Joule Thief inductor cores

Knowing little about magnetic's, I'd say no gap.. its a transformer. And the material choice depends on the frequency. Higher freq -> higher resistivity. (the higher resistivity ferrites also have lower permeability.)

In practice just grab what ever toroid is around.

George H.

Ok thanks!

Uhhh...

Without doing the analysis myself I can't say, but you want to check to see how much flux is building up in the critter, and make your decisions accordingly. I haven't memorized which switching topology wants what, but some benefit from a gapped inductor, and some don't.

It depends on whether the energy is being stored in the core between half- cycles (in which case you want a gap) or getting transferred to the load at the same instant that it's going into the primary. The former stores the energy in the flux, and benefits from a gapped core, because an air gap can store way more energy than ferrite. The latter can have a plain ol' ungapped core.

...

Ok thanks!

I tried building one a couple of years ago, and when it didn't work I thought it was because I had the wrong kind of core... (from a choke?)

Michael

"Tim Wescott" >

Energy IS stored in the magnetic field, according to E = L/2 x I squared.

( Note the similarity with capacitors were E = C/2 x V squared )

Gapping the core reduces the L value but allows I to increase by a similar amount - but since I is squared, energy goes up enormously.

... Phil

Oops, thanks for the correction Tim. I sometimes think I shold just keep my mouth shut. But I can also learn, if I say somethng stupid.

George H.

What "Joule Thief?" That circuit dates back to vacuum toobs and is called a "blocking oscillator."

I would suspect it would be more efficient with a non gapped core, but as you see from web examples, it isn't very finicky about the inductor. As for core material, that is dependent on frequency of operation. Up to about 20 KHZ laminated steel cores and powdered iron above 20, above 50, and ferrites rule. (there will always be exceptions to that general statement)

I've been using inductor cores salvaged from CFL's for boost converters working at 100 KHZ...

Very old TV sets used blocking oscillators with laminated steel cores to provide vertical deflection. If they lost their sync signal, they kept oscillating - important to the CRT circuits because the phosphors would burn if the electron beam stopped moving.

The wrong kind of core would probably just make things inefficient; it'd have to be really bad to make the thing refuse to work entirely.

I think the dotted end of the coil is wrong in the Wikipedia article. If you put the dot next to the collector and next to the base, then you have a Hartley oscillator that uses the stray capacitance of the transistor and coil to establish "resonance" (in quotes because it's not _very_ resonant).

Oh neato! Thanks!

An autotransformer should work fine too, right?

The J110 jfet seems to be obsolete. I'm reading the specs on the J111 but am not sure what the continuous drain current is. Is it 20 mA (Idss)?

Michael

The joule thief is not resonant, it's a blocking oscilator. it starts by VCC flowing throught the base winding into the base and blocks when collector current stops increasing or the core saturates.

Some manufacturers consider de J110 obsolete. Some others still offer them. The SMD variant seems to be widely available. Look at Mouser and RS for instance.

petrus bitbyter

...

Thanks

Look for joule thief on this site

lot of information

On Wednesday, February 5, 2014 4:25:05 PM UTC-8, David Eather wrote: ...

Wow sure is, thanks!

It's a shame he only covered BJTs tho.