EE core transformers and parts

[Red Squirrel]

I'm doing a small digikey order and I always like to throw in some misc stuff to play around with, figured it would be fun to play with proper transformer parts.  EE cores and bobbins.  So I choose somewhat randomly a E size and proper bobbin that goes with it.  So I do my winding around bobbin, and insert two E's into it, correct?

I also see that there is a yoke accessory, where does this go?  I presume it's to hold the two E's together tightly, do I need to buy 2 per transformer or 1?  Or do I need it at all? I presume it's simple, but just want to be sure I'm on the right path.

If curious this is what I'm looking at, and the associated bobbin: http://www.digikey.ca/scripts/DkSearch/dksus.dll?Detail&itemSeq=193000970&uq=635947135066268151

[station240]

Link doesn't work.

See if you can a datasheet from the manufacturer. I know one company has a separate PDF for the core accessories.

[Red Squirrel]

Odd that it does not work, but here is a datasheet.  http://en.tdk.eu/inf/80/db/fer_13/e_30_15_7.pdf

They use same one for most accessories from what I saw, it shows dimensions and what not.

Also is there a core size that is more common than others?  I pretty much just picked that one at random.  I have not read up that deeply into the magnetics theory yet, but it looks quite involved and size can affect everything such as what frequency you can use etc.  Obviously if I had a specific project in mind I'd want to size correctly but this is pretty much just to play with and try stuff.

It looks like the yoke fits on both sides, and pushes down on the back of the E, so need two per transformer, but just want to be sure that's how they're installed.   

[amspire]

Also is there a core size that is more common than others?  I pretty much just picked that one at random.

There is not a "common" core size. The higher the frequency, the smaller the core needed as the magnetics only has to store the energy needed for a few cycles -  or perhaps only one cycle. So a core for a 1MHz switching circuit only needs to store 1/100th of the energy of a similar switching circuit running at 10KHz. So you commonly see many sizes. When you get to the large ferrite cores, the price skyrockets, so over 55mm starts to get uncommon.

Then you have all the different ferrites you can choose. Some are high permeability - some are high frequency.

As long as you have the core and bobbin, you are set, as you can easily just run tape around the core.

One of the things that is not obvious is the importance of the gap between the cores - I don't know if you are buying gapped cores or not, but you can easily add your own gap with a bit of paper or tape between the cores. Obviously, the size of the gap is critical. No gap means you get a very much higher inductance which sounds good, but if you have a gap, the gap stores most of the magnetic energy. Since ferrite saturates and air doesn't, a gap means you can store much more energy in the core, and it will behave in a much more linear fashion. Ungapped cores will very significantly decrease in inductance as the current increases - gapped cores have a pretty constant inductance up to the point the ferrite saturates.

Have fun!

[Red Squirrel]

Ah cool I missed the part about needing two, I'll order them, and then figure it out from there.  From the pictures I have a general idea how they work.  Since I'll be mostly experimenting I probably won't use tape or any adhesives until I decide to incorporate it into a project.   Definitely won't be using 60hz main with these, probably 100khz or what not.  May even experiment with using them as chokes and such too.   Ordered some toroids too to mess with. 
 

[amspire]

You need tape or the clips because the cores have to be help together pretty tightly. Definitely avoid gluing unless you are at a point that you will never modify the core again.

[SeanB]

If yopu need higher current than your wire can handle just use multiple lengths of wire, and make sure you wind them on as equal lengths. Easiest is by calculating the length requied, then adding around 10cm more and measuring off that length that number of times. Then hold the one end of the wires together in a drill chuck, and the other end ( not tangling them) and wind up gently so you get a single insulated twisted composite wire to wind with.

As well kapton tape of assorted widths, some a little wider than the former for interturn insulation, some thinner for holding ends down during winding, and some very small diameter high temperature sleeving to get inner windings out with more robust isolation than the solder through varnish provides. Sleeving can also be used as a spacer at the ends of windings if you are not winding a full bobbin length, or to separate windings on the same layer.

As well definitely get a small amount of polyurethane varnish to paint over the windings to do a final encapsulation so the windings have better environmental protection.  Just make sure it will not react with the winding insulation. Can als be used after you put the core yokes on to both hold them and glue the core halves together so they do not whistle as much.

[Red Squirrel]

Actually was going to ask about magnet wire, what is the main difference between that and regular wire?  Thinner insulation so you can do more turns?  For low current I'll probably be using some 18 awg telecom wire as I have a lot. Though in a real project I'd probably buy the proper magnet wire.

[SeanB]

Normally the PVC insulation melts at around 70C on regular wire, as opposed to magnet wire which is designed to run at up to 130C. As well the insulation is lower loss, which can be an issue if you go over around 500kHz as then the PVC losses will start being appreciable.  Biggest issue is the insulation takes up half the winding space on its own.

[Red Squirrel]

Ah I see, so for experimenting at relatively low currents I should still be fine with the regular wire then?  I might get magnet wire later depending how far I get with this though.   

As for using more wire per run, is this to counter the skin effect?  Guessing at the khz range it becomes an issue right?  I've read briefly on that and you can even buy special braided wire, but I'd probably just do as suggested and basically make my own.   Digikey does sell spools of magnet wire too so I might just break down and get one.

I do eventually want to get into high current SMPSes (like say 2000w @ 54vdc) but I'll start small first.  

[Red Squirrel]

I ended up caving and getting a spool of 26 awg magnet wire.  I figured if I don't, and I start playing with these and realize I'm getting bad performance with regular wire, then I'll be kicking myself for having to make another order.  That, and adding the spool bumps me into the free shipping territory. 

Funny thing is I have yet to play with half the stuff I ordered from my last digikey order... lol.  So now I'll have even more stuff to accidentally blow up. 

[Ice-Tea]

In addition: magnet wire is awesome for breadboarding stuff. Burn off the enemal with your iron and you can solder on components. At the same time, the wire can touch whatever and it won't short out. You can do this not only for the ends of the wire (where you expect to be able to solder on stuff) but everywhere along the line. Easy way way to quickly cook up a circuit 

[SeanB]

In addition: magnet wire is awesome for breadboarding stuff. Burn off the enemal with your iron and you can solder on components. At the same time, the wire can touch whatever and it won't short out. You can do this not only for the ends of the wire (where you expect to be able to solder on stuff) but everywhere along the line. Easy way way to quickly cook up a circuit 

Done that years ago with 40SWg wire to connect to a watch PCB. Really fine wires, but still does work to this day as an alarm clock.

[Red Squirrel]

I use telecom wire for breadboarding, that works well too.  But good to know.

Got all my stuff today, and I'm glad I got those clips too because they hold the two Es tightly without any tape.  Great for playing around before using anything permanent/semi permanent like glue or tape.  For a permanent design I'd tape too of course.
 

[mazurov]

You can disassemble harvested E-core transformers/inductors very easily. Heat it up on a preheater to ~150C, then pull out the halves. Every dead CFL bulb has one ~1mH inductor wound on 10 mm -ish ungapped core in the controller; I've repurposed more than a couple by separating the core, filing the gap in the center leg and gluing the halves back together.

You can also add gap to ferrite toroid core by breaking it apart (ends up in 4 pieces usually) and gluing it back.

One good source of magnet wire is Chinese AC motors. Another is old TV sets.  High-frequency coils can also be wound with copper clad kapton tape instead of wire. 

[CatalinaWOW]

Don't know if it is still true, but at one time magnet wire was designed to be baked after winding.  The insulation would fuze together making the coil mechanically rigid.  Helped reduce mechanical noise (hum).

[Back2Volts]

Do you guys realize that E-core transformers are way obsolete ?    Nowadays everybody uses eCore transformers