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Polar relays
Latching relays - under construction

A polar relay, also known as a latching or bistable relay, is a special type of (electromechanical) electrically operated switch (relay). It maintains either contact position indefinitely without power applied to the coil(s). It can be pulse-operated, which has the advantage that it only consumes power when the relay is being switched. Polar relays can be used for a variety of applications, and are usually found in vintage teletypewriters, telegraph equipment and cipher machines.

Polar relays on this website
Original Trls-43a polar relay
T.rls.63a polar relay (A)
T-rls-46a polar relay (B) - left angle view
Nomenclature
The name polar relay comes from polarised relay and refers to a relay of which the sensitivity has been improved by placing the armature between the poles of a permanent magnet [1]. A latching relay is a relay that maintains either contact position indefinitely without power applied to the coil(s). Classic polar relays, such as the ones used in vintage telegraph equipment, can be seen as a combination of both relay types. In literature they are known under various names, including:

  • Polar relay
  • Polarized relay
  • Latching relay
  • Latch
  • Telegraph relay
  • Bistable relay
  • Impulse relay
  • Keep relay
  • Stay relay
In many cases, the contacts of the switch are named after the MARK (M) and SPACE (S) signals, as defined in the telegraphy standard. In German litarature, these signals are known as ZEICHEN (Z) and TRENNUNG (T) respectively. The COMMON (C) contact is known in German as ALLGEMEIN (A).




Operation
Most polar relays has a minimum of two coils, one for each of its two possible states. In the dia­gram below they are denoted M (mark) and S (space). Driving coil M (Im) manipulates the relay's armature to the left, and closes contact (m). The C-shaped core of the relay is pre-magnetised, so that the arma­ture sticks in the last state once the current through the coil is removed. Briefly driving a cur­rent through coil (S), moves the armature to the right and closes contact (s).

Polar relay with two coils
Move the mouse over the image to see the alternate state

Instead of using two separate coils, the above can also be realised with a single coil, or with two series-connected coils with a centre contact. In that case, all we have to do, is reverse the current through the coil(s) to force the armature to its alternate position.

Trls-64a polar relay with seven coils

To complicate things, some polar relays have more than two individual coil windings. A good example is the Siemens Trls-64a polar relay, that is used in the Siemens T-56 TCD. It has seven individual windings (w1-w7), two of which are connected in series (w6, w7). The diagram above shows the Trls-64a. In reality, the seven individual windings are wound onto a single spool.

The extra windings can be used for al kinds of features, but one of the most interesting appli­ca­tions is the use as a hybrid, or fork circuit. In a teleprinter connection device (TCD), a hybrid is used for echo cancellation. In the case of the Siemens T-56 TCD, five of the seven coils are used for this purpose. Hybrids are further discussed below.


Symbols
Below are a couple of examples of symbols that are used in circuit diagrams. The basic symbol of a polar relay is based on that of a regular relay with a diagonal line representing each individual coil winding. The double arrow symbolises the two stable states of the relay. Although single-coil polar relays exist, in most cases they internally have two coils that may be connected in series.

Examples of symbols used in circuit diagrams

In all cases it is important to observe the numbering of the coil wires, as they indicate the win­ding direction. In addition, dots may be added to show the default current direction and contact position. More complex polar relays, with more than two coil windings can be drawn as a single symbol, such as the four-coil one shown in the leftmost example below, or as separate coil pairs as shown on the right. Note that they are sometimes scattered throughout the circuit diagram.

Examples of symbols used in circuit diagrams


Usage
Polar relays are suitable for a variety of applications. They can be used as a latch when converting serial to parallel data, or as a receiving magnet (RM) in telegraphic equipment. But they can also be used as part of a so-called hybrid, or fork circuit, to eliminate the echo on a telex line. Here are some popular examples:

Receiving magnet
Polar relays are often used as the receive magnet (RM) of a teleprinter or a TCD. The circuit diagram below shows a simplified receiver path of the 4-wire double-current telex subscriber line. It is commonly shown as a single-coil polar relay, although in reality it is often contains two series-connected coils. Reversing the current through the RM alters the state of the contacts.

Receiving path of a 4-wire double-current subscriber line


Digital latch
In early teleprinters, polar relays were used to electronically convert the serial bits of a baudot character to parallel data, so that it could be printed on paper. The bits are first stored in capacitors, after which the energy from the capacitors is used to create a pulse that sets the desired state of each of the five polar relays. The first teleprinter that had this feature was the Siemens T-36, soon followed by the Siemens T-52 cipher machine (Geheimschreiber).

Reception path of the T-36 teleprinter

The simplified circuit diagram below shows how this works. At the left is the receiving line (input). As the data is received in serial form, a timing unit decides when each bit is sampled. The timing unit is started upon reception of the start bit. In the example, we have 6 timing elements, numbered 1 to 6. At t1, the first bit is sampled. Its value is stored temporarily in capacitor C1. At t2, the second bit is sampled, and so on, until all five bits are held in capacitors C1-C5.

5-bit latch (serial to parallel converter)

As t6, the 5 switches marked t6 are closed, and the charges are transferred from capacitors C1-C5 to the polar relays (Y1-Y5). At this point, switches y1-y5 represent the bit pattern of the re­ceived character. This is then used to translate the bit pattern to a character that can be printed.

Hybrid circuit
One of the most fascinating applications of a polar relay, is the use as a hybrid, or fork circuit. In telegraphy, hybrids are used to eliminate the echo on a long haul subscriber lines. They are ne­ces­sary, as otherwise the echo interferes with the transmitted cha­rac­ter, causing the wrong cha­racter to be printed on paper. In this application, a polar relay with multiple coils is used, al­lowing different parts of the circuit to contribute to its operation. Hybrids are an integral part of a tele­printer connection device (TCD). As they are complex, they are explained on a separate page.

 More about hybrids




Repair
A polar relay is an extremely sensitive device, and is difficult to maintain, repair and align. The distance between its contacts is no more than 0.04 mm (!), which as barely visible by the human eye. Opening a polar relay is therefore not encouraged and alignment is only recommended if you known exactly what you are doing. It is better to replace it by a known good one if possible.

T-109 relay tester
In the ultimate case that you decide to repair and/or adjust the relay yourself, it is recommen­ded to use a suitable relay tester, such as the Siemens 9.T.mse.109a (T-109) shown in the image on the right. It was released in 1961 and has sockets for the popular Siemens types.

 Siemens R-109 relay tester

  



Siemens polar relays
Siemens polar relays, also known as telegraph relays, are commonly found in German vintage tele­graph equipment, such as teleprinters and teleprinter connection devices (TCD). Note that there are many models, versions and variants. If you are looking for a specific relay, the fact that the shape and model number match, does not mean that you are dealing with the same relay. The full identification of a relay consists of a model number and a version number, in this format [2]:



Model
The table below shows the various types of switches that are available. The bistable relays are shown with a yellow coil. The other relays are monostable, but may optionally have a neutral centre position. All relays are available in single-pole (SP) and double-pole (DP) variants.

Type Switch Description
43 Old type bistable relay (German: Flachrelais)
63 Bistable transmission relay with high contact pressure
64 Bistable reception relay with high sensitivity
65 Monostable SPDTCO relay (default off, neutral centre position)
66 Bistable DPDT relay (double switch)
67 Monostable SPST relay (regular relay)
68 Monostable DPDTCO relay (default off)
69 Monostable DPDT relay (regular double relay)
Suffix
The table below shows the meaning of the suffix of the 6-series relays (i.e. T.rls.63, 64, etc). It consists of one or two lower-case letters. The suffix may be followed by an extra numeric suffix (usually a subscript), e.g. T.rls.63a1. The meaning of this subscript is currently unknown. The most common suffix is 'a'. The meaning of the suffix on old style relays is currently unknown.

      Connector  Maximum switch ratings
Suffix Contacts Armature Pins Type Vs Is Ps
a Platinum A Friction spring 16 Plug 50-80V 10-500mA 30 W
b Platinum A Friction spring 16 Solder 50-80V 10-500mA 30 W
c Platinum A Rigid 16 Plug 50-80V 10-500mA 30 W
d Platinum A Rigid 16 Solder 50-80V 10-500mA 30 W
e Platinum B Friction spring 16 Plug 60-200V 0.5-2A 100 W
f Platinum B Friction spring 16 Solder 60-200V 0.5-2A 100 W
g Platinum B Rigid 16 Plug 60-200V 0.5-2A 100 W
h Platinum B Rigid 16 Solder 60-200V 0.5-2A 100 W
p Silver Friction spring 16 Plug 6-220V 1mA-5A 30 W
q Silver Friction spring 16 Solder 6-220V 1mA-5A 30 W
u Silver Rigid 16 Plug 6-220V 1mA-5A 30 W
v Silver Rigid 16 Solder 6-220V 1mA-5A 30 W
w Platinum A Friction spring   8 1 Plug 50-80V 10-500mA 30 W
x Platinum A Rigid   8 1 Plug 50-80V 10-500mA 30 W
ab Gold C Friction spring 16 Plug µV-12V µA-10mA 1 W
bb Gold C Friction spring 16 Solder µV-12V µA-10mA 1 W
cb Gold C Rigid 16 Plug µV-12V µA-10mA 1 W
db Gold C Rigid 16 Solder µV-12V µA-10mA 1 W
ag Gold D Friction spring 16 Plug µV-100V µA-100mA 10 W
bg Gold D Friction spring 16 Solder µV-100V µA-100mA 10 W
cg Gold D Rigid 16 Plug µV-100V µA-100mA 10 W
dg Gold D Rigid 16 Solder µV-100V µA-100mA 10 W
wp Silver Friction spring   8 1 Plug 6-220V 1mA-5A 1 W
  1. Fits old 8-pin banana socket

T.rls.43   T.rls.39
Old style telegraph relay

Trls-43a is a polar relay that was used by the German manufacturer Siemens in telegraphy equip­ment like the T-36 teletypewriter (telex) (1931). Five such relays were used to store a 5-bit serial data word before it was printed. Before the T-36, this was done mechanically. It was also used in the Siemens T-52 cipher machine (Geheimschreiber), which had a T-36 chassis at its core. In German technical literature, they are also known as Gepolte Relais or Flachrelais (flat relay).

The image on the right shows a typical Trls-43a relay of which the protective bakelite cover has been removed. At the right is the 8-pin plug that mates with a socket inside the machine. At the center are the two coils; one for each state. At the left are the armature and the contacts.

The core of the coils (the outer ring) is mildly pre-magnetised, so that the wiper contact is held in position once the current to the coils it cut off. In practice, there were many problems with this relay, as it became unpredictable over time and had to be re-adjusted repeatedly.
  

The reliability issues plagued the T-36 teleprinter, as a result of which Siemens returned to mechanical serial-to-parallel convertors for its successor: the T-37. It also caused problems with the T-52 Geheimschreiber, in which Trls-43a relays were also used as latches. Note that there are different versions of this relay. As far as we know, the Trls-43a is used in the following devices:

Today, the surviving T-36 and T-52 machines all have problems with this relay, caused by loss of pre-magnetisation of the core and contact bouncing. Although they can be re-adjusted with the right equipment and a lot of patience, their alignment will be lost after days or sometimes even hours. For this reason, Crypto Museum has developed a solid state replacement for the Trls-43a.

 Solid state version of the Trls-43a

Versions
Below is an overview of the currently known versions of the T.rls.43. The specified current (Ic) is the typical current that should be running through the two (series connected) coils when testing the relay at a switching frequency of 25 Hz. The rightmost column shows examples of its use.

Relay D Ic Coil resistance in Ω Examples
T.rls. T.Bv. mm mA I II III IV V VI VII VIII Remark
39b2 4/155 ? ? ? ?             T-36
39b2 4/266 ? ? ? ?             T-36
43a 4/302 0.1 ? ? ?              
43a 4/308 0.1 ? 800 800             T-52
Note that is most cases, the T.Bv. number is not printed on the relay itself. As a result, it may be difficult to determine which version of the relay is used in the equipment. If you find new old stock T.rls.43a relays in their original packaging, the T.Bv. number is usually printed on the box.

If you come across a device in which one of these relays is used, and that is not listed here, please let us know. Also, if you come across other polar relays, we'd like to hear from you.


Specifications
  • Model
    T.tls.43a
  • Manufacturer
    Siemens
  • Years
    1930
  • Coils
    2
  • Contacts
    8 (7 used)
  • Resistance
    see table
  • Frequency
    150 Hz
  • Distance
    0.1 mm
  • Dimensions
    90 (108) × 55 × 20 mm
    (without bakelite cover)
  • Weight
    170 g
Connections
Below is the pinout when looking into the socket. Note that in some equipment the socket has a bridge between pins 1 and 2. This means that, in that case, the two coils are series connected.

  1 Bridge to 2 w1b
  2 Bridge to 1 w2a
  3 Unused n.c.
  4 Allgemein (A) Common (C)
  5 Coil (with 1) w1a
  6 Coil (with 2) w2b
  7 Zeichen (Z) MARK (M)
  8 Trennung (T) SPACE (S)

Trls-43a on this website
Siemens T-36 teletypewriter
The Siemens T-52 'Geheimschreiber'
Solid state reproduction of the Trls-43a relay
T.rls.63
New style transmit relay

The Trls-63a was developed in the 1950s as a transmit relay, and is much more reliable than the Trls-43a featured above. It is housed in a dustproof transparent plastic enclosure. At the bottom are two guide pins and 16 flat contacts, some of which are unused. At the top is a metal bracket. This type also known as Telegrafenrelais (telegraph relay) or Kleinpolrelais (small polar relay).

There are many different types of this relay, as shown in the table. They have between two and eight coils, each with different coil resistances. The first two coils are wired to pins 1,4 and 5,8 respectively, and are often connected in series.

A special variant of this relay – T.rls.63w – was offered as a replacement for the older T.rls.43a (and similar). Instead of the 16 flat contacts and two guide pins, it has eight 4 mm banana plugs at the bottom, arranged in the same pattern as the old style relays. This allows them to be used as a direct replacement for the T.rls.43a.
  

This relay is suitable for switching frequencies up to 100 Hz. This is less than the 150 Hz of the older T.rls.43a, but still sufficient for application in telegraph equipment. Furthermore, as the contact spacing of these relays is smaller (0.04 mm instead of 0.1 mm) and the plastic enclosure is dustproof, this family of relays is much more reliable that its predecessors. They were used in a variety of equipment, including teleprinters, cipher machines and teleprinter connection devices.

Versions
Below is an overview of the currently known versions of the T.rls.63. The specified current (Ic) is the typical current that should be running through the two primary coils (connected in series) when testing the relay at a switching frequency of 25 Hz. These relays have between 2 and 7 coils, of which the DC resistance is specified below. The rightmost column shows examples of equipment in which a specific version is used. At the bottom are some rarely seen variants.

Relay D Ic Coil resistance in Ω Examples
T.rls. T.Bv. mm mA I II III IV V VI VII VIII Remark
63a 3302/1 0.11 12 120 120 120 120 26 26 2k    
63a 3302/4 0.11 5 720 3k8 780            
63a 3302/5 0.11 1.36 9k 9k              
63a 3302/11 0.11 1.77 6k2 6k2 400            
63a 3302/18 0.11 15 105 105 105 105          
63a 3302/19 0.11 7.5 210 210              
63a 3302/21 0.11 4.68 1k 1k 3k            
63a 3302/28 0.11 12 77 1k6              
63a 3302/30 0.11 17.4 46 46              
63a 3302/33 0.11 1.82 4k9 4k9              
63a 3302/36 0.11 15 70 70             T-56
63a 3302/37 ? ? ? ?              
63a 3302/50 0.11 34.3 46 46              
63a 3302/55 0.11 6.25 900 900 900 900 900 900      
63a 3302/57 0.11 12 110 110 110 110 1k9        
63a 3302/60 0.11 15 70 70 50 50          
63a 3302/63 0.11 15 70 70 600            
63a 3319/63 0.05 15 70 70 600            
63a 3302/81 0.11 15 70 70 380            
63a 3302/89 0.11 6 470 740 1k6            
63a 3302/139 0.11 7.5 70 70              
63a 3314/11 0.08 1.77 6k2 6k2 400            
63ag 3307/20 ? ? 3.8 315              
63ag 3302/55 ? ? 900 900 900 900 900 900      
If you come across a device in which one of these relays is used, and that is not listed here, please let us know. Also, if you come across other polar relays, we'd like to hear from you.


Specifications
  • Model
    T.tls.63a
  • Manufacturer
    Siemens
  • Years
    1950
  • Coils
    2-7
  • Contacts
    16
  • Resistance
    see table
  • Current
    15 mA
  • Frequency
    100 Hz
  • Distance
    0.04 mm
  • Dimensions
    83 (96) × 39 × 28 mm
  • Weight
    170 g
Connections
Below is the pinout of the T.rls.63a when looking into the socket. Note that the 16 pins are num­bered 1-13, plus the letters Z, A and T. The lettered contacts are connected to the SPDT switch.

  1 Coil (with 4) w1a
  4 Coil (with 1) w1b
  5 Coil (with 8) w2a
  8 Coil (with 5) w2b
  Z Zeichen MARK (M)
  A Allgemein Common (C)
  T Trennung SPACE (S)

Equipment in which the Trls-63a is used
Siemens T-56 Fernschreib-Endsatz (teleprinter terminating unit) with signalling
T.rls.64
New style receive relay

Trls-64 was developed in the 1950s as a receive relay. It is the companion of the T.rls.63 and it a lot more reliable that its predecessors T.rls.43 and T.rls.54. It is housed in the same dustproof transparent plastic enclosure as the T.rls.63a transmit relay featured above. In German literature this relay is also known as Telegrafenrelais (telegraph relay) or Kleinpolrelais (small polar relay).

There are many different versions of this relay, as shown in the table. Although most versions have two coils (of which the coil resistance can be different), some versions have up to 8 coils. When known, the coil resistances are shown in the table. The two primary coils are always present and are connected to pins 1,4 and 5,8 respectively. When tested in a relay tester like the T-109, only the two primary coils are used.

In the T-56 teleprinter connection device, The 7-coil T.rls.64a — T.Bv.3402/1 version, is used as an echo-cancelling hybrid, also known as a fork.
  

This relay is suitable for switching frequencies up to 100 Hz. This is less than the 150 Hz of the older T.rls.43a, but still sufficient for application in telegraph equipment. Furthermore, as the contact spacing of these relays is smaller (0.04 mm instead of 0.1 mm) and the plastic enclosure is dustproof, this family of relays is much more reliable that its predecessors. They were used in a variety of equipment, including teleprinters, cipher machines and teleprinter connection devices.

Versions
Below is an overview of the currently known versions of the T.rls.64. The specified current (Ic) is the typical current that should be running through the two primary coils (connected in series) when testing the relay at a switching frequency of 25 Hz. These relays have between 2 and 8 coils, of which the DC resistance is specified below. The rightmost column shows examples of equipment in which a specific version is used. At the bottom are some rarely seen variants.

Relay D Ic Coil resistance in Ω Examples
T.rls. T.Bv. mm mA I II III IV V VI VII VIII Remark
64a 3402/1 0.04 2.8 120 120 120 120 26 26 2k   T-56
64a 3402/4 0.04 1.16 720 3k8 780            
64a 3402/5 0.04 1.32 9k 9k              
64a 3402/7 0.04 1.52 670 4k9              
64a 3422/7 0.04 1.52 670 4k9             T-109
64a 3402/9 0.04 1.4 475 475              
64a 3402/14 ? ?                  
64a 3402/16 0.04 1.21 5k8 5k8 5k8            
64a 3402/21 0.04 1.1 1k 1k 3k            
64a 3402/24 ? ?                  
64a 3402/35 0.04 0.46 3k 2k5              
64a 3402/39 0.04 5.22 15k 800 140            
64a 3402/40 0.04 2.8 110 110 110 110 1k9        
64a 3402/46 0.04 0.4 6k5 6k5 230 240          
64a 3402/56 0.04 5.6 150 150 150 150 280 280 280 280  
64a 3402/59 0.04 2.9 62 62 2k2            
64a 3402/65 0.04 1.1 60 900 1k8            
64a 3402/67 0.04 2.8 120 120 120 120 55 55 2k2    
64a 3402/73 0.04 1.1 1k2 1k2 3k1            
64a 3402/89 0.04 1.4 470 740 1k6            
64ad 3428/1 ? ? 120 120 120 120 26 26 2k    
If you come across a device in which one of these relays is used, and that is not listed here, please let us know. Also, if you come across other polar relays, we'd like to hear from you.


Specifications
  • Model
    T.tls.64a
  • Manufacturer
    Siemens
  • Years
    1950
  • Coils
    2-8
  • Contacts
    16 (7-16 used)
  • Resistance
    see table
  • Current
    2.8 mA
  • Frequency
    100 Hz
  • Distance
    0.04 mm
  • Dimensions
    83 (96) × 39 × 28 mm
  • Weight
    170 g
Connections
Below is the pinout of the T.rls.64a when looking into the socket. Note that the 16 pins are num­bered 1-13, plus the letters Z, A and T. The lettered contacts are connected to the SPDT switch. The two primary coils are always present and are always connected to pins 1,4 and 5,8 respec­tively. The use of the remaining pins depends on the configuration of the selected relay.

  1 Coil (with 4) w1a
  2 ?  
  3 ?  
  4 Coil (with 1) w1b
  5 Coil (with 8) w2a
  6 ?  
  7 ?  
  8 Coil (with 5) w2b
  9 ?  
  10 ?  
  11 ?  
  12 ?  
  13 ?  
  Z Zeichen MARK (M)
  A Allgemein Common (C)
  T Trennung SPACE (S)
Equipment in which the Trls-64a is used
Siemens T-56 Fernschreib-Endsatz (teleprinter terminating unit) with signalling
Other models
The other relay types (model 65 onwards) are rarely used in telegraph equipment, and are there­fore not further described on this page. For completeness however, the known models and ver­sions are listed in the table below. The sepcified current (Ic) is the typical current that should be running through the two (series connected) coils when testing the relay at a switching frequency of 25 Hz. The rightmost column shows examples of equipment in which this relay type is used.

Relay D Ic Coil resistance in Ω Examples
T.rls. T.Bv. mm mA I II III IV V VI VII VIII Remark
66a 3602/54 ? ? 250 15k             1
66a 3603/54 ? ? 250 15k             1
67a 3704/96 ? ?                  
68c 3807/114 ? ? 7k 7k             2
92b ? ?                    
If you come across a device in which one of these relays is used, and that is not listed here, please let us know. Also, if you come across other polar relays, we'd like to hear from you.

  1. Double-Pole Double-Throw switch (DPDT) (i.e. double switch).
  2. This relay has a neutral centre position.

Literature
  1. Dr.-Ing. Fritz Schiweck, Fernschreibtechnik
    Lehrbücher der Feinwerktechnik, Band 9.
    CFW Leipzig (Germany), 1942.
Documentation
  1. T-Relais 63 und 64 - Prüf- und Einstellvorschriften
    Test and calibration procedures (German).
    DK 621.318.562.08. Deutsche Bundespost, May 1972.

  2. Prüfvorschriften fur Telegraphenrelais (incomplete)
    Siemens & Halske, undated.

  3. Relays T rls 63a, 64a and 67c - Supervisory regulation (Swedish)
    Försvarets Materielverk, Teknisk Order MT 857-176. 26 April 1976.

  4. Die gepolten Relais T rls 63...69
    Reprinted from Postleitfaden Band 6 - Fernmeldetechnischen Atlas.
    Rel 3101. Siemens & Halske. Undated (but pre-1966).
References
  1. Wikipedia, Relay
    Visited 12 September 2024.

  2. Wolfram Zucker, Kleinpolrelais von Siemens
    26 June 2019. Visited 26 September 2024.
Further information
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© Crypto Museum. Created: Thursday 12 September 2024. Last changed: Friday, 27 September 2024 - 10:37 CET.
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