Joule Thief - Back EMF explained

[Bluecast]

Hello Everyone,
I'm new to the site, I have been working with electronics for over 25 years, but most of it has been in the building and repair end, not much on the design/experimentation and invention front.
I have been digging deep (like going to school again) focusing on understanding the Joule Thief circuit. I have been looking at Lidmotor, thedaftman, jeannacav, chilliqueen along with several other video's on youtube, to educate myself.
It wasn't until I started viewing Lidmotors records, that I heard about 'Back EMF' which is what makes the Joule Thief work? This is what I don't understand. There doesn't seem to be a focus on explaining this and there's not much info I could find on 'back emf' which really pertained to the joule thief circuit. Most 'back emf' info has to do with motors. What exactly is going on with the ferrite toroid from the two joined wires, to the two which aren't joined? I have theorized that the electrical current flowing through the wires creates and electromagnetic field, which is magnifying somehow. Can anyone confirm or explain this? I did a little experimentation using a strong magnet from a junked computer hard drive. When I brought the magnet close to the toroid, it attached itself and the LED goes out. So, I'm not sure if this is the magnet field being disrupted? Or possibly some sort of induction short? Creating a virtual short in the Toroid winding? I'm more inclined to think the magnetic field is disrupted.
I'm focused on understanding how this process works so I can apply it to battery operated lights, used by video camera's (which typically run down quickly). I look forward to your input.

-Bluecast

[SilverToGold]

Back EMF is not really the correct term for the effect you're looking at. Back EMF is always lower than the voltage applied to the coil and never greater. The effect of the voltage spike is from the rapidly collapsing magnetic field. If you go look at Peter Linemann's videos' on motors secrets, he explains it very throughly in engineer-able terms.

Free Energy, Products, Books and DVDs

My guess is that the reason your thief quit working is that the strong magnet saturated your core and that little current from the battery just has no chance of overcoming that field in the core that was already at it maximum.

But magnets if properly used could probably magnify the effects of the Joule thief. Check out this little device to see what I'm talking about.

2SGen, an amazing tiny Solid State Generator by JL Naudin

[boguslaw]

Simply : coil is not a coil , it is resonating system with inductance and capacitance. When you charge it with current and disconnect it will oscillate using own capacitance which is small.The so called back-EMF is a ring-down of oscillation inside coil. Tesla knew that and was able to produce coil which would oscillate 3 years when disconnected.

[Allcanadian]

@Bluecast
A good resource for the joule theif can be found by googling, "self-oscillating boost converter", which is what the joule theif is called technically. As well in my opinion the term "Bemf" is not the correct term for the phenomena, I prefer the term "Inductive Discharge" which is more self explanatory and better resources on this effect can be found if you google this term.
To understand the process I would google, boost converter, self-oscillating boost converter and water hammer for a great analogy.
Basically the joule thief uses a secondary coil and transistor to automatically switch a boost converter.
Regards
AC

[SilverToGold]

Tesla knew that and was able to produce coil which would oscillate 3 years when disconnected.

Can you provide a reference from Tesla stating this? Never heard this from him. thx

[Bluecast]

@ SilvertoGold - yes, Back EMF is the term that has been floating around, but I wasn't sure it was the right term, hence the posting here. I looked over the 2SGen model and really, that is similar to what I was attempting to do. When I introduced the magnet into the Joule Thief circuit, I was wondering if an opposing magnetic field, would increase or focus the existing field within the toroid's field. While 2SGen isn't exactly what I was looking for, it does explain the need for the proper amount of magnetic field, so possibly my idea could still be the case. I've been on the Free Energy site and will be ordering some things off there eventually. Thanks.

@ Boguslaw - I'm not really sure how your answer applies to my question, but that's interesting information.

@Allcanadian - I looked up the "self-oscillating boost converter" and while the description does sound similar to the Joule Thief circuit, there are some disqualification's for this being what a Joule Thief is technically. The first thing I noticed after reading the description, is that it sounds incredibly complex (which the Joule thief isn't). I looked up several circuit diagrams and IT IS complex, by about 4 times that of a Joule Thief. The other thing is that the Joule Thief isn't "self-oscillating". There's nothing in the circuit which causes oscillation (as fare as I can tell, please prove me wrong if there is some somewhere). The Term Inductive Discharge probably is a more accurate description of what's happening, since the Back EMF really deals more with motors. My thoughts however were split between it being an EMA (Electro-Magnetic Amplification) and something dealing with an increase in capacitance for the circuit (which is measurable).

The other thing that doesn't sit right is the term "secondary coil". In the Joule Thief Circuit, there really isn't a secondary coil. The coils are connected, interchangeable single ends and therefore it is actually a double wound single coil or a dual wound triple interface inductor. This would be the equivalent of taking a single wire on a core, finding the middle, and making that the third contact point.

Good points though and definitely along the right line of thinking. Thanks for the input.

[Joit]

Hi,
Silvertogolds explaination does match at all pretty well.
It is the collapsing Field, what goes back to the source.
Just what i have learned there too, that it is the speed of the collapse, lol.
It is not the Term Bemf, what do match, because this would be the resistance from the Load.

I did do a experiment with a stroboscope light, like you are using it for testing the Spark at Cars.
This light has an little Arrow on the clamb, that you see, in wich direction the Current goes.
When i connected it to the Plus line, the Pulse comes from the Source, like a Battery, till the Coil.
When i connect that clamp to a charge battery, what is connected over the Diode to the Coil on Emitter side,
then the Pulse goes opposite how the Diode is pointing.
The Pulse even goes into the the Minus side of the Batterie, and goes out against, how the Diode is pointing.
So it is is the Field what collapse and goes back to the Source.
But the Potential what you create there is higher, when you connect a capacitor to Emitter - Diode - Cap and Minus leg at Plus, you can charge it up to about 100 V around.

With your Magnet on your Core, you did stop the flowing of the Field from the Coil, because it was to strong, i did do something similar but there was still not really a different, only, that it did not work.

Just be carefully, when you wanna charge Batteries from Videos or Cameras, that you disconnet them from the Rest of the Circuit, because the most Circuits can not handle the Spikes, and can be damaged.
How efficient the JT works is hard to say, some guess about 60%, some more, some less.
but it may depends, how it is build. But not sure, if it can light a Light longer then the normal Circuits. It maybe works for Led, but i dont know how it is at the rest from a ie Camera Circuit.

[boguslaw]

Can you provide a reference from Tesla stating this? Never heard this from him. thx

Nikola Tesla On His Work With Alternating Currents -- Chapter IV

"I have designed circuits in connection with an enterprise in 1898 for transmission of energy which, once started, would vibrate three years, and even after that the oscillations could still be detected. "

[Watson]

Hello Everyone,
I'm new to the site, I have been working with electronics for over 25 years, but most of it has been in the building and repair end, not much on the design/experimentation and invention front.
I have been digging deep (like going to school again) focusing on understanding the Joule Thief circuit. I have been looking at Lidmotor, thedaftman, jeannacav, chilliqueen along with several other video's on youtube, to educate myself.
It wasn't until I started viewing Lidmotors records, that I heard about 'Back EMF' which is what makes the Joule Thief work? This is what I don't understand. There doesn't seem to be a focus on explaining this and there's not much info I could find on 'back emf' which really pertained to the joule thief circuit. Most 'back emf' info has to do with motors. What exactly is going on with the ferrite toroid from the two joined wires, to the two which aren't joined? I have theorized that the electrical current flowing through the wires creates and electromagnetic field, which is magnifying somehow. Can anyone confirm or explain this? I did a little experimentation using a strong magnet from a junked computer hard drive. When I brought the magnet close to the toroid, it attached itself and the LED goes out. So, I'm not sure if this is the magnet field being disrupted? Or possibly some sort of induction short? Creating a virtual short in the Toroid winding? I'm more inclined to think the magnetic field is disrupted.
I'm focused on understanding how this process works so I can apply it to battery operated lights, used by video camera's (which typically run down quickly). I look forward to your input.

-Bluecast

So far, not a single "explanation" has given the proper term, Counter EMF or CEMF. An explanation of an inductor should explain CEMF. An inductor opposes a change in current with a counter EMF. In a JT, when the transistor switches off, the inductor wants the current to keep flowing, so it makes a counter EMF that causes a voltage of opposite polarity across itself, and hence the voltage across the transistor increases. When the voltage reaches the 3 to 4V forward voltage of the white LED, it starts to conduct and it puts out the pulse of light.

My simplified explanation of how a JT works is here, with links to more detailed explanations.

[SilverToGold]

Nikola Tesla On His Work With Alternating Currents -- Chapter IV

"I have designed circuits in connection with an enterprise in 1898 for transmission of energy which, once started, would vibrate three years, and even after that the oscillations could still be detected. "

Thanks Boguslaw, sounds like that once he sends signals into the earth, it would resonate for over 3 years. Makes sense since he said that earth was near a perfect conductor, kind of like his bell in a vacuum.