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As you people are wondering how even scientists have named this phenomenon in such an interesting way. In this project I’m going to tell you what a Joule Thief Circuit is how easily it can be made and also a bit of how it works. Coming back to the name ‘Joule Thief’ actually signifies that we are going to steal something and that is power. This power is said to be stolen as we are going to extract it from dead pencil cell (AA) batteries. Actually it isn’t theft because these batteries that we feel have gone dead are never completely exhausted, there’s always some residual power left inside them after our usage and this is power is sufficient to run a Joule Thief Circuit.

Now regarding the construction of this circuit, it is very easy to make and can be made with a few basic components. The main components are transistor (2N4401, BC337, 2N3904), LED, Toroid Bead (ferrite core), Thin wire (it should be insulated as found in a motor) and of course dead 1.5V AA type pencil cell that you use daily. Amongst these components the one that is truly tough to find is a toroid bead. You can get it in an electrical equipment store or else you can find it in an old CFL. But take care while removing it from the CFL because you will have to open the entire CFL and you might break the glass. So after opening the CFL there will be a PCB on which you may find your toroid bead. Also another option is that you can take it out from an old motherboard of a PC. It is actually a hollow cylindrical bead made up of magnetic material (test it with a magnet), so that it can act as a core for the inductor that we are going to make for this circuit. The circuit connections are given below:

joule thief circuit connections

The process of making this mainly consists of soldering all the parts according to the circuit diagram given and winding the toroid bead with the thin insulated wire.

For the detailed information regarding how to make it you can go to this link:

So if you have made it then you’ll be able to see how nicely a dead battery can make the LED glow so bright. But our journey doesn’t end here since we have to know what wonders are actually taking place inside this small piece of circuitry. Just for finding things out, I had actually taken an LED that turns ON at a minimum voltage of 3V. This means that no matter even if I try to directly connect the LED to a brand new 1.5V pencil cell, then also the LED won’t glow (and believe me I did try connecting the LED to a new cell and it didn’t glow even a bit). Only if you would connect two new 1.5V cells in series, the LED will glow. You might be thinking that there would be some kind of voltage amplification involved over here. Well after you’ve made it, take a DMM (digital multimeter) and check the output voltage across the LED. You will be shocked as I was to find out that the DMM shows you a voltage of just 1.5V or even less than that. Now you check the input voltage across the battery and you will find that it is more or less same as compared to the output voltage. Then how is the LED glowing? Initially I had tried glowing the LED by connecting it directly to the new 1.5V cell, at that time it didn’t glow; but if I connect the LED to this circuit it glows brightly having the same output voltage (1.5V). After getting these results I started reading about this on the internet and also consulting various people as to why this was happening. Then I came to know that actually the transistor is the reason for all this. The inductor made with the toroid gives an instantaneous voltage boost to the transistor and the transistor here is acting as a switching device.

So what happens is that the DMM is misleading us to false conclusions. The DMM always shows the average voltage across the output terminals. Thus, in reality there would be some voltage amplification (which is enough for the turning the LED and in our case greater than 3V) and then the output waveform would also have a particular duty cycle for which it gives a higher voltage. For finding out the exact values of the amplified voltage and for what percentage of the duty cycle we are getting the high voltage you need to check it on a DSO (digital storage oscilloscope). Here is an image of a sample output waveform that you might get in a DSO for this circuit.

Joule Thief Waveform

As you can see the frequency of the switching by the transistor is very high (38.46 kHz). Hence, due to persistence in human vision we cannot make out, but in reality whenever the spike in the voltage occurs (the transistor turns ON) the LED glows for a very small amount of time and then it turns off for the remaining 60 to 70 % of the cycle. Since this happens at a very fast rate our eyes can’t judge it and we think that the LED is glowing for the whole time. The brightness will be decided by the amount of current that flows through the transistor and thus transistors like 2N4401 or BC337 are preferred as they can provide more current.

Hence, we have made a device which actually runs from very small power taken from dead batteries and don’t worry that since it is a dead battery, the LED will be ON for a long time. I have tested it and it keeps on glowing for a few hours. So what I actually made out of this concept is an emergency torch. Just that instead of one LED I connected a few more LEDs in parallel and made a bunch of it in a packed case and there it is I had a wonderful torch which ran on dead batteries. So remember not to throw out these dead batteries, they can come handy during emergencies.

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