My first joule thief... success using scavenged SMPS common mode choke.

[Evil Lurker]

So last night I whipped up a joule thief using a common mode choke scavenged from an old junk ATX PSU. This particular type of choke looks like a transformer rather than a choke... in the picture below the choke is the thing in between the power factor correction transformer and the small toroid on the secondary side of the PSU.



The reason I selected this particular combination was to allow me to wind one or more high voltage secondaries hopefully capable of driving a CFL light on the same hunk of ferrite because lets face it, large ferrite toroids are not exactly easy to scavenge and putting 300+ turns on a toroid without an automated winding machine borders on sado-masochism. Then again going thru the process of stripping down a transformer is also a total pain in the arse but at least there is less chance of the wire kinking up during the winding process.

Oh and if your wondering how in the hell I got the thing apart I stick em my SMPS transformers in the oven. You see the plastic used in these bobbins is some insane thermoplastic crap that can easily take temps in excess of 500F. The ferrite however cannot take these temperatures but as long as you heat the transformer to a point below it's Curie temperature (wherein the ferrite can lose its magnetic properties) but above the point where the glue and tape holding them together becomes soft enough to pull apart everything is all good. I have several EER size 35 transformer cores and bobbins I intend to play with using this process and even a few 5v standby transformers (those are hard to shuck without damaging the bobbin).

Anyhoos, the basic topology I used can be found below.



The only changes were to use 3 turns instead of two for my feedback loop and a 20 ohm trimpot. Yes they do make those... bout as common as frog hairs but you can scavenge them off CRT yokes. Heck I even scavenged the 2n3904 transistor off an old analog satellite tuner circa 1983. Winding the transformer was a piece of cake... I yanked out the pins using some pliars and replaced them with some new pins scavenged from a PC board male header. For portion of the bobbin that was used for the thief I used the bottom partition only for the windings Started off by taking a hunk of magnet wire a couple feet long and scraping off about 1cm of the epoxy coating it with a knife about a third of the way down the wire. This part was to be my center tap... I made a loop around one pin on the bottom of the transformer and soldered it on and proceeded to with my ouput winding. I think I got around 8 turns with the wire gauge I was using before I had to go up a layer to complete the winding with 4 additional turns. This left me with just enough room to put my feedback winding on top of the output winding in the opposite direction. I'm not for sure if this is the proper way to wind the transformer because to be honest I have never seen a SMPS transformer used in a JT until I built mine last night. I wish I could post pics but my camera's batteries are well being "repurposed" at the time.

Finally FWIW, see those toroids in the PSU picture above? FYI those are actually powdered iron cores, the green/blue ones are type 52 material and the yellow/white ones are type 26. I believe the smaller toroid in that pic is a size T-80 and the larger pi filter toroid is a size T106. I have had good luck using those in a 555 timer PWM PC fan controller using a SEPIC based topology but from what I understand they are unsuitable for use in joule thieves. It has to do with core saturation and energy retention... powdered iron cores hold energy and don't saturate easily... whereas in a joule thief saturation of the core is necessary. Actual ferrite cores can be had out of ATX PSU's though... they are again on the input filtering being used as common mode and are "boxed" having a snap apart plastic shell and two separate windings on either side. You can get the toroid core out of those fairly easily by snipping off the windings and plastic shell. Also, watch out for the smaller toroids in some PSUs.. I have several that are actually some weird plastic shell encasing a rolled ribbon of some metal.

End of rant for now.