Transformer Oscillator

Many of you will notice that this is just a slight variation of a blocking oscillator aka Joule thief with 180dgs shift from the transistor and another 180dgs shift from the transformer. What many won't notice are the few major impact modifications. The first one of which is the base configuration. As normal the base current is controlled by the 220k resistor but that much resistance is bound to attenuate the AC voltage to the point where the oscillations will cease. By adding the 10uF cap and the 6.8k resistor we're making an easier path for the AC to shunt to. With it we can control the overall gain (the strength of the positive feedback). The second and most important modifications is the 2.2uF cap. Without it the output is bound to swing deformed and without any coherent shape peaking at a few hundred volts. By adding it we're making a resonant tank circuit which tends to oscillate at a near perfect sinewave effectively turning a joule thief from a step up convertor into an oscillator.

published 10 years ago