as you possibly can. You can use thicker wire for the secondary, and you'll need to wind fewer turns, because a huge core can induce more volts per turn.

GeordieBoy

Thu May 08 2008, 11:58AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Agree with everything Mr Conner said, plus the fact that a larger core will have a bigger winding window to allow space for decent margins and insulation between layers.

-Richie,

Firefox

Thu May 08 2008, 12:32PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

Thanks for the replies. I think I may step the voltage down by a magnitude if the calculations prove 12kV too daunting. Unfortunately, the secondary windings and primary windings were impossible to remove separately. As for winding the many layers, could I put the primary on the small part of the E and then put multiple secondaries on the branches, and then connect the secondaries in series? Would the center being more than twice as thick be a problem doing this? If it would make it easier/a better transformer, I could remove the center of the E and make it a C-I core instead.

flannelhead

Thu May 08 2008, 01:50PM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

At least don't try to run a regular mains transformer in reverse. I did that once with a small 220V/10V transformer. It gave very nice and screamy sparks for a while but then the insulations broke down.

Not to hijack the thread but what causes the high voltage when a mains trafo is run reversed? I ran just 12V on the primary (secondary, remember I was running it reverse.) The voltage was pulsed by a 555 and a MOSFET. Is it just because it's pulsed DC, not AC?

Arcstarter

Thu May 08 2008, 04:13PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

spark wrote ...

At least don't try to run a regular mains transformer in reverse. I did that once with a small 220V/10V transformer. It gave very nice and screamy sparks for a while but then the insulations broke down.

Not to hijack the thread but what causes the high voltage when a mains trafo is run reversed? I ran just 12V on the primary (secondary, remember I was running it reverse.) The voltage was pulsed by a 555 and a MOSFET. Is it just because it's pulsed DC, not AC?

Actually it is because of inductive action(thank you powerlabs!). I don't really understand how it works but it does, and that was all i needed to know!

Back on topic... If you want to get enough wire for a good transformer than go to parawire.com and ask for magnet wire which is double build insulation so it can withsatnd relatively high voltage,(thank you andyman!) and then get yourself a juicy core.

Oh i almost forgot, you can make a powerful flyback driver that can run on mains voltage and wind around 100 turns on a pvc pipe and put it on a flybacks core. Find out what would be the best amount of turns for primary and viola, you have yourself a high voltage transformer. I made one and used a 555 flyback driver with 120volts in and i got around 6kv out at 210 watts. And that was only 60 or so turns!

quicksilver

Fri May 09 2008, 03:11PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I have some information that you may find useful. I've now taken apart 30+ Microwave ovens for the parts (I actually am getting as many as I can haul away). I found that certain brands and years of manufacture have some consistency. For a LARGE transformer with a very large window you are looking for a Sharp made in the mid to early 1990's. These will be very nearly 5" square @ about 1.75" thick & have a window for the secondary that is certainly about 2" square & the quality is outstanding. ...VERY easy to manipulate and work with the windings.

Although there is some variability in the components of some manufacturers, there is some dependability on the quality level before Chinese stuff was the rule. There are some that are seriously not taking apart due to astoundingly poor quality in what few components exist.

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