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Flyback transformer cores

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LAN_403

diy

Tue Dec 20 2011, 11:58AM

Registered Member #4277 Joined: Tue Dec 20 2011, 11:41AM
Location:
Posts: 4

Hello everyone!

I read the forum from a quite a long time, but never thought of registering. I am interested in high-voltage and recently got a few CRT monitors. I took the flyback transformer of one of them out. Tried to take out the core, since I don`t need the rectifing part. Sadly, the core broke in 4 big and a few smaller pieces. I am pretty sure i can glue it, but I have a few questions...

Do I put the glue between the pieces or around the crack?
Does it have to be perfectly restored?
How can i get the core out, whithout cracking it so much?
What wire (gauge) should I use for the secondary and the primary?
If I drive it with a ZVS can I directly feed a Cockcroftâ€“Walton multiplier?

Thanks!

Sorry for bad English, it`s not my native language.

Inducktion

Tue Dec 20 2011, 03:26PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

1. Personally, I usually try to put super glue between the cracks, so that they have a nice solid connection.

2. The core does NOT have to be perfectly restored. However, it's usually best to get most of the ferrite back into the core.

3. To be honest, there's not too much a trick to getting the core out correctly; I've heard of people boiling them in water for a little while to loosen the epoxy, but I've never tried it personally.

4. For the secondary, anywhere from 28 gauge to 32 gauge wire ought to work. With thinner gauge, you can shove more turns on the core, at the expense of less current. For the primary, 14 to 18 gauge wire would work.

5. Because a ZVS driver outputs true AC (a sine wave) you can directly feed it into a CW multiplier.

And your english is quite fine good sir. :P

Turkey9

Tue Dec 20 2011, 09:23PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

The best way I've found to get a core out without cracking it is to keep looking until you find one that will just slide out. Some transformers have the cores glued in with a sort of epoxy and others don't. If you take apart enough monitors you'll find one that comes out in perfect condition.

diy

Wed Dec 21 2011, 03:13PM

Registered Member #4277 Joined: Tue Dec 20 2011, 11:41AM
Location:
Posts: 4

Thank you for answering me! I have a few other questions...

Do, and if "yes" - how, diffrent type capacitors affect the Cockcroftâ€“Walton multiplier? Will there be any diffrence if I use a Polyester (PET) Capacitor or a ordinary Ceramic one?

Are there any ways to make the ZVS better? I think - IRFP250/260 can be changed with IRFP740 and you can use higher voltages. What else can be done?

Forty

Wed Dec 21 2011, 06:43PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

Soaking the transformer in acetone for a few hours or over night has been pretty successful for me in getting the cores out undamaged.
using litz wire and/or putting the primary inside the secondary improve the output power a bit.
I've heard the zvs can be a bit finicky about what type of mosfet you use. I've had success with other types though. just make sure it can handle plenty of current, voltage spikes about 4x the input voltage, have a low Rds on rating, and be fully avalanche rated.
also, when running at higher voltages, it is better to separate the supplies to the gate and primary sections. power the gate side with the usual 12v (ie through the ~470 ohm resistors) and power the rest with the higher voltage (ie through the inductor)

diy

Sun Dec 25 2011, 05:12PM

Registered Member #4277 Joined: Tue Dec 20 2011, 11:41AM
Location:
Posts: 4

Are there any special requirements for winding the secondry?

I will probably use the wire from a inductor that I got from the CRT.

Merry Christmas!

Platinum

Sun Dec 25 2011, 06:27PM

Registered Member #3926 Joined: Fri Jun 03 2011, 08:32PM
Location: UK.
Posts: 525

When winding the secondary just make sure you have good insulation every set of turns.

diy

Sun Dec 25 2011, 06:44PM

Registered Member #4277 Joined: Tue Dec 20 2011, 11:41AM
Location:
Posts: 4

By "set" you mean layer?

Inducktion

Sun Dec 25 2011, 09:51PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Yes. I used a combination of Teflon tape and electrical tape to wind my own secondary. It worked pretty well.

The Lightning Stalker

Sun Dec 25 2011, 10:42PM

Registered Member #4230 Joined: Sat Nov 26 2011, 05:50AM
Location: Socketville
Posts: 53

Due to impatience and perfectionism, I have developed my own special technique for removing transformer cores without damage. Make sure you have a couple pairs of adjustable pliers at the ready. They have to be adjusted right for the size core you're extracting.

Take a piece of sheet metal such as the bottom of a coffee can and put it over a burner on your stovetop. Do not let the flame hit the ferrite directly, or it will shatter.

Put the heat on just a bit higher than the lowest setting.

Find the spot where the heat hits the metal and put the core directly on it. You want to heat slowly and as evenly as possible.

When the core half reaches 400-900ÂºF the epoxy will begin to soften. You can test for this by scratching at it with a knife or screwdriver.

When the epoxy is soft enough, grab the transformer with 2 pairs of pliers and gently but firmly pull out the hot core half. You'll break a few until you get the feel of it.

Flip the coil over and begin heating the other core half, then extract it with the same procedure.

If it won't come out, put it back on the heat for a while and try again. How hot it has to be depends on a lot of things such as the type of epoxy or eurethane, how much of it there is, how deep it goes into the form, how tight the core fits in the form, etc.

If you heat it too slowly and for too long, the form and windings will begin to heat up with the core and you'll end up with a melted form, cooked magnet wire, or both.

When you get good, most times you can remove the core with no damage to the form or windings. Then it's easy to remove the windings and reuse the form.

A lot of times one or both core halves will be recessed and/or pins will be in the way. In this case, you need to use an alternate heating method. You can put a small block of aluminum in there to bridge the gap. You can also heat it with a high wattage resistor that will fit in there or a soldering iron. You can also heat it with a pencil torch, but you have to keep the flame moving and you risk melting the form or catching it on fire.

For a while I've been wondering about inductive heating methods. It would require high power at RF frequencies, especially for ferrite. Of course this wouldn't work on coils with shorted windings.

The heat source doesn't matter as long as it mostly heats the core and not the form and windings slowly and evenly enough to keep it from shattering.

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