Vibrator Units

Author: J.B. Hoag

A common source of power for portable electronic circuits is the 6-volt automobile-type storage battery. This low voltage may be stepped-up to a suitably high value by means of a vibrator and transformer unit, as shown in Fig. 11 J, after which it is rectified either by means of a diode, as in (a), or mechanically, as in (b), and then filtered in the usual manner.

Fig. 11 J. Power supply units for mobile or portable use

In (a), when the battery is first connected, the vibrating reed is drawn down by the magnet coil. A pulse of current passes through the lower half of the primary of transformer T and, at the same time, the magnet coil is shorted and hence de-energized. The reed then moves upward to close the upper circuit. But then the magnet is again energized and draws the reed down, etc. Condenser C₂ (0.005 to 0.03 μfd. 1,500 to 2,000 volt d.c. rating) absorbs the surges of current in the secondary of T, and hence serves both to protect the rectifier tube and to smooth the current. In order to prevent r.f. interference or " hash " when the circuit is used in a receiver, the entire unit must be carefully shielded in a metal can and hash filters are used. One of these consists of: r.f.c. (50 turns, No. 12 wire, half-inch in diameter) and condenser

C1 (0.5 to 1 μfd., 50 volts rating). The other hash filter consists of r.f.c. (2.5 millihenry choke) and condenser C₃ (0.01 to 0.1 μfd.).

The circuit of Fig. 11 J(a) is of the so-called non-synchronous type while that of (b) is of the so-called synchronous type. In the latter, the double-diode rectifier is replaced by an extra pair of contacts on the vibrator.