The joule thief

[Armagdn03]

Im not very good yet at visualizing the way a diagram works, So I was wondering if someone could give me a walk through on this one.

**broken link removed**

there it is an oscillator that uses the back EMF of the coil to light the led which could not be lit alone by the power cell. says it works at about 50khz. The two coils are actually wound around the same core with the same ammount of turns so that its kinda like a decoupling transformer.

neet little circuit though!

 

[ljcox]

A rough analysis is as follows.

The A coil and the transistor Collector - Emitter capacitance in parallel with the LED capacitance forms a tuned circuit.

When the power is turned on, current starts to build up in both the A and B coils. Initially the current through the A coil is going into the LED.

Due to positive feed back from A to B (via the inductive coupling from A to B), the base current rises rapidly thus causing the collector current to also rise rapidly. When it reaches a peak, the collector current becomes steady, thus no voltage is induced into the B coil. The transistor starts to turn off thus starting a rapid change due to the positive feecback and the energy in the A coil is dumped into the LED.

Note that your claim that the back EMF is a high voltage is wrong.

If we assume that the voltage across the LED is 2 Volt and the battery is 1.5 V, the Back EMF will be 0.5 Volt. (Kirchoff's voltage Law)

Then the cycle starts again, ie. the current through A starts to rise, etc.