JOULE THIEF - or how to power with very little energy

[Yuril]

Hello Econologists!

I have not seen a subject devoted to Joule Thief. I think it has its place here, what do you say?

Basically what is Joule Thief?

The Joule Thief is a small electric circuit which allows, from a low DC voltage (example: 1V), to power a device that operates on a higher voltage (example: 3V).

My knowledge in Electronics / Electricity is rather average so if someone has a more technical definition of Joule Thief I am a taker.

Here is the diagram of a simple Joule Thief:

- A batteries (even a battery that looks dead to you)
- A resistance from + or - 1Kohms
- A NPN transistor
- A LED which operates on a voltage of more than 1.5V
- And an Ferrite torus wind with 2 wires. (ten turns)


To better understand here is a video:
http://www.youtube.com/watch?v=gTAqGKt64WM


If I create this subject it is mainly to share our experiences on this circuit between Econologist.
- Have you ever tried to make one? Success? Failure?
- Why did you build one? History to try or for experimentation?
- Are you going further than this simple Joule Thief? Power other device than an LED? With bigger voltages?
- Etc. ..



My experience:
I'm just starting out, I made a Joule Thief from the Diagram (see above), in place of a 2N3904 transistor I put a BC549B and replace the LED with a block of 3 LEDs operating on a voltage of 3,6V.
My battery is of type Alkaline AAA LR03 of 1,5V but not new, it has a voltage of 1,1V.
I did a duration test, it's been 4 hours that the LEDs are on, the brightness has dropped a bit for the moment and the Voltage across the battery is 0,46V.
to be continued... Then I will test with a new battery.

On the net you will find all kinds of experience on the Joule Thief.
Ferrule Joule Thief without Tore, Power from sources other than batteries and even what is called Super Joule Thief which uses High Voltage to power larger bulbs ...

[Yuril]

Arf I just wanted to EDIT I ​​know what happened. = __ = '

Edited by Remundo: We can not edit your posts after some time: they are then frozen and final, unless there is an intervention of a Moderator

[Yuril]

Okay thank you Remundo.

Continuation of the duration test:
Well it's been a little over 13 hours that the Joule Thief is working, the LEDs are always on and the Voltage across the battery is 0,36V.
I am already won over by the Joule Thief for its operating time with a half-user battery.
I imagine the LEDs will be off when I get up on Saturday morning ...

An example of what can be done with a Joule Thief:
http://lafamillecreative.blogspot.fr/2011/12/une-lampe-de-poche-immortelle.html
And an improved version
http://lafamillecreative.blogspot.fr/2012/02/amelioration-de-la-lampe-de-poche.html

[BobFuck]

It's nice to finish the batteries, but if the goal is to light, it's not worth a real rechargeable lamp...

[Yuril]

The goal of the Joule Thief is not specially to light, it's just that it is often tried with an LED because low consumption.

If you want to know the rest of my duration test:
the LEDs have been on for 36 hours for a voltage across the battery of 0.35V.

I admit that I don't really understand this almost constant tension over the last 24 hours, with the same brightness, it seems to me.

[sherkanner]

Sounds pretty similar to a boost type DC / DC converter (minus the capa).

http://fr.wikipedia.org/wiki/Convertisseur_Boost

The principle is the same, providing a voltage higher than the supply voltage. It is generally done at the expense of the power (we will pump a little more amps to provide the higher voltage, respecting U = RxI).
The efficiency is generally quite high, around 90% and more, but the output signal is quite noisy. Ideal for a diode or a simple element, more complicated for a more sensitive circuit or a large filtering is generally added.

[Yuril]

I do not know.

Yes it sounds like me, anyway the function for which these arrangements have had to create is the same.

Now it is fitting that is more efficient and has the best performance?

[julier]

Yes it sounds like me, anyway the function for which these arrangements have had to create is the same.

This assembly already has the advantage of simplicity!

And another advantage is that it avoids the power generally lost in the diodes of the converters (troublesome especially when the output voltage is low; avoided in the "synchronous" converters which use an additional transistor to play the role of the diode). Here, since the load itself is a diode (LED), there is no need for another diode.

On the other hand, as disadvantages:

the output current is irregular, it works with small LEDs, on the other hand large powerful LEDs like Luxeon 3W strictly forbid transiently exceeding the maximum power.

the value of the output current is unpredictable: it depends on the type of transformer (which material, in particular)

Principle of operation:
- at the start (voltage> 0.6V), the battery turns on the transistor
- phase 1: transistor passing, the current increases in the transformer, therefore the magnetic flux increases in the torus, so a voltage appears on the base of the transistor, which makes it well passing (positive feedback: it's binary)
- end of phase 1: the torus is completely magnetized, the current continues to increase (and even faster and faster: that's not great for efficiency), but the magnetic flux no longer increases. The tension on the base decreases
- start of phase 2: the transistor no longer has enough base current (it depends on its gain: not easy to calculate), so it no longer conducts current as well. So the current drops in the transformer
- phase 2: the current drops in the transformer, therefore the magnetic flux too, therefore negative voltage on the base: the transistor is completely blocked. The current which continues to flow in the transformer is therefore evacuated towards the LED, which starts to shine.
- when there is no more current in the transformer, we start again.

The highest yields are typically:
- either with synchronous converters (but they are complex)
- or with resonant converters, such as the Boyer circuit, widely used to power the fluorescent tubes of LCD screens. The simplest version requires 2 transistors and a transformer with 2 windings, 1 with a midpoint. Efficient, but also difficult to calculate. [/ List]

[Yuril]

Thank you for all these details Julier. ^^

So the Joule Thief is practical for powering small LEDs rather inadvisable to power other devices?