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I am basically a rank newbie. The only electrical project I have ever undertaken was to take a light fan and rip it apart and glue magnets in it and with a pair of vise grips spin it around and generate about 16 V on a multimeter.

I am interested in building a basic Joule thief to power small LED lights off of low power sources like solar garden lights. I am also investigating alterative low power sources that produce very small amounts of electricity, but would need to "boost the "juice." Anyway, from what I have seen these folks are building the "core" of a Joule thief and they say you have to use a "ferrite core".

Pardon my ignorance, I do not know a lot of electronic jargon so I am not sure what a ferrite core is. It sounds like and looks a ring magnet like what one would find in a music speaker or microwave oven. Is this what folks are using for the basic ferrite core? Would a round magnet work for a Joule Thief?

I asked Google and never got a proper satisfactory answer.

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    \$\begingroup\$ Have you first searched this site? You should find that phrase here more than a few times. But the circuit is pretty basic and it is hard to fail when making one. Yes, repeated reverse breakdown will eventually reduce its effectiveness and may destroy it someday -- but who cares? Just stick in another BJT. (A good commercial design would be more involved.) The core is called a "toroid" and a ferrite type is probably better to use. If you have options, shoot for a type 61 perhaps. If not, don't worry about it. Just get it and try. Do wind the wire correctly, though. \$\endgroup\$
    – jonk
    Jan 31, 2019 at 0:19
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    \$\begingroup\$ When you try to google something like this, there are many excellent explanations at different levels of detail and I'm guessing you just felt overwhelmed by the terminology, but when that happens, if you actually want to learn electronics, find the simplest explanation you can find and look up every word you don't understand, and repeat as necessary for those words. At any rate, if you read up on "inductance" and "inductor core material" you should more easily find direct explanations. That said, a joule thief is best for a weak DC power source that is expected to weaken or stay the same. \$\endgroup\$
    – K H
    Feb 1, 2019 at 0:31
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    \$\begingroup\$ If you're planning to use a better power source than the already "dead" alkaline batteries that the joule thief is typically paired with, you might want to instead learn how to build a boost converter. If you want to learn, you might want to slap together a joule thief just because it's simple and it'll get rid of your old AAs, but after that, I'd recommend building a buck converter, then a boost converter, then maybe a boost/buck and/or a cuk converter. Doing it in that order should make it easier as each one builds on the knowledge from the last. \$\endgroup\$
    – K H
    Feb 1, 2019 at 0:34
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    \$\begingroup\$ Solar garden lights use a specialized IC, often a YX8018. These use a simple inductor, rather than a transformer, and they do all of the heavy lifting regarding solar charging and using the battery to run an LED. If that's the kind of thing you want to mimic, then just buy some YX8018 devices. They are pretty cheap and they will work fine for all the complexities involved in solar charging as well as operation. If you are looking for some kind of general purpose, efficient micropower system, where you imagine that a Joule Thief is the right approach, perhaps look elsewhere. \$\endgroup\$
    – jonk
    Feb 1, 2019 at 1:38
  • \$\begingroup\$ You don't necessarily have to use ferrite. A powdered iron core can be smaller and produce the same output. Suggest getting a professional 100uH toroid inductor and adding the secondary to it, which can be thinner than the primary because it's just a sense winding and not a true transformer (it's a coupled inductor). \$\endgroup\$
    – MicroservicesOnDDD
    Apr 11, 2021 at 21:16

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A ferrite core, is something you put in the centre of your coil (a core) made of ferrite (a ceramic with particular magnetic properties). Ferrite has a very high permeability (it can very easily be temporarily magnetised by an external field), and has a high saturation level (it can be magnetised a very great deal).

You can buy ferrite cores (or coil formers) in lots of places, or you can recycle one from some other piece of equipment. The lumps on computer cables are hollow ferrites.

Your joule thief will run at relatively high frequencies so soft iron cores (that are used in low frequency transformers) won't work very well.

you can read about ferrite cores on wikipedia: https://en.wikipedia.org/wiki/Ferrite_core https://en.wikipedia.org/wiki/Ferrite_bead

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    \$\begingroup\$ Thanks for all of the replies. Like I said I am a newbie, and other than the internet and maybe one old neighbor I don't know basically anything about electronics and there is no one to help locally .What is the difference between an inductor and a ferrite core? The picture on Wikipedia looks the same.. Also, living in a remote location new electronic parts are hard to come by, but recycled stuff is easy to source. What would you recommend in the discarded electronics that I can tear apart and I might find a ferrite core? thanks \$\endgroup\$
    – nite hawk
    Jan 31, 2019 at 5:31
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    \$\begingroup\$ @nitehawk A ferrite core is simply the magnetic material being used. It becomes an inductor when you wrap turns of wire around it. The easiest core to get is the one used for reducing EMI on mains cables (the bump you see clamped on the cable. It certainly wont be the smallest but you will be able to make it work. I'd suggest you take the core out of the plastic holder and Superglue the two halve together. \$\endgroup\$
    – Jack Creasey
    Jan 31, 2019 at 5:36
  • \$\begingroup\$ Thanks for your patience.. I am learning, it is just a whole new learning curve.. \$\endgroup\$
    – nite hawk
    Jan 31, 2019 at 6:48
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    \$\begingroup\$ A ferrite magnet is made of a slightly different material which rememberss its magnetization, and is permanently magnetized during its production. It would make a bad core for an inductor. (But that sort of ferrite was used as a type of RAM in early computers) \$\endgroup\$
    – james
    Jan 31, 2019 at 9:21
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The ferrite core for a Joule Thief would not be magnetized.
But you are correct however that the same type of ferrite material is indeed used to make magnets.

You want to purchase unmagnetized ferrite cores to use as inductors or transformers. Something like this might be a lifetime supply for your joule thief experiments. You can buy just the cores, but IMO you are better buying Inductors such as shown that already have 1 winding in place so you only have to wind the secondary.

enter image description here

68 or 100 uH should be adequate for your purpose.

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  • \$\begingroup\$ You can build an amazing Joule Thief with these. The secondary, used as a sense winding, can be significantly thinner (my guideline is thick enough not to break easily). To read inductance of the primary, I use this cheap (open-source, I think) device: Mega328 Transistor Tester. We should enhance this device for the Joule Thief to better test ESR of caps and coils. \$\endgroup\$
    – MicroservicesOnDDD
    Jun 15, 2020 at 11:24
  • \$\begingroup\$ In general, I have found the secondary should have the same number of turns as the primary. \$\endgroup\$
    – MicroservicesOnDDD
    Jun 15, 2020 at 11:25

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