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           The Ericsson 500 Point Selector Switch

-- Exchanges based on this switch share many common elements with the 7A rotary system discussed elsewhere on this site. 
 

The LM Ericsson 500 point switch, developed in Sweden, saw worldwide usage in telephone exchanges starting from 1923 (Norra Vasa, Stockholm, Sweden). The switch required a “Register" or "Sender” controller (as in the 7A rotary and panel offices) to direct its movements based on the dialed digits. Although it had both rotary and linear movements, it wasn't a direct-dial controlled switch, making it distinct from the Strowger-type.  

It was sometimes called a pancake switch [Chronicle] because of its flat nature. It had a detached terminal frame. This feature was similar to a Strowger since the switch frame and its terminal banks could easily be separated.  

 

The switches (and their associated sequence switches) were mechanically powered by rotating shafts continuously driven by small motors, common to several racks. The motor drive, with shaft S, is apparent on the far right side on Fig 2. [Chapuis] states that this is the only telephone switch to use polar coordinates because the arm's contact position on the wire  bank is defined by an angle and a length

Circa 1919 Knut Kåell of Ericsson invented the switch with some ideas borrowed from a similar switch designed by Axel Hultman. The first Hultman design was a 10,000 line switch and it proved impractical. Mr. Kåell slimed down the Hultman design and the result was the Ericsson 500 point switch. As it matured, it was installed worldwide. 

Ericsson 500 point selector switch 1940's

Fig 1, LME 500 point selector switch,1940's

 working principles diagram of a Ericsson 500 point selector

Fig 2, working principles of a 500 point selector [Diagram]

Here is some help in deciphering Fig 2.
 

  • The electro-magnets MH and MV may engage, one at a time, the continuously rotating spindle S. Engaging MH causes CW rotation and engaging MV causes CCW rotation of TS.  

  • If the locking magnet CV is actuated, the selector is released for rotation and the rotary disc (TS), together with the contact aim, will start turning.
     

  • If the locking magnet CR is actuated, the contact arm (KA) is released for radial movement.
     

  • The wire-bank (labeled 1-25) has 20 triplets of contacts for each of 25 rotary positions. Hence the name, Ericsson 500 Switch.  The wire bank is permanently attached to the mounting frame (not shown) but the selector is removable. 
     

  • Three movable contacts are on the end of the radial arm KA. These are the typical T/R/S leads associated with a talking path. The contacts are labeled, a, b, and c. Notice that that the wire bank has 2 contacts (a, b) on one side and 1 contact (c) on the opposite side. 
     

  • An external motor powers both the linear movement of arm KA and the rotation of disc TS.  An external Register, similar to the Register in the 7A rotary system, governs their motion direction and start/stop. There is a revertive switch (not shown in Fig 2) on the selector frame that provides real-time position information to the Register. 
     

Ericsson 500 switches at Norra Vasa 1922

Fig 3, Ericsson 500 style switches at Norra Vasa, 1923, Sweden

ericsson 500 switches in a rack with radial arm rest

Fig 4, Switches mounted in a rack. Note protruding radial arm rest and terminal banks. Note motor bottom left. Source: C. Berglund, LM Ericson.

This switch had line finder, selector and final selector versions as did the 7A rotary system. For more than 50 years successive versions of AGF (Ericsson) exchanges were installed worldwide. In 1974, 5 million subscriber lines were in operation [Chapuis]. 

Today this switch is rare and the author does not know of a museum with a demo. However, Wim der Kinderen from the Netherlands has made a excellent video of the Ericsson 500 switch. In the video, Mr. Kinderen added a rotary motor to replace the normal exchange-based motor drive. Also, he used an Arduino computer to control the switch actions. The Arduino + program code mimics what the Register logic would have done for switch control in an exchange. 

 

References

Chapuis, Robert, 100 years of Telephone Switching (1878-1978), Part 1, 1982 (page 183)

Chronicle: John Meurling and Richard Jeans, The Ericsson Chronicle, Infomationsforlaget, Stockholm, 2000

Diagram: L.M. Ericsson Review, Vol 1, 1924 
 

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