The Strowger Step-by-Step Switch

Relays. A relay is defined^ref.1 as "an electromechanical device by means of which a change of current or potential in one circuit can be made to produce a change in the electrical condition of another circuit." The relays to be considered are operated by current changes.

The magnetic circuit of a relay consists of the coil core, frame or base (sometimes called "heelpiece"), and armature, all made of good magnetic materials annealed to provide a good magnetic path. The coil is wound with many turns of insulated copper wire having the desired resistance and number of turns to meet circuit requirements. In dial-telephone applications it is sometimes necessary to have relays that are slow to close or slow to release. This time delay can be produced by locating a copper slug, or sleeve, on the core. This acts as a short-circuited turn, and any change in magnetic flux linking this slug will cause current flow in the slug, which will produce magnetic flux in opposition to the flux change. A copper slug on the armature end of the coil core will make the relay slow to close and also slow to release. A copper slug on the opposite, or heel, end will make the relay slow to release but will not affect the closing time.

The contacts of the telephone-type relays are usually of platinum, palladium, silver, or other special metals on nickel-silver or phosphor-bronze springs. The spring assemblies are actuated by the movement of the armature and provide the desired circuit switching connections.

Bank Assembly. The step-by-step switch bank assembly is made up of two banks of contacts shown at the bottom of Fig. 25. The upper of the two is called the sleeve bank and consists of 100 contacts arranged in 10 horizontal rows of 10 contacts each. These contacts are arranged in a semicy-lindrical form so that a pair of spring wipers mounted on the switch shaft may make contact with any of the 100 contacts.

Figure 25. A Strowgcr step-by-step connector switch. (Courtesy Automatic Electric Co.)

The lower of the two is called the line bank and has 200 contacts also arranged in 10 horizontal rows or levels, each consisting of ten sets of two contacts, the two contacts of each set being placed one above the other with a thin insulator between them. Another pair of wipers on the switch shaft makes contact with the line bank. The two pairs of wipers are so arranged on the shaft that when the sleeve wiper is raised to a given level of the upper bank contacts, the line wiper is raised to the corresponding level on the lower, or line, bank.

Switching Mechanism. The switching mechanism of the step-by-step switch consists of the vertical, rotary, and release magnets and associated armatures which control the motion of the shaft and wipers of Fig. 25. The vertical armature, actuated by the vertical magnet, steps the shaft up one step for each dialed impulse where it is held. The rotary magnet actuates the rotary armature to step the switch in the rotary direction where it is held at the desired position. When the release armature is operated by the release magnet, the shaft and wipers are allowed to return to the normal unoperated position.