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Old 8th Apr 2019, 3:06 pm   #1
Station X
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Default Making transformer bobbins?

Does anyone have any hints for making transformer bobbins? I'm talking simple bobbins with a rectangular hole through the middle for the laminations and two side cheeks.

I've seen YouTube videos where the centre part is made by winding SRBP round a former and gluing with what looks like PVA. Not much on the subject of side cheeks though. I assume these would be made of thicker material pushed on to the centre part.

An alternative might be to 3D print the bobbin in two halves and glue them together. I don't have the facilities to do this though and I'd have to farm the job out.

Any advice gratefully received.
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Old 8th Apr 2019, 3:49 pm   #2
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Default Re: Making transformer bobbins?

ERBGF sheet and araldite? Perhaps copper laminated version soldered together with strategic cuts in the copper to avoid shorted turn effects?
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Old 8th Apr 2019, 6:24 pm   #3
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Default Re: Making transformer bobbins?

Plasticard is brilliant for this sort of thing, glue with polystyrene cement or (if your joints are nice and tight) the solvent only "glue".
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Old 8th Apr 2019, 6:55 pm   #4
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Default Re: Making transformer bobbins?

I use old credit cards!

A centre rectangle folded to fit around the laminations leg, with cheeks cut out to fit over the edges of the centre. All held together with superglue.
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Old 8th Apr 2019, 7:25 pm   #5
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Default Re: Making transformer bobbins?

Hi Graham, as described above, but beware of superglue as it can loose its grip under heat and may also affect the enamel.

SRBP would be the material of choice as it is not a thermoplastic

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Old 9th Apr 2019, 11:52 pm   #6
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Default Re: Making transformer bobbins?

Fibre sheet is also good. It's available in a number of thicknesses. It's easy to cut with a sharp craft knife.
If only "thin" sheet is available you may need to stick two pieces together for side cheeks.
It's easy to drill, is excellent insulation and it's cheap. Car shops sell it for making gaskets.
For larger transformers I use circuit board fibreglass that has the copper etched off. I glue it with good quality epoxy.

Joe
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Old 10th Apr 2019, 7:45 am   #7
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Default Re: Making transformer bobbins?

After destroying several painstakeingly made cardboard bobbins (when the coil winder got to the ends, the wire being tight pushed the sides out) I hit on the idea of using copper clad board. I thought great, very strong, soldered together nicely, just the job. One big problem though as some of you will be screaming at the screen, yep, it's one big shorted turn, so I "cut" a path through on one side, however as I was taking out the tails/terminations through holes drilled into the board, theses rubbed and thus shorted to each other.

Graham, these lads sell single bobbins or they did - http://www.ascocomponents.co.uk/ and I got some samples from here a few years ago - https://www.formers.co.uk/

Andy.
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Old 10th Apr 2019, 8:43 am   #8
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Default Re: Making transformer bobbins?

Great links Andy - duly saved in my favourites folder!

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Old 10th Apr 2019, 11:03 am   #9
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Default Re: Making transformer bobbins?

I've made bobbins and central tubes from 0.007" Nomex (synthetic aramid paper, OK to 220C) but have also used cardboard from a cereal packet for a quick temporary job - which lasted 12 years before doing it 'properly! A couple of turns of adhesive Nomex tape covers the join.

The flimsier the bobbin or former, the better the mandrel needs to be when winding, else the pressure of the windings distorts the bobbin.

The open corners of this construction has advantages and drawbacks! Advantage is that it allows excellent varnish penetration. Down side is that insulation from laminations is not good - although the winding and the core will never overlap, there is minimal distance between them, so side cheeks may need to be considered. But it's a very flexible technique (helped by having a friendly machine shop nearby which makes central mandrel supports for a small fee).

The photos show an intervalve transformer, not a mains transformer, if anyone is worried about primary/secondary creepage distance!
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Old 10th Apr 2019, 7:30 pm   #10
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Default Re: Making transformer bobbins?

I'm following this thread with interest - for a future project i need a modulation-transformer to match a push-pull pair of OC35/NKT404 transistors to a single-ended RF stage. I'm thinking of using copperclad FR4 with short-circuit-stopping slits for the bobbins, if only because I can earth the copper to minimise the chance of a few tens of Watts of RF coupling-back into the audio power-stage.
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Old 10th Apr 2019, 7:55 pm   #11
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Default Re: Making transformer bobbins?

Some tips in here on making transformer bobbins: https://ludens.cl/Electron/trafos/trafos.html
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Old 14th Apr 2019, 8:34 pm   #12
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Default Re: Making transformer bobbins?

Thanks for all your replies. It looks like some kind of resin bonded material is to be preferred.

I'm going to have a go with thin plywood and Araldite first though, mainly because I have some nice thin plywood left over from model boat building.

First step will be to construct a mandrel, probably of hardwood, to form the centre box around. I'll keep you updated.
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Old 15th Apr 2019, 8:47 am   #13
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Default Re: Making transformer bobbins?

Hi Graham, beware of moisture in your ply!

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Old 15th Apr 2019, 9:48 am   #14
David G4EBT
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Default Re: Making transformer bobbins?

Pricers for Tufnol/Paxolin in various thicknesses on e-bay are quite reasonable.

For example,TUFNOL Whale Sheet Phenolic SRBF (synthetic resin bonded fabric), A4 sized sheet x 1.5mm thick, 5.00 post free:

https://www.ebay.co.uk/itm/P1-SRBP-B...-/252426091086

I've used a company called Trent Plastics several times. They often sell sheets more cheaply with surface blemishes such as scratches and sometimes call it 'refurbished' or 'used', but it's fine where appearance isn't an issue. For example, 300mm x 300mm x 2mm thick Whale sheet, 10.39 post free:

https://www.ebay.co.uk/itm/TUFNOL-Wh...-/162272259505

They have other thicknesses available up to 50mm thick.

Phenolic sheet sticks really well with 'original' Gorilla Glue (polyurethane).

Hope that's of help.
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Old 15th Apr 2019, 9:54 am   #15
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Default Re: Making transformer bobbins?

Quote:
Originally Posted by Ed_Dinning View Post
Hi Graham, beware of moisture in your ply!

Ed
Thanks Ed.

This is superior quality plywood and has been kept in a dehumidified building for many years.
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Old 15th Apr 2019, 2:36 pm   #16
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Default Re: Making transformer bobbins?

I've always used single sided copper clad board, never had a problem. First I solder it togeter then epoxy glue then Dremel a few cuts in the copper so it isnt a single turn winding.

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