Author Topic: Chokes and transformers  (Read 3067 times)

0 Members and 1 Guest are viewing this topic.

Offline tron9000Topic starter

  • Frequent Contributor
  • **
  • Posts: 423
  • Country: gb
  • Still an Electronics Lab Tech
    • My Hack-a-day project page
Chokes and transformers
« on: July 16, 2015, 10:55:03 am »
I'm looking for a small transformer for a joule thief, rather than roll my own, its for a kit and the target audience is assumed not to have much dexterity or access to winding equipment, plus limited time.

There are a lot of of common mode chokes that do fit the criteria and have built a joule thief using a choke out of an ATX power supply (see attached) - ok so it works, but is using a CM choke the correct usage for it?

or is there really no difference between a 1:1 transformer and a CM choke? are the cores not made of different materials?
Partsbox.io - orangise your parts!
"If you're green you can only ripen. If you're ripe you can only rot!"
 

Offline Ian.M

  • Super Contributor
  • ***
  • Posts: 12805
Re: Chokes and transformers
« Reply #1 on: July 16, 2015, 12:03:52 pm »
My first Joule thief used a 24T centertapped winding on a 6mm x 4mm dia ferrite bead.  If you supply an appropriately sized ring core and a length of magnet wire, anyone not physically disabled should be able to wind the transformer with no problems.   Start by twisting a loop for the CT in the middle of the wire then wind both ends in opposite directions.   If the core aperture is large enough, its quite easy to do a neat winding, and not too tedious up to a few tens of turns or so.  Alternatively provide two lengths of differently coloured magnet wire and have them lightly twist them together and do a bifilar winding to halve the number of passes required through the core.  Its still best to start from the middle though.

You only need to go to a prewound transformer if you want a greater inductance for a higher operating voltage to drive a series string of LEDs fom >1.5V in, or a lower operating frequency.
« Last Edit: July 16, 2015, 12:28:22 pm by Ian.M »
 

Offline tron9000Topic starter

  • Frequent Contributor
  • **
  • Posts: 423
  • Country: gb
  • Still an Electronics Lab Tech
    • My Hack-a-day project page
Re: Chokes and transformers
« Reply #2 on: July 16, 2015, 12:34:26 pm »
My first Joule thief used a 24T centertapped winding on a 6mm x 4mm dia ferrite bead.  If you supply an appropriately sized ring core and a length of magnet wire, anyone not physically disabled should be able to wind the transformer with no problems.   Start by twisting a loop for the CT in the middle of the wire then wind both ends in opposite directions.   If the core aperture is large enough, its quite easy to do a neat winding, and not too tedious up to a few tens of turns or so.  Alternatively provide two lengths of differently coloured magnet wire and have them lightly twist them together and do a bifilar winding to halve the number of passes required through the core.  Its still best to start from the middle though.

You only need to go to a prewound transformer if you want a greater inductance for a higher operating voltage to drive a series string of LEDs fom >1.5V in, or a lower operating frequency.

Ah I think I understand - length of wire, fold it in half, pass the fold through the ferrite ring, keep passing the fold through till your run out of wire, cut fold. That would be a quicker way for fabricating one!
You can buy 1:1 coupled inductors, they are likely to be cheaper and smaller then a CMC.

Search filtered to exclude anything below 100uH.

http://canada.newark.com/webapp/wcs/stores/servlet/Search?catalogId=15003&langId=1&storeId=10196&categoryId=800000005314&pageSize=25&beginIndex=1&showResults=true&aa=true&sf=502,781&pf=811335882,811335906,811335907,811335909,811335917,811335939,811335974,811335981,811335994,811336025,811336045,811336087,811336112,811336133,811342381,811342392,811342396,811342418,811342429,811342453,811342498&min=811336132

A couple of vendors sell multi inductors in which you can set turns ratio depending on how you hook them up. I think one is called versa pak or something like that, last I looked at them they weren't cheap though.
True, but I'm going traditional single side, solder side-component side, construction, so I was only looking for THP parts, thanks though.
Partsbox.io - orangise your parts!
"If you're green you can only ripen. If you're ripe you can only rot!"
 

Offline Ian.M

  • Super Contributor
  • ***
  • Posts: 12805
Re: Chokes and transformers
« Reply #3 on: July 16, 2015, 12:54:36 pm »
No. the cut the fold method does NOT get you a center tapped winding unless you check continuity after cutting and pair it with the opposite end of the other half.  That's why I suggested two different colours magnet wire.

Even so its much easier if you tack down the middle of the wire to the outside of the ring core with a narrow strip  of sellotape, then wind each end in opposite directions as it halves the length of wire you have to pull through each time.  With bifilar winding, a 100T CT transformer on a 1" ring with a 1/2" aperture can be wound starting from the middle in about two minutes as each end only needs 25 passes.
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 21606
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: Chokes and transformers
« Reply #4 on: July 17, 2015, 02:15:35 am »
CMCs are made to carry the least magnetizing current possible, so they are unsuitable for this type of circuit.  You need much too large a part.

Gapped ferrite cores are ideal.  Anything wound on a gapped shape (U or E cores with some gap -- add shims if necessary), or a rod, dogbone or bobbin style, will do nicely.

Or use an internally gapped material, like powdered iron toroids.

The inductivity (inductance per turn^2) will be relatively small, which means you need relatively many turns for a given operating frequency.  That's fine, 50 turns of hair-fine wire (enough to power a single 20mA LED) is easy.  Use a light touch. ;)


If you want to make a bigger one, you can do it like this,



That's a #6 powdered iron (permeability = 35, reasonable losses).  Current draw is about an ampere (from a fresh alkaline or charged NiMH), running the LED at about 1W.

If you're finding a lot of toroids with one color on three sides and another color on the fourth, try looking it up here,
http://www.micrometals.com/material/pcprop.html
or here
http://www.micrometals.com/materialchart.html

#26 and #52 are the most common; they're all but useless for this application, because they have very high losses.  #3, 8 and 15 are the best.

More likely, you'll find an SMT ferrite core with a bobbin construction, which will be tedious to rewind, but most suitable for the purpose.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf