The concept discussed here remains exactly identical to the usual joule thief configuration, we just replace the normally used 5mm LED with a 1 watt LED.
Of course this would mean the battery getting drained pretty much earlier than a 5mm LED, but it's still economical than using a two 1.5 cells and not including a joule thief circuit.
Let's try to understand the proposed circuity with the following points:
If you see the circuit diagram the only seemingly difficult part is the coil, rest of the parts are just too easy to configure. However if you have a suitable ferrite core and some spare thin copper wires, you would make the coil within minutes.
The above design may be improved further by attaching a rectifying network using a diode and a capacitor, as shown below:
Parts List
- R1 = 1K, 1/4 watt
- C1 = 0.0047uF/50V
- C2 = 1000uF/25V
- T1 = 2N2222
- D1 = 1N4007 better if BA159 or FR107 is used
- Coil = 20 turns each side using 1 mm enameled copper wire over a ferrite ring which accommodates the winding comfortably
The coil may be wound over a T13 torroidal ferrite core using a 0.2mm or 0.3mm super enameled copper wire. About twenty turns on each side will be quite enough. In fact any ferrite core will, a ferrite rod or bar will also serve the purpose well.
After this is done, its all about fixing the parts in the shown manner.
If everything is done correctly, connecting a 1.5 V penlight cell would instantly illuminate the attached 1 watt LED very brightly.
If you find the circuit connections to be alright yet the LED not illuminating, just interchange the coil winding terminals (either the primary ends or the secondary ends) this would fix the problem immediately.
How the Circuit Functions
When the circuit is switched ON, T1 receives a biasing trigger via R1 and the associated primary winding of TR1.
T1 switches ON and pulls the entire supply voltage to ground and in the course chokes the current across the primary winding of the coil so that the biasing to T1 dries up, shutting off T1 instantaneously.
The above situation switches OFF the voltage across the secondary winding triggering a reverse emf from the coil which is effectively dumped across the connected LED. The LED illuminates!!
However the shutting of T1 instantaneously also releases the primary winding and restores it to original condition so that the supply voltage now can pass across to the base of T1. This initiates the whole process yet again and the cycle repeats at a frequency of around 30 to 50 kHz.
The connected LED also illuminates at this rate, however due to the persistence of vision we find it illuminated continuously.
Actually the LED is ON only for 50 percent of the time period, and that's what makes the unit so economical.
Also because TR1 is able to generate voltages that may be many times greater than the supply voltage, the required 3.3V to the LED is sustained even after the cell voltage has dropped to about 0.7V, keeping the LED well illuminated even at these levels.
Video Demonstration
How to wind the Torroid Coil
The torroidal coil can be built according to the following steps:
Take a super enameled copper with, maybe around 0.5mm, thickness of the wire is not criticaal, it simply decides its maximum capacity to handle the load current. For our application, 0.5mm looks good enough.
Wind two 20 turns of winding on the core, as shown in the following diagram:
Don't worry about the winding polarity, meaning the direction of the winding, whether clockwise or anticlockwise...don't worry about it, just complete the winding as shown in the above figure.
If suppose the LED fails to illuminate, or the circuit fails to oscillate, you just have to swap the winding terminals of either side of the winding connection with the circuit, but not both sides, swap any one side winding terminals.
For example, suppose you connect the coil upper side terminal with the R1 (1k) and lower side terminal with the transistor base, and you find the circuit is not working, now you just have to interchange the winding terminals....meaning connect the terminal which was connected to R1, with the T1 base, and connect the base winding terminal, with R1....got it?
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