# Joule thief : number of turns around the ferrite core

Hi everyone,

I have to build a simple Joule thief circuit for school. Here it is :(http://upload.wikimedia.org/wikipedia/commons/8/83/Joule_thief.png). The resistance of the resistor is fixed to 1 kOhms, the voltage across the "dead battery" is about 1.5V, the transistor needs about 0.7V to open and finally, the led needs 2.0V to light up. So I have to figure out (not experimentaly) the number of turns around each inductor so that the led has enough voltage across it to light up at best... and that's where I'm stuck with calculations. With all the formulas that I know : Lenz's law, Ohm's law, etc... I feel like the problem is quite hard to solve (but maybe I can get a good approximation). Does anyone have an idea on how to solve my problem?

berkeman
Mentor
Hi everyone,

I have to build a simple Joule thief circuit for school. Here it is :(http://upload.wikimedia.org/wikipedia/commons/8/83/Joule_thief.png). The resistance of the resistor is fixed to 1 kOhms, the voltage across the "dead battery" is about 1.5V, the transistor needs about 0.7V to open and finally, the led needs 2.0V to light up. So I have to figure out (not experimentaly) the number of turns around each inductor so that the led has enough voltage across it to light up at best... and that's where I'm stuck with calculations. With all the formulas that I know : Lenz's law, Ohm's law, etc... I feel like the problem is quite hard to solve (but maybe I can get a good approximation). Does anyone have an idea on how to solve my problem?

Welcome to the PF.

The circuit does not look like it will work without some modifications. Where did you get it? What is the overall goal of this school project?

I got it from wikipedia (the circuit of the school is exactly the same), I think you get more info on how the circuit works there : http://en.wikipedia.org/wiki/Joule_thief . By what is the overall goal of the school project, do you mean what is the purpose of the circuit? If so, the circuit can generate "more voltage" (in fact, in convert DC to AC with higher amplitude) to light up a led that is connected to a "dead battery" that is, a battery that does not have enough voltage to light up the led if you connect it directly to the led.

... I have to figure out (not experimentaly) the number of turns around each inductor...
As I understand it it does not really matters. The principle of it's operation based on the saturation of the ferrite, and then the discharge of the stored energy through the LED - the number of turns (within a wide range) would determine only the operating frequency.

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Sounds right! But then how do you determine the minimum number of turns around the coil for the LED to light up :/ ?

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NascentOxygen
Staff Emeritus