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What kind of a toroid (specifically) do you need to make a joule thief? Answered

I've literally looked everywhere for what kind of a toroid you need for a joule thief, but no one gives a specific number range or type; the most specific was, "high permeability ferrite toroid". I've been searching high and low for a reasonably priced (including shipping) high permeability toroid for over a week, and I've ended up empty-handed. What's considered "high" permeability?

The best one within my price range seems to be a pack of FT50A-75 toroids that I found on eBay. I looked up the specs:
Inner Diameter:  0.312  inches
Outer Diameter:  0.500  inches
Height:  0.250  inches
AL Value:  3000  µH/100 turns

Which would be 300nH per single turn?

Is that high or low permeability? Will it work on a joule thief?

Am I making a mountain out of a mole hill? Can I just buy the cheapest ones I can find on eBay? (Even though the seller even specified "low permeability" in the description?)

And I don't have any old electronics that I could try to take apart to salvage it from.

Please don't say to just buy it and try it... It's about 6 bucks, but I'm in a situation where I really need to save every dollar I can. I really can't afford to just spend money on something that might not work. If it doesn't work - and I'm usually unlucky, so we can assume that it won't - I'll have to buy more, for probably a more expensive price, and that's more money that I just can't risk. Money is a big factor, so what's the cheapest toroid that I can get?



Best Answer 5 years ago

Probably any will do, buy the cheapest one for starters, if you are sure that you can't get your hands on a broken PSU. As much as I have read, joulethieves are very foolproof gadgets to make. I haven't seen a scientific approach, so probably they just work on every attempt made of anyone's junkpile parts.
I have myself made one with a tiniest toroid I could get and I made lots of turns of thin enamelled wire - instead of making 20 like the tutorials state I guess I made 40 or so. And I have also seen one out of PSU toroid and 20 turns work. So the numbers do not matter much. There is much bigger chance to mess up the ends of wires (really, pay attention to them) than to get unsuitable component.

For the wires, I got a bit from my school, but I accidentally got two different types; they're both 22 AWG but one is stranded and one is solid... Does it matter? Can I just use both in the joule thief? And what do you think about the toroids in the picture?

Stranded and solid core wire differ mostly in their mechanical properties not electrical - stranded wire is better for applications where it is bent repeatedly during usage, it does not break as easily as solid core wire. Joule thief will work with both.

I'm no expert of detecting toroid material nor have I much understanding how much iron and ferrite could differ. But these ones on the picture look just like the random one I gave to my friend who successfully made a joule thief out of it. Google image search returns images of painted green things on both "ferrite toroid" and "iron toroid" search.
If they work in a power supply why shouldn't they work in a joule thief? Neither one is a precision device. Start experimenting already and you will find out soon :)

P.S I just took a look inside two different CFL lightbulbs and both of them had a ferrite colored toroid. These were smaller than in your power adapter. Therefore might be hard to wind 20 turns of 22AWG wire. But there is also small transformer in each CFL - if one can gently break it then the enamelled transformer wire might do the thing. But it would be trickier, so first get a hang of it with your current supplies. And be careful when opening up CFL-s, the glass part contains a bit of poisonous mercury and should remain intact.

I also bought some of those FT50-75 cores. They make very good cores for a JT. The single turn measured over 3 microhenrys on my LC Meter IIb, which is high permeability. I got four for two dollars, from a website, not eBay. I bought a lot of toroids from surplussales.com. Their T231212T tiny little core, only a quarter inch across makes a good JT core, and so do their YJ41003TC cores. Both are reasonably high mu.  Both cost less than fifty cents apiece.
You may be making a mountain out of a molehill.  I make JT coils by winding fifteen feet of 24 AWG two conductor telephone wire or speaker wire onto a AA cell, remove the cell and wrap it in tape or tie it with wire ties in a few places to make it a 'donut'.  This should have an inductance of 30 to 50 uH and will work adequately for a JT.  It's more important to get the right transistor.  Use a PN2222A or 2N4401 or a BC337-25 and you will get the best performance.

You are the one with the blog, right? I was looking around and noticed a nice BC337-40 that gives even greater gain than the 25 or the 16 and can grab 40 of them off of ebay for around $4.50 delivered.

Wouldn't a higher perm torroid mean the battery voltage could be less to get it to ring?

The BC337-25 will drive the LED at about 20 mA, which is about maximum, with almost no problems at 1.5V with a 1000 ohm resistor.  The BC337-40 will probably not oscillate at 1.5V with 1000 ohms, but will start at lower voltage, such as 1.2 or 1.0V.  Or you can increase the resistor and it will work reliably at 1.5V.  If you can 'grab' 50 of each, you can experiment and see the difference.  On rare occasions I've seen the -25 have the same problem with a fresh AA cell at 1.6V, so I may have to use a 1.2k or 1.5k resistor.

I use the Fair-Rite 2643002402 medium permeability and the 2673002402 high mu toroid cores, and I see very little difference,  I don't have to use as long a wire with the high mu cores.

The coil might ring with a capacitor across it to tune it, but a Joule Thief has no capacitor.  I hope this info will help you have best of success in your JT projects.

What would you suggest for being able to run a LED 3.5Vdc @350ma via a Nicd?

You'd find toroids that would work in old PC powersupplies - most dumpsters will have one these days.....

I don't think I'm really up to dumpster-diving for old PC power supplies in every dumpster I can find... I actually kind of doubt I'll find one amongst all the other garbage (and it's not like I know the locations of all my local dumpsters...).

So is that a no for the toroid I gave? Something like that wouldn't work?

Try the local PC repair shop - my local one is always happy for me to have dead supplies for fans and things. The ones in there should be ideal for a JT.

I went yesterday and I apparently missed all their garbage by a few days - they got it all picked up. But the guy did manage to find some power adapters for me. So I found some toroids... They're green though, and I forget where but I remember reading somewhere (when searching about how to get toroids, etc.) that the green ones might not be ferrite? And also that the dark grey ones were high permeability... So it seems like these green ones may not be? Would these toroids work? And do you have a suggested way of removing them safely? Also, how do you remove the stuff that's already coiled around it?

I meant to include the image in the above comment, but I messed up:

photo - Copy.JPG

The bigger one on the top (or right?) already has two windings and should work okay for a Joule Thief.  The smaller green one has two windings, but it is much lower inductance, and the Joule Thief may work but it will probably be running at a much higher frequency.  That's probably not a problem if you don't want to listen to your AM radio.  I would remove the existing wire and wind more turns on it.

You seem to want to ask a lot of questions.  If you get the parts together and make a few you will get the hang of it.  Like Nike says, "Just do it."

I'd slice them off with a sharp knife, slid along the surface, not scoring them.

Suck it and see. What's it cost you ? High mu isn't necessarily a problem.

Oh, I just heard that you need to use high permeability ferrite toroids. Getting a high permeability one seemed harder than I thought it would be which is why I asked. Seriously, is there no way to determine if it's ferrite?