Chokes and transformers - Page 1
Forum section: electronics / beginners

author: tron9000
author_rank: Frequent Contributor
timestamp: July 16, 2015, 10:55:03 am
content: 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?

author: Ian.M
author_rank: Super Contributor
timestamp: July 16, 2015, 12:03:52 pm
content: 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.

author: tron9000
author_rank: Frequent Contributor
timestamp: July 16, 2015, 12:34:26 pm
content: Quote from: Ian.M on July 16, 2015, 12:03:52 pmAh 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!Quote from: AcHmed99 on July 16, 2015, 12:23:58 pmTrue, but I'm going traditional single side, solder side-component side, construction, so I was only looking for THP parts, thanks though.

author: Ian.M
author_rank: Super Contributor
timestamp: July 16, 2015, 12:54:36 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 17, 2015, 02:15:35 am
content: 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.htmlor herehttp://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