Re: Whats different between these two circuits?



In <a231f34f-d2dd-4f2c-a7f0-9c2c45e595b0@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
readeraz wrote:

Ringing choke circuit and blocking oscillator are different names of
oscillator.
Are they same meaning or they are different circuit in practice?
How to distinctly identify them?

Both resemble variants of the Hartley oscillator, with the capacitor
across the tapped inductor removed.

A blocking oscillator has the oscillation feedback path through a
resistor and capacitor in parallel. During the half-cycle where the
transistor conducts, positive feedbck is through the capacitor. Once the
capacitor has become excessively charged to maintain enough feedback to
keep the transistor saturated, this half-cycle ends. This half-cycle is
usually the shorter one. During the other half-cycle, the transistor is
off until the capacitor is discharged sufficiently by its paralleled
resistor to allow current to flow through the base of the transistor.

In a ringing choke oscillator, the oscillation feedback (assuming a
bipolar transistor) is through a resistor. The "transistor-on" half cycle
is usually the longer one. That half-cycle ends when either the tapped
inductor saturates or the transistor comes out of saturation. The
transistor-off half cycle's onset reinforces itself with the feedback
winding's voltage changing in a direction to reduce the transistor's
collector current. In fact, once collector current starts decreasing, the
transistor usually quickly slams off, and a high voltage pulse can occur.
If a ringing choke oscillator is not carefully designed, the transistor
may be destroyed by breakdown from high voltage pulses resulting from
suddenly interrupting current flowing through the inductor.
The transistor-off half cycle ends when the transistor's collector
current has decreased to zero and has become steadily zero. There may be
a delay for stray capacitance charged by the high voltage pulse to
discharge before the feedback winding produces voltage in the forward bias
direction, but that is usually short.

- Don Klipstein (don@xxxxxxxxx)
.



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