RELAY OVERVIEW



 

General Information

THE TELEPHONE RELAY

INTRODUCTION
A Relay is an electrically operated switch for opening and closing circuits.  In the operation of telephone and telegraph equipment many of the complex switching operations are performed by relays.  In a large automatic exchange there maybe as many as 120,000 relays and the completion of a successful call can depend upon the correct functioning of 200 or more relays.

ELEMENTS OF A TELEPHONE RELAY
See picture above

OPERATION
When current flows through the winding a magnetic field is built up; the magnetic flux path is shown above.  The armature is attracted. to the pole face thereby operating the Lever Spring(s).  When current flow ceases the magnetic field collapses and the tension of the springs cause the armature to release.  The residual air gap prevents a closed. magnetic circuit and ensures prompt release when the operate path is disconnected.

PO 3000 TYPE RELAY
This is the standard general purpose relay and incorporates the following features:-

  1. A fixed residual stud of Phosphor - Bronze, 4., 12 or 20 mils thick (A,. B and C respectively) or an adjustable residual for more stringent adjustments.
  2. Twin contact domes are pro1ei for greater reliability.
  3. The standard contact material is Silver, used to disconnect currents up to 300 mA at 50 Volts.

    For currents up to 1 AMP, Platinum (pt) or more recently Palladium (Pd) contacts are used. These two contact materials are identified as follows:-


     
  4. Four types of contact action are available.
  5. Up to Four coil windings maybe provided.
  6. A maximum number of 9 Springs can be fitted each side of the Buffer Block.


COIL TAG IDENTIFICATION & SPRING NUMBERING


LABELLING
A guide to the adjustments of a relay is given by the PO Code Label.


If WHITE label relay has 14 MIL springs.
If GREEN label relay has 12 MIL springs.
If RED label special adjustments in some respects refer to adjustment card.

HIGH IMPEDANCE
Relays connected to Speech wires in circuits providing transmission bridge facilities must offer High Impedance to speech frequencies or they will shunt the ac signal.  To achieve this the relay must have high Inductance, this is done by fitting three Nickel-Iron sleeves.  Identified by 3N on cheek.

 

 

 
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Last revised: November 11, 2021

FM