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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 amount 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. == Circuit operation == 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: * t, time, a variable * Tclosed: instant at the end of the closed cycle, beginning of open cycle. Also a measure of the time ''duration'' when the switch is closed. * Topen: instant at the end of the open cycle, beginning of closed cycle. Same as T=0. Also a measure of the time ''duration'' when the switch is open. * Vb, source voltage e.g. Vbattery * Vp, voltage ''across'' the primary winding. An ideal switch will present supply voltage Vb across the primary, so in the ideal case Vp = Vb. * Vs, voltage ''across'' the secondary winding * Vz, fixed load voltage caused by e.g. by the reverse voltage of a Zener diode or the forward voltage of a light-emitting diode (LED). * Im, magnetizing current in the primary * Ipeak,m, maximum or "peak" magnetizing current in the primary. Occurs immediately before Topen. * Np, number of primary turns * Ns, number of secondary turns * N, the turns ratio defined as Ns/Np, . For an ideal transformer operating under ideal conditions, Is = Ip/N, Vs = N×Vp. * Lp, primary (self-)inductance, a value determined by the number of primary turns Np ''squared'', and an "inductance factor" AL. Self-inductance is often written as Lp = AL×Np2×10−9 henries.〔AL represents the geometry of the coils (their length and area and separation, etc), the geometry of the magnetic path through the magnetic material (if present) -- its area and length -- the magnetic material (if present), and fundamental physical constants. Ungapped "cores" in continuous magnetic materials have AL ranging from 1000 to 10,000; gapped cores have AL ranging from 100 to 1000. Rods, "plugs", half-cores etc have AL in the 10 to 100 range. A similar formula exists for the secondary inductance Ls. For reference see the Ferroxcube "big catalog" pages 7-13 dated 2008 Sep 01. How to determine inductance of coils without magnetic material can be found in Chapter 10 Calculation of Inductance in Langford-Smith 1953:429-449.〕 * R, combined switch and primary resistance * Up, energy stored in the flux of the magnetic field in the windings, as represented by the magnetizing current Im. A more-detailed analysis would require the following: * M = mutual inductance, its value determined by degree to which the magnetic field created by the primary couples to (is shared by) the secondary, and vice versa. coupling. Coupling is never perfect; there is always so-called primary and secondary "leakage flux". Usually calculated from short-circuit secondary and short-circuited primary measurements. * * Lp,leak = self-inductance that represents the magnetic field created by, and coupled to the primary windings only * * Ls,leak = self-inductance that represents the magnetic field created by, and coupled to the secondary windings only * Cwindings = interwinding capacitance. Values exist for the primary turns only, the secondary turns only, and the primary-to-secondary windings. Usually combined into a single value. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Blocking oscillator」の詳細全文を読む スポンサード リンク
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