Hi Everyone,

Can someone help he out in understanding the attached circuit?
It is from a strobe. The only bit I am not too sure about is around the transformer at the top, mainly the winding above "blue" (cable colour).

So, as I understand it, the 39V zener VR2 sets up a bias on Q1 making current flow thought the winding until the emitter goes above 39V, at which point Q1 stops conducting, Magnetic field in T1 collapses and the whole things starts again.

My questions are;
1. Is my thinking above correct?
2. Is the transformer just a one primary and a centre tapped secondary?
3. Is the >39v for the zener is been made between the 28V input and the negative voltage coming form the "blue" winding?
4. How is the emitter even going to see a positive voltage, in respect to the base, when it is directly connected to ground?

Thanks

 

Hmm, maybe the question is a little open ended.
Is there anything anyone can tell me about how that bottom part (around the blue winding) of the transformer may work?

 

hi mark,
Its a self oscillating circuit, the blue winding is a feedback winding, required to make the circuit oscillate.
E

 

Q1 and the transformer make an oscillator. The transformer has a primary connected to the transistor collector, one secondary connected to CR7 which produces a high rectified voltage across C8/C9, another secondary winding which feeds back to Q1 base. When the voltage from that secondary winding goes positive it feeds via R8 and CR6 to the base of Q1 to turn on the transistor. When the collector current saturates and the magnetic field stops increasing the voltage from the blue wire begins to fall and become negative the transistor switches off, CR4 conducts and charges C7. VR2 prevents the base voltage going more than 39V (if it is a 39V zener) positive of the voltage on C7. So as the voltage on C7 becomes more negative it will begin to reduce the positive drive to Q1 base and so limit its conduction and limit the output voltage. This roughly regulates the voltage across the feedback winding and hence regulates the output voltage.

 

Thanks Albert, That clears a few thinks up for me. I looks like my guess was no where near right..

Just one more thing I don't quite get. How does the blue winding go negative? When the core saturates, how does the current reverse?

Thanks.