Vibrator Power Supply Design - Vibrator Power Supply Construction - II

Figure 59 illustrates improved means of "hash" filtering, with alternate means of supplying the required amount. Considering first the primary circuit, it will be noted that shown as dashed lines are two resistors connected from each interrupter contact pin on the socket to the reed, or ground, terminal. These are quite effective in eliminating a type of spontaneous transient "hash" of rather strong impulse strength known as "pop-hash" because of its intermittent character. A primary timing capacitance is also shown as an alternative which sometimes is effective. In other applications it is required because of the value of input voltage. The value of this capacitor, Ci, is usually between 0.1 and 0.5 mfds., but may be as large as 1.0 mfds. in difficult cases. The value of Ri will ordinarily be from 50 to 150 ohms for 6-volt applications These resistances are an additional input load upon the battery, and the minimum resistance permissible depends upon wattage size of the resistor that can be accommodated in the physical space available
The RF by-pass capacitor C3 is here shown as an improved type which has proven very satisfactory in this type of application. The unit has two tabs at each end of the foil, so that by making proper connections, the current is conducted to the capacitor foil without any intervening length of lead being interposed. Thus, no lead inductance is inserted to decrease the by-passing action. The value of this capacitor is usually 0.5 mfd., but 1.0 mfd. may be used.
The primary RF choke, CHi, may be of several types of construction. This is usually a layer-wound inductance of an odd number of layers, and because the current is high, the wire size should be rather large, generally from #12 to #16, to give a low voltage drop. The inductance, measured on a 1000-cycle bridge, will usually be from 8 to 30 micro-henries in this type of choke.
Several expedients have been developed to increase the inductance to a more favorable value, without increasing the voltage drop in the choke or increasing the physical size Powdered-iron cores, which increase the inductance by as much as three or four times, and still maintain low RF losses, have been successfully utilized. It has been found that silicon-steel laminated cores are a so satisfactory in this respect, and many chokes for interference elimination work have been made in this manner. This type of choke is increasingly effective in the frequency ranges below 500 kc. The disadvantage to such chokes is their higher distributed capacitance, which reduces their effectiveness at the higher frequencies. Special "pie" wound chokes are now available and are very effective over wide frequency ranges.
If the choke inductance is too high, it will affect the vibrator performance. A value of 50 micro-henries should be the maximum for 6-volt input systems. As the input voltage goes up, and the commutated current decreases, a larger value of inductance might be tolerated. In this connection, the relay mentioned in a previous chapter as a device for improving the starting of higher-voltage vibrators, also acts as an excellent choke.
Referring again to Figure 59, it will be noted that an individual choke, CH3, has been provided in the heater circuit to prevent the introduction of "hash" into the RF section by this means. Another choke, CH2, has been provided in the battery lead, which in conjunction with the "spark-plate" capacitor, is primarily intended to prevent the entrance of automotive ignition interference into the receiver. However, it also serves to eliminate any remaining "hash" from the battery lead which might be radiated to the antenna circuit. This "spark-plate" type of capacitor is unique in that it is usually built up of ordinary materials and is assembled directly to the chassis or case in its construction. Tin-plated sheet steel and "fish-paper," or fiber, are cut and assembled to form the capacitor. The current flows in at one end of the plates and out at the other, to eliminate any inductive effects.
The power supply unit MUST be fused to insure satisfactory service. The value of the fuse must be such that under maximum input voltage conditions and loading the fuse will not over-heat and open. Neither should the resistance be so high as to cause'an excessive voltage drop. However, it must be of low enough rating so that an abnormal load, such as might be caused by a shorted output tube, electrolytic or paper capacitor, or vibrator, will cause it to "blow" quickly.
The output RF choke, CH4, and by-pass capacitor, C6, are of standard construction. Because of the low current being handled, the choke can be a universal-wound type with small-size wire. The nominal value is about 1 milli-henry, and is satisfactory for the usual frequency range of most receivers. For high-frequency RF bands, a single-layer choke of much smaller inductance will be more effective. A mica capacitor on the output of the RF filter is often desirable for better high-frequency performance.
When the heater type of rectifier tube is replaced with the cold-cathode type, such as the Type 0Z4, additional filtering may be required. The lower corner of the diagram shows capacitors C7 added in the circuit when this tube is used. These are not always required but are occasionally very helpful. The values of such capacitors are approximately .001 mfds. or less.
Choke, CH6, and capacitor, Cs, may be required in the center-tap lead if this is not connected directly to ground. In this case a capacitor may also be used on the output side of the choke if necessary. The loads into which the filters work will often determine the location of the capacitors in the circuit.
Timing capacitor C2 and resistor R2 have been shown in the manner discussed throughout the text. This system offers the smallest value of capacitance and the fewest number of components to accomplish the required duty. However, other methods of connection can be used when required.

The connections noted at C2a and R2a are one version of the alternative arrangements. In this arrangement, the two capacitors must each be twice the capacitance of C2 to have the same over-all value in the circuit. Since they are in series, the voltage rating may be lower. A certain amount of hash-suppression is accomplished by connecting the resistor to ground from the center-point. This resistor could be eliminated entirely when the interrupter type of vibrator is used, but is used as shown with a self-rectifying vibrator. When used with the self-rectifying vibrator, this resistor prevents stored energy of the capacitor from discharging across the rectifier contacts, since the reed of the vibrator is at ground potential. It will be noted that the stabilizing action of the resistor is not present in this arrangement, as the location is not in the LC circuit.
The second alternative arrangement is that shown in the version indicated* by capacitors C26 and resistors R20. Here the resistors are placed in series with each capacitor and the center-point grounded. The same value of capacitance will be required in each capacitor as in the case of C2a. This circuit is of importance only in the case of a self-rectifying vibrator. Here the capacitors act as point-capacitance across the rectifying contacts, with the re¬sistors serving to dissipate the energy stored in the capacitors when the contacts close. However, with the resistances now in series with the parallel LC circuit, they will act as stabilizing factors.
The values for single resistors R2 and R2a should be from 5,000 to 25,000 ohms. The values for the dual resistors R2j should be from 5,000 to 10,000 ohms. The general rules for "hash-elimination" work can be summarized as follows:
1. It is advisable to originally include all possible "hash" filtering elements that are known to be of assistance. Then with the circuit free from interference, one component at a time can be changed, or eliminated, and the point at which the interference is objectionable can be easily identified.
2.    Provide complete and adequate magnetic and electrostatic shielding of the components and the complete power unit.
3.    Select the proper electrical and electrostatic grounds in both the receiver and the power unit so as to avoid all coupling and radiation between the units.
4.    Provide complete and adequate filtering in the leads to and from the power unit.
5.    Provide correct orientation of the receiver coils and transformers, where necessary, and shields if required.
Obviously, if the power unit is intended to power equipment other than radio communication, the interference filters are not required, unless nearby radio equipment is affected by interference from the unfiltered power unit. In the event of the latter condition, then the power unit must be filtered the same as if it were powering the nearby equipment.