Author Topic: adventures in homemade flyback bobbins.  (Read 1097 times)

Offline oneKone

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adventures in homemade flyback bobbins.
« on: May 24, 2018, 12:15:36 PM »
Having no option but to put my drsstc build on hold i still wanted to work on a high voltage project, but remain cheap at the same time. I've had a few flyback cores sitting for a long time so i decided to make use of one of them. i was looking online when i saw the internals of a flyback (how it was wound) so i thought i'd give it a go.

for a driver i'm using mads circuit except rather than using max4420 i'm using a ucc37325 and 2 pairs of irf9540/540 to handle the switching. The igbt of choice is what i had laying around..... a cm300 

 the OD of the bobbin is 40mm with a winding depth of 7.5mm, this should give 225 turns per section but it was wound by spinning a drill and getting the wire to go from left to right within the section. So as a pure guess i'm going to say the amount of turns would be in the 1500 range. The first bobbin i tried didn't have enough of a wall between the core and the windings as well as i didn't give the oil long enough time to penetrate, this resulted in a very short life. The next bobbin wall thickness was increased by 2.5mm and the oil was given 8hrs to penetrate. this is the current bobbin i am running now. The oil i'm using is synthetic engine oil.

I think the next step would be to get a cheap vacuum pump and epoxy the next bobbin rather than oil. 

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Offline Mads Barnkob

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Re: adventures in homemade flyback bobbins.
« Reply #1 on: May 25, 2018, 08:28:24 PM »
That is one heck of a killer-inverter for a small high voltage transformer!

Is the bobbin 3D printed, kind of looks like it. If it is, isn't a part of the failure then that the material is not solid, but full of holes and air inside?

If you want to whole-cast a transformer in epoxy, be sure to mix it quite thin so that it can get in-between everything a little easier.
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Offline the_anomaly

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Re: adventures in homemade flyback bobbins.
« Reply #2 on: May 25, 2018, 09:42:01 PM »
Nice! I wound a few of my own as well but chose a different approach for bobbin:





I started with a piece of PVC pipe.  I wound a layer ~180turns of 26awg wire and then wrapped an insulating sheet around the layer before winding the next layer.  Each layer added a larger radius.  IIRC I managed ~12 layers before calling it done. 

Wish I had pics of them running.  I made two, one produced something around 14KV which I fed into a Cockcroft–Walton multiplier and the other about 20KV with no multiplier.

I put them under mineral oil and used a vacuum pump to remove the air.  10 minutes under 500 microns and I was able to run them for hours with no problems.

If you decide to epoxy pot, you might need to include ultrasound to help shake the bubbles out.  I like https://www.epoxies.com/products/potting-encapsulating-and-casting/ for potting compounds.  Choose as low a cP as possible; above a certain point, ultrasound is required in addition to a vacuum.


Offline oneKone

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Re: adventures in homemade flyback bobbins.
« Reply #3 on: May 26, 2018, 05:57:23 AM »
@Mads,
haha yeah... its extremely overkill, i should have some smaller igbts waiting for me at work. The bobbin is 3d printed but the infill was 100% so it's "solid". When i make my newer ones i'll machine the bobbin from acetal or teflon if i have some.

@The_anomaly,
That secondary is clean! did you wind left to right, insulate then wind right to left? Also thank you for the information about potting.

Offline the_anomaly

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Re: adventures in homemade flyback bobbins.
« Reply #4 on: May 26, 2018, 03:03:30 PM »
Yes I did.  I kept the winding going as I wrapped a layer of insulation and when the previous winding was completely covered by a small margin, I continued winding in the other direction. 

Once you get a vacuum pump and desiccator jar, you will be impressed how much air it removes!

Offline oneKone

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Re: adventures in homemade flyback bobbins.
« Reply #5 on: June 10, 2018, 04:18:20 AM »
after having luck with me second bobbin i thought i'd wind a smaller one using 0.14mm wire. It had 6 segments with a rough estimation of 2500 turns. After leaving it in warm oil overnight and gradually running hotter arcs through it (my choice of making sure any trapped air bubbles will rise to the surface) as a bed in process all seemed fine. I then moved the 2 electrodes further apart and that's when failure happened. On the third segment it arced over... My best guess is that using such thin wire with many turns had too high resistance, or that i was simply generating too high voltage per segment.


My second attempt i used 0.3mm wire with an approximate of 1200 turns. Having the same bed in process led to a working core.



at the moment i'm feeding  45v dc into the half bridge which then is connected to an isolation transformer with 5 turns primary side and 10 turns secondary side on a flyback transformer. this ups the voltage to about 60v (i think it may have been higher but can't remember what the voltage was under full load). I have also made a new driver that's still based on mads flyback driver except has 2 irf9540/540 output stage, the reasoning for this is that i had the parts laying around and i can driver pretty much whatever brick i want.


Knowing this method of winding flybacks actually works i'm going to order a cheap vacuum pump and try potting in some cheap resin and paraffin wax. If all fails with using cheap mediums i'll continue submerging the bobbins in oil and use the pump to accelerate the process. 
« Last Edit: June 10, 2018, 04:22:15 AM by oneKone »

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Re: adventures in homemade flyback bobbins.
« Reply #5 on: June 10, 2018, 04:18:20 AM »

 


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