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Astable Blocking Oscillator; With small variations of the Monostable Blocking Oscillator circuit, it can be provided to work into astable mode. The astable blocking. A blocking oscillator is a simple configuration of discrete electronic components which can .. ff and “The astable blocking oscillator” p. ff. Figure shows the diagram of an astable blocking oscillator with an. RC circuit in series with the base of the transistor. This circuit includes, in addition to the.

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Now, the current flows through the capacitor which lets the capacitor discharge.

This is done to simplify the presentation of the material. A resistance R is connected in series to the base of the transistor which controls the pulse duration. The more necessary is the Zener diode across the emitter junction. Induction into the secondary will be via the secondary turns through which astabble mutual linked flux passes; this induction causes voltage to appear at the secondary, and if this voltage is not blocked e. Control voltage or current to the switch is now free to “flow” into the control input and close the switch.

The name is derived from the fact that the transistor or tube is cut-off or “blocked” for most of the duty-cycleproducing periodic pulses. Because of the transformer’s “winding sense” direction of its windingsthe voltage that appears at the secondary must now be negative. Calvert Created 25 November Last revised 28 April The transformer seemed the most appropriate of those I had on hand, but may not be optimum.

The secondary winding of the transformer can be fed to a speaker, a lamp, or the windings of a relay. This turns the transistor Q 1 OFF. A pulse-shaping circuit is shown at oscillaotr output.

Now, the diode which was ON, has some voltage across it, which gets applied to the transformer primary, which is induced into the secondary. The transformer is phased so that any increase in collector current pulls the base up, further increasing the current. Look at the output waveform from transistor Q2, as shown in figure In reality these waveforms do have rise and fall times transient intervals. A steady state exists when circuit operation is essentially constant; that is, one transistor remains in conduction and the other remains cut off until an external signal is applied.


To initiate the circuit, a negative trigger pulse is applied at the collector. Square and rectangular waveforms are normally used to turn such circuits on and off because the sharp leading and trailing edges make them ideal for timing purposes. This cumulative action, throws the transistor into cut off condition, which is the stable state of the circuit. For a 6SF5, only a couple of volts will do. At this point the cycle repeats itself.

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 transformer primary and the diode are connected in the collector.

We know that the collector current is. A high-voltage transistor such as the MPSA06 avoids any problem with collector breakdown, but the strong ringing is not desirable.

Pulse and Digital Circuits by Manmadha Rao G., Rama Sudha K., Venkata Rao K.

Anyways, this disadvantage can be eliminated if the resistor is placed in emitter, which means the solution is the emitter timing circuit. This is easier to see when a capacitor “commutates” the control voltage or current; aastable ringing oscillation carries the control voltage or current from negative switch open through 0 to positive switch closed.

The flow of current through collector further lowers the collector voltage and through inductive coupling of pulse Transformer increases the base voltage.


This feature of blocking the output while being an oscillatorgets the name blocking oscillator to it. When an input signal to one amplifier is large enough, the transistor can be driven into cutoff, and its collector voltage will be almost V CC.

In this osciillator condition the transformer’s flux will be steady.

Blocking oscillator

This magnetizing current I m will “ride upon” any reflected secondary current I s that flows into a secondary load e. To have a clear understanding, let us discuss the working of Diode controlled Astable blocking oscillator.

No extra components are necessary, since the tube can tolerate large negative swings of the grid with no difficulty whatever.

In blocing third case, the magnetic “core” material saturates, meaning it cannot support further increases in its magnetic field; in this condition induction from primary to secondary fails. At T1 this voltage abruptly falls essentially no time used to its previous static value. As the left side of C2 becomes more negative, the base of Q1 also becomes more negative. Many electronic circuits are not in an “on” condition all of the time.

Figure shows a breakdown of the square wave and is the figure you should view throughout the square wave discussion.

Blocking oscillator – Wikipedia

Unfortunately, at the same time this reduces the reverse breakdown voltage of the junction to only about 5 V. The negative alternation could be represented as the longer of the two alternations. This section will discuss basic periodic waveforms. Explain the operation of a sawtooth generator.

The usual cure for turn-off transients, a diode across the offending inductance, does not work here because it destroys the necessary inductive action.