CONVERTERS RINGING
No's 4, 5, 6 & 7

1. Scope of Instruction
This Instruction describes the Converters, Ringing, Nos. 4 to 7, which are static devices for converting single-phase a.c. mains to a 25 c/s a.c. supply for the purpose of ringing magneto bells at P.B.X.s.

2. General
Table 1 gives details of size, fixing centres and approximate weight of the items together with the drawing and diagram references and bell ringing capabilities.

The output of the Converters, Ringing, Nos. 4, 5 and 6 is a nominal 60v, and the output of the No. 7 a nominal 50v.  Each converter will ring up to four Bells No. 59A in series with a 2uF capacitor.

Bell No. 64E, which has a loud-sounding gong, works very efficiently with these converters.  Bell No. 67A, which has a heavy movement, also responds reasonably well if a 2uF capacitor is connected in series.

The numbers of Bells No. 59A, each connected in series with a 2uF capacitor, which can be rung in parallel by the output of the converters is given in Table 1.

3. Operation
Consider the converter transformer as having two cores with identical secondary windings and a common primary winding.  These cores saturate in turn on alternate positive half cycles of the half-wave rectified mains supply.  During the time a core is saturated no energy is transferred to the output. During the time a core is not saturated (i.e. it is working normally) a pulse of energy is transferred to the output.  The saturation pulses are at 50c/s. The secondary windings are connected in series opposition relative to the primary winding, therefore these output pulses will be alternately in opposite senses, giving rise to an output at a frequency of 25c/s.  The optimum value of the capacitor which is connected to the secondary windings of the transformer is determined by the behaviour of the converter at low mains-input voltage and the starting of oscillations on load.  Unlike a normal LC resonant circuit, the value of this capacitor does not influence the frequency of the output current.

When the loading on the output terminals is increased beyond full load, the output voltage falls rapidly and the production of 25c/s ceases.  On removing the excess load, the oscillations restart.  This provides a useful self-protecting feature.

4. Uses
The Converter, Ringing, No. 4 is for use at P.B.X.s with up to four operators' positions, as laid down in General, S 3801.

The Converter, Ringing, No. 5 is for use at P.B.X.s where there are 5 to 20 operators' positions or at 20-line Multiphone switchboards.  Electrically, it can be considered as the equivalent of the Converters, Ringing, Nos. 2A and 2B.  Physically, however, there is some difference as both the No. 4 and No. 5 are for wall-mounting.  By the use of two Mountings D 91065, a Converter, Ringing, No. 4 or No. 5 may be mounted on a standard 19in, rack, such as Racks, Apparatus, No. 42.

The Converter, Ringing, No. 6 is for use at P.B.X.s where there are more than 21 operators' positions or at Multiphone switchboards having 21 to 60 lines. It can be considered as an equivalent of the Converter, Ringing, No. 3.  Physically there are some differences and the Converter, Ringing, No. 6 may be mounted on a wall or direct on a standard 19in rack.  The Converter, Ringing, No. 7 is for use at non-multiple installations.

5. Installation
The subscriber will provide a standard 3-pin B.S. wall-socket adjacent to the position for mounting the converter, mains energy free of charge to the Department and an efficient mains earth connexion.  For the connexion from this supply to the unit, Cord, Flexible, 250V, T.R.S. or P.V.C. 3-core, 0.0006 sq. in. should be used.

Modern equivalents to the GPO Ringers

A Power Unit No. 50A with a Converter Ringing No. 7 fitted to the front left
Both covers have been removed for clarity

Last revised: April 10, 2024