Motion Picture News (Oct-Dec 1930)

October 11, 1930 Motion Picture News 49 THE* Projectionists' Round Table THE impedance characteristic of the series resonant circuit is untilized in practice to bypass current around some other unit in the associated system. This characteristic can be illustrated by showing the relation between frequency and impedance and between frequency and current flow. Take as the basis, a circuit such as that shown in Figure 63A. The individual reactance variations of L and C are shown in Figure 64, illustrating that at some value of frequency. ioohms ^VW\_ lOO MfD. 10 HENfcYS u t = ioov 60 CYCLES FIG. 63 A Xl = Xc, and the total impedance is that of the resistance R. For all other frequencies higher and lower than resonance, the remainder of Xl — Xc is some finite value, either negative or positive in design, depending upon whether the capacity reactance or the inductive reactance predominates. This impedance characteristic is best illustrated by means of the curve shown in Figure 65. Here we find the impedance plotted on the ordinate axis and the frequency plotted on the abscissa. The latter is classified in three ways, "Resonance" at the point O, and a range "lower" than resonance and a range "higher" than resonance. At resonance the impedance is lowest, but it does not reach zero because of the resistance R illustrated in the original circuit. For some frequency either side of this resonant point the impedance shows an increase. Now, if this curve is inverted, it can be used to indicate the current w,hich would flow through that circuit when it is connected to a source of constant voltage but variable frequency. This is shown in Figure 65A. The ordinate now bears the current designation and the abscissa bears the frequency designation. The latter is identical to that shown in Figure 65. If these two curves are associate, the relation between impedance and current flow becomes evident, and the same is true of the relation between current flow and frequency. The current is maximum at resonance and decreases for frequencies above or below resonance, although the voltage at the source remains constant at call frequencies. Now this curve is of value when we consider the application of the series resonant circuit to bypass current around another element, as for example in figure 65B. Here we note a source of voltage, this time the phonograph pickup unit, indicated as PH. This device is connected to the primary of the input transformer connected to the amplifier. The primary winding is designated at TP. If we assume that this generator (PH) causes the flow of current through TP at various frequencies and we desire to reduce the current through TP at one frequency, we can connect a series resonant circuit LC in shunt with TP and realize upon the operation shown in Figure 65A. That is to say, the current flowing through LC, due to the voltage generated in PH, will ^By John F. Rider* depend upon the frequency of the voltage. If it is the resonant frequency and the impedance of LC is lower than that of TP at that frequency, most of the current will flow through LC. For frequencies higher and lower than the resonant frequency, the bypassing effect of LC will decrease. If the frequency of the voltage is sufficiently far removed from the resonant frequency of LC, the presence of LC will have no effect upon the current passed through TP. Thus, the series resonant circuit will serve to filter one frequency and keep it out of the associated circuit. Such is the purpose of the scratch filter and, as a matter of fact, such is the usual design of the scratch filter. Now, if we assume that the circuit LC in Figure 65B has no resistance, it constitutes a practical short circuit upon TP at the resonant frequency, be that value 500, 2,400 or 8,000 cycles, and no current flows through TP at that frequency. Sometimes it is not necessary, in other applications, to bypass all current. In some instances, as for example in speaker circuits, the required condition is the reduction of the current through speaker winding, thereby reducing the volume of sound at that frequency. Perhaps this reduction is 50 per cent. In other words, because of the characteristic of the amplifier or the speaker, the normal current through the FReatUENcr R<*.G4 speaker must be reduced from .005 ampere to .0025 ampere. Such a series resonant circuit enables the reduction of the current at the predetermined frequency without interfering with the current through the speaker at other frequencies. In such cases, it isnecessary that the resonant circuit have the correct impedance with respect to the impedance of the speaker coil so that the required division of current take place. Such a condition is shown in electrical form in Figure 65C, where R will represent the impedance in ohms of the filter circuit and Rl will represent the impedance of the speaker coil. By correctly apportioning the filter impedance, any division of current may be secured. The correct impedance for the filter system is secured by the use of a separate variable resistance R in Figure 65D, or if the D.C. resistance of L is sufficient, by utilizing this resistance as the resistance R. Thus, if the impedance of the speaker coil is 10 ohms at 5.000 cycles and the current is as stated, the value of the shunt impedance to reduce the speaker current at 5,000 cycles or .0025 ampere would be 10 ohms. An approximation of the division of the current between the impedance of the load and the impedance of the filter can be secured from the simple relation. Zr ItX Zr-f-Z where Zr is the impedance of the filter and Z is the impedance of the load. It is the original total current which flowed through the speaker before the filter was applied. Thus if It is .005 ampere, Z is 10 ohms and Zr is 10 ohms, the current through the speaker coil is 10 .005 X = .0025 ampere 10+10 There is another means of applying the series resonant circuit for the purpose of controlling the current feed into a circuit. This is shown in Figure 65E. Here the circuit is in series with another element and, while the impedance characteristic of the resonant circuit remains as before, the effect of its use is changed. As was shown in Figure 65A, the current through the circuit is maximum at resonance. Hence is LC in Figure 65E are of such values that the presence of the other winding has no effect upon resonance, the LC circuit will allow the flow of current at resonance into the winding, but will reduce the current at all other frequencies. Voltage Step-Up in Series Resonant Circuit. — -As to the property of voltage step-up secured with the series resonant circuit, it is applied in certain audio amplifiers to secure better or greater response upon one frequency or a narrow band of frequencies. It is a bit premature to discuss this subject at this time, but more detailed data will be published supplementary to what is included at this time. The circuit employed in the audio amplifier is like that shown in Figure 63A, except that the input circuit of the amplifying tube is connected across the inductance L, as in Figure 65F. The voltage is secured from some source of variable frequency. By proper apportionment of the values of C and L, the voltage across L is maintained substantially constant LOWER t HIQHEPR.ESONANCE FK5.65A over a large range of frequencies higher than the resonant frequency of LC. At resonance there is available across L, some value of voltage in excess of that available at E, so that This is Lesson 20 in The Rider Series on Sound Projection