Blocking oscillator
A blocking oscillator is a simple configuration of discrete electronic components which can produce a free-running signal, requiring only a resistor, a transformer, and one amplifying element. The name is derived from the fact that the transistor (or tube) is cut-off or "blocked" for most of the duty-cycle, producing periodic pulses. The non-sinusoidal output is not suitable for use as a radio-frequency local oscillator, but it can serve as a timing generator, to power lights, LEDs, Elwire, or small neon indicators. The simple tones are also sufficient for applications such as alarms or a morse-code practice device. Some cameras use a blocking oscillator to strobe the flash prior to a shot to reduce the red-eye effect.
When it comes to the components involved in this circuit, specific types of each component are needed to have it work to its full potential. The transformer is a vital component. For example, a pulse transformer creates rectangular pulses, which are characterized by fast rise and fall times with a flat top. There are a seemingly endless number of combinations of voltages, transformers, capacitors, transistors and resistors that can be used to vary and model the circuit.
Due to the circuit's simplicity, it forms the basis for many of the learning projects in commercial electronic kits. The secondary winding of the transformer can be fed to a speaker, a lamp, or the windings of a relay. Instead of a resistor, a potentiometer placed in parallel with the timing capacitor permits the frequency to be adjusted freely, but at low resistances the transistor can be overdriven, and possibly damaged. The output signal will jump in amplitude and be greatly distorted.
The circuit works due to positive feedback through the transformer and involves two times—the time Tclosed when the switch is closed, and the time Topen when the switch is open. The following abbreviations are used in the analysis:
A more-detailed analysis would require the following:
When the switch (transistor, tube) closes it places the source voltage Vb across the transformer primary. The magnetizing current Im of the transformer is Im = Vprimary×t/Lp; here t (time) is a variable that starts at 0. This magnetizing current Im will "ride upon" any reflected secondary current Is that flows into a secondary load (e.g. into the control terminal of the switch; reflected secondary current in primary = Is/N). The changing primary current causes a changing magnetic field ("flux") through the transformer's windings; this changing field induces a (relatively) steady secondary voltage Vs = N×Vb. In some designs (as shown in the diagrams) the secondary voltage Vs adds to the source voltage Vb; in this case because the voltage across the primary (during the time the switch is closed) is approximately Vb, Vs = (N+1)×Vb. Alternately the switch may get some of its control voltage or current directly from Vb and the rest from the induced Vs. Thus the switch-control voltage or current is "in phase" meaning that it keeps the switch closed, and it (via the switch) maintains the source voltage across the primary.
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