Normally we find Joule Thief Circuits using a toroidal ferrite core with two separate winding, meaning it has a separate primary and secondary winding just like transformers. However, in the article we will discuss about a design which eliminate the requirement of having two separate winding, rather makes use of a single continuous winding with a center tap.
This alternate joule thief design can be identically used to drive Blue and white LEDs that works with just 1.5 volts or for illuminating all 3.3V LEDs, and it looks pretty amazing and too good to be true.
If we go through the datasheet of a blue or a white LED, we can easily find that these devices need a minimum of 3 volts to light up optimally.
However the present design employs just a single 1.5 V cell for producing the same as with a 3 V battery.
That’s where the whole configuration becomes very special.
The Importance of Inductor
The trick lies with the inductor L1 which in fact becomes the heart of the circuit.
The entire circuit is built around a single active component T1, which is wired up as a switch and is responsible for switching the LED at a very high frequency and at a relatively high voltage.
Thus the LED is never switched ON continuously rather stays ON only for a certain portion of the time period, however due to persistence of vision we find it switched ON permanently without any oscillation.
And because of this partial switching the power consumption also becomes partial making the consumption very economic.
This LED Joule thief circuit may be simulated with the following points:
How it Works
As can be seen in the diagram, the circuit involves only a single transistor T1, a couple of resistors R1, R2 and the inductor L1 for the main operation.
When power is switched ON, transistor T1 is forward biased instantaneously through the left half winding of L1. This pulls the current stored inside L1 through the collector of T1 to ground which is technically twice the value of the applied supply voltage.
The grounding of L1 instantly switches off T1since the action inhibits the base bias current of T1.
However the moment T1 switches OFF, a peak voltage twice the value of the supply voltage, generated as a result of a back EMF from the coil is dumped inside the Led, illuminating it brightly.
The condition however stays only for a fraction of a second or even less when the T1 switches ON once again, because its collector is no longer pulling the base drive to ground during that instant.
The cycle keeps on repeating, switching the LED as described above at a very rapid rate.
The LED consumes a nominal 20 mA in the switched ON condition, making the whole proceeding truly efficient.
Making the Coil L1
The making of L1 is by no means difficult at all, in fact it does not carry much criticality, you may try a number of versions by varying the number of turns and by trying out different material as the core, of course they all must be magnetic by nature.
For the proposed circuit, one can use the wire from a discarded 1amp transformer. Use the secondary winding wire.
A 3 inches nail may be selected as the core over which the above wire needs to be wound.
Initially you may try winding about 90 to 100 turns over it, don’t forget to remove the center tap at 50th winding.
Alternatively, if you a have some lengths of telephone wire in your junk box, you may try it for the design.
Tear apart one of the wires from the twin section and wind it over an iron nail having a length of about 2 inches. Wind at least 50 turns and follow the procedures as explained above.
Rest of the things may be assembled with the help of the given schematic.
Switching ON power to the assembled circuit will instantly illuminate the LED and you can use the unit for any relevant desired application.
Parts List
You will require the following parts for the proposed 1.5 white/blue LED driver circuit:
- R1 = 1K5,
- R2 = 22 Ohms,
- C1 = 0.01uF
- T1 = BC547B,
- L1 = as explained in the text.
- SW1 = Push to ON switch.
- LED = 5 mm, Blue, white LED. UV LEDs can aso be driven with this circuit.
- Supply = From 1.5 penlight cell or a button cell.
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