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420
Radio Broadcast
FIG. 20
Another charger made on the vibrating principle
add to each other, giving the effect shown by the heavy hne. It will be seen that the attraction for the armature increases and decreases once during each cycle. This will tend tQ make the armature vibrate back and forth once per cycle, which will close the contacts at the same rate. Using proper polarities, therefore, the battery will be connected to the secondary of the transformer every time the current flows in the proper direction.
There is one point of difference between this form of rectifier and the electrolytic and gas types. These two types inherently prevent the possibility of current flowing in the wrong direction and so discharging the battery. In the vibrating form of rectifier, on the other hand, this feature must be obtained by proper adjustment. This will be understood by reference to the second curve in Fig. 18. The dotted line represents the battery voltage, and the solid curve indicates the alternating voltage. If the contacts should be allowed to remain closed until the alternating voltage is less than the battery voltage, the battery would discharge, and the time of charging would be greatly increased. The third curve in Fig. 18 shows the battery charging current with proper adjustment. In this ideal case, the vibrator is assumed to make contact at the instant that the alternating voltage equals the battery
voltage and to break contact when they are again equal.
Figs. 19 and 20 illustrate typical types of vibrating rectifiers. These are both provided with ammeters so as to show the proper direction of the current.
There is one type of rectifier on the market that automatically takes care of the proper polarity and thus prevents improperly connecting the battery to the line. A schematic diagram of this charge, is shown in Fig. 21. It will be seen that there are two coils used, one being connected directly across the battery to be charged. This coil is used instead of the permanent magnet and its polarity, of course, depends upon which way the battery terminals are connected to the charger terminals. As the armature of the vibrator always closes the circuit between the line and the battery when the sum of the attractions due to the constant pull (in this case caused by the coil connected tQ the battery) and the alternating pull is greatest, the battery will always be charged regardless of which terminals of the charger it is connected to. With this type of rectifier, therefore, it is impossible to connect the battery improperly.
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Fuse
FIG. 21
A typical circuit arrangement of the vibrating type of battery charger
The only disadvantage of this type of charger is that low battery voltages will evidently cause a small amount of attraction due to the battery magnet, and if the battery is very badly discharged, it is possible that the rectifier will not function. If the battery is kept charged, however, this trouble should not occur.
From the above, it will be readily appreciated that the advantages of a "B" storage battery can be enjoyed by all, even those located in districts served with alternating current.