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A few questions on homemade flybacks

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LAN_403

Platinum

Sun Dec 02 2018, 06:58PM

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

Good evening 4hv'ers I have a few questions regarding the making of a homemade flyback transformer that is going to be used to drive a tesla coil. I have a surplus of 28awg magnet wire (about 750grams) and was wondering if I could use this wire to wind a flyback tranformer which will be driven using a full bridge off rectified mains to power a SGTC, is this wire suitable for the secondary of a flyback? What should I use as interwinding insulation?

Also could this wire be used for the secondary of a SGTC with a power rating of about 1-1.5KW

The core I'm using for the transformer will be 60x55x15, 2 of them to form a pair, this core should handle 1.5KW easily? Should I add an air gap?

One last question, to rectify the flyback should it be half wave? As is one stringnofndiodes on one leg of the output?

I'm hoping for about 10KV for use in a tesla coil so will aim for about 500 turns, switching frequency is 30khz

Thanks

Flachzange

Sun Dec 02 2018, 07:52PM

Registered Member #61569 Joined: Sat Apr 15 2017, 05:12PM
Location: Germany
Posts: 28

The thickness of wire you have should be very good.
Even thinner also works great for these power levels.
However you should not wind it in a layer-wise winding. Several disc windings are a lot better. Like these:

As an insulation a material with low dielectric losses should be choosen (tan delta).
I have build several high frequency transformers using PTFE as an insulation and it works great.
If you are using it as an transformer no airgap should be used. If it really is a Flyback then you should include an air gap.
The transformers i have build are rated for 50kV output at 10 kHz and therefore the insulation is the biggest problem.
I have submersed the entire transformer in shell diala S4, otherwise Corona will destroy everything within seconds.
If you are using rectangular ferrite cores then you should consider to round the corners to reduce the electrical field.

If you are interested then i can post some pictures from my transformers.

Platinum

Sun Dec 02 2018, 08:20PM

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

Hi Flachzange, thanks for the helpful reply. I have plenty of PTFE which is a bonus. I will not use an air gap as it is not running in flyback mode. I will have to construct a bobbin to wind the secondary coil to use a disc secondary layout as opposed to layer windings. If possible could you post some pictures of your transformers?

Do you think I could use 28awg for a tesla coil? As I have found many conflicting wire sizes to coil output powers, I have read 0.5mm for 4 inch diameter and also 0.28 for 4 inch.

Thanks

Patrick

Mon Dec 03 2018, 06:00AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

watch out for inductive heating of plastic, but yes the pancake coiled method is a good place to start. if you have to many blow ups, use triple or quadruple varnish insulation and FEA of the field distribution.

Platinum

Mon Dec 03 2018, 12:58PM

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

Any advice on the other questions asked?

Also would a static gap be better than a rotary gap? which would be constructed from a variable speed angle grinder I plan to use 5/32s 2% thoriated tungsten I already have on hand.

Thanks

johnf

Mon Dec 03 2018, 06:02PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Flybacks are limited to about 3-400watts max after that snubbing of the drive device becomes extremely tricky.
so as suggested use transformer action half or full bridge and if you want an easier path use a current mode controller

Patrick

Mon Dec 03 2018, 06:29PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

yes. i have advocated for and second his comment on the use of current mode controllers and regulators on many occasions. They also allow for parallel current sharing so you can use as many flybacks as you want.

Platinum

Mon Dec 03 2018, 09:19PM

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

My apologies for calling it a flyback, it is basically just a ManganeseZinc U core which will form a pair, dimensional sizes will be 120 x 55 x 15 (pair) I plan to wind the transformer with 28awg and run off a full bridge, this will not be a true flyback which I hope will hold up to power levels of >1KW as I have seen small LOPT cores running this sort of power easily.

Blackcurrant

Tue Dec 04 2018, 02:15PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

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