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Secondary: segmented/multi flanged bobbin

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baleworker

Wed Jul 23 2008, 09:38PM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

Hello everyone!

So I decided to make to an EHT secondary myself.
I finally found pictures of dissected potted flyback as well as of a modern ignition coils and it looks like their bobbins consist of multiple sections, separated by donut shaped walls. So the whole coil is like a bunch of smaller coils connected in series.

Here are some question I might want to know the answers to:

1. Have anyone here designed the similar coil? what is the best way of pulling the wire from one section to another?

2. How to compute a required thickness of insulation? i.e. what thickness dielectric should I use to insulate secondary from the core, primary, etc

3. The coils I saw had fatter and deeper sections in the middle. why? Does it give any benefits to have less turns closer to the sides?

4. Is it any different if I put primary under the secondary or on the side?

Thanks a lot!

Mates

Wed Jul 23 2008, 11:25PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

In fly-back, those windings are separated by diodes...It means those are really many coils connected in series

Dr. Dark Current

Thu Jul 24 2008, 09:53AM

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

If you make the multi-segment bobbin, you MUST put HV high speed rectifying diodes between the individual segments (and there should be more than 4 segments), otherwise the AC corona will eat the insulation between last (highest voltage) sement and core.

If you don't want to use the diodes, you can wind the secondary in "stacking layers", each layer separated by a sheet of insulation material. This is how the older AC flyback transformers are made.

-J.M.

EDIT: "4. Is it any different if I put primary under the secondary or on the side? "
Yes it is. With the primary under secondary, you'll get very low leakage inductance which means more external inductance will be needed for ballasting. If you put it on the other side, leakage will be quite high and less external inductance will be needed. However it is possible the leakage will be so high that the required output current won't be reached, in this case you'll need to put the primary closer to the secondary (physically closer on the core).

baleworker

Thu Jul 24 2008, 10:53PM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

I will pot it with epoxy under vacuum. Will corona be still an issue?

I was planning on full bridge rectifier. If I use diodes, how the current/voltage waveforms are going to look like on the output?

Btw, I came up with some crazy idea of excentrically machinned grooves, so then i pull the wire from one section to another, they dont have to cross, and starting point of each section never goes close to the end point of the same section, just smoothly translates into the next segment. Should work out very well. Just got the cores and over 1Kv per mil rated CPVC for the bobbin today :)

Does 1kV/mil dielectrical strength means 20 mils is enough to insulate 20kV? That does not sound right !!!

Dr. Dark Current

Fri Jul 25 2008, 08:41AM

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

baleworker wrote ...

Does 1kV/mil dielectrical strength means 20 mils is enough to insulate 20kV? That does not sound right !!!

As i said, at high frequencies many insulators do not behave as insulators anymore. It depends on how lossy your dielectric is and how high is your frequency. I guess for 24kV at 50khz, you will need at least 10mm of actual insulation material between the last secondary section and any other grounded object (core,primary).
The strength you quoted is for DC. If you include the diodes like i said, you will need a lot less insulation material.

baleworker

Wed Jul 30 2008, 01:50AM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

Here are two bobbins I made, I know the sectional one is not wound up to the standards, but what can you do? Its like first 3-4 layers are nice but then it just gets very messy. May be put a layer of mylar film after every other layer?

Anyway, i hope after vacuum potting, each section (3x4 mm copper crosssection) will be able to hold at least up to 1.5-2kV. Will consider connecting sections thru diodes if this design gives any problems.

1217382422 1596 FT50554 Bobbin Sectional

Dr. Dark Current

Wed Jul 30 2008, 07:38AM

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

Oops, looks like you didn't regard my advice, how thick is the bobbin between the last (highest voltage) segment and the core?
I will be really surprised if the coil as you wound it now will stand more than some 10kV without flashover between last segment and core.
Otherwise the bobbins look nice.

Electroholic

Wed Jul 30 2008, 07:55AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

the segment to segment insulation is also quite weak looking.

Marko

Wed Jul 30 2008, 11:39AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Where are you going to put your primary coil? Primary coil should be under the secondary coil if you want to reduce the leakage inductance.

If you are going to put it at the end of secondary or separate leg of the core you might have problems.

baleworker

Wed Jul 30 2008, 01:00PM

Registered Member #1596 Joined: Fri Jul 18 2008, 08:43PM
Location: USA
Posts: 65

There is no primary, it goes right underneath on a separate bobbin. So you have core->bobbin->primary->bobbin->secondary.
At the thinetst point insulation between secondary and primary is 3mm CPVC + 2mm potting compound.

I dont understand how can it arc if there is no air at all? You gotta have air to have corona!
sgments are insulated by 1.5mm CPVC, that should be more than enought for 1.5-2kV difference, I am more worried about arcing inside of the segment itself.

If I put diodes, how the output waveform going to look like? I want to have a full bridge rectifier, but its not gonna be possible if I put diodes between the segments.

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