Why R1&2, D1&2 need in this Circuits? How this circuits work?
Answer:
R1&2, D1&2 is used to prevent high voltage
R1 and R2 produce the voltages that are fed to the step up transformer.
D1 and D2 are used to keep opposite cycle voltage appart from each other.
D1 and R2 prevent Q2 from receiving too much voltage when Q2 turns off. If not for this snubber network, as it is called, Q2's collector would rise significantly above 12V when it turns off, partly from the inductive effect of the transformer and also because at that time, the other side of the transformer is being pulled down by Q1. This will make the left side of the transformer rise to 24V, I believe. I would guess that R2 is pretty small, 10 0hms or less, just guessing. Since D2 is connected to 12V, it wont conduct until Q2's collector gets above that.
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