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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.
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Polar relays on this website
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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:
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- Polar relay
- Polarized relay
- Latching relay
- Latch
- Telegraph relay
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- Bistable relay
- Impulse relay
- Keep relay
- Stay relay
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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).
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Most polar relays have 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).
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Polar relay with two coils
Move the mouse over the image to see the alternate state
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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.
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Trls-64a polar relay with seven coils
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To complicate things, some polar relays have more than two individual
coil windings. A good example is the
Siemens T.rls.64a T.Bv.3402/1,
that is used in the Siemens T-56 TCD. It has 7
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.
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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.
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.
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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 an
echo-cancelling hybrid, or fork circuit.
Here are some popular examples:
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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.
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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).
The simplified circuit diagram below shows how this works. At the left
is the input. As the data is received in serial form,
a timing unit decides when each bit is sampled. In early teleprinters this
was a rotating commutator, whilst in later machines the timing was achieved
electronically. The timing unit is started upon reception of the start bit.
In the example, we have 6 timing elements,
numbered t1 to t6. 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.
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.
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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 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. A hybrid is
an integral part of a teleprinter connection device (TCD).
As its operation is complex, it is explained on a separate page.
➤ More about hybrids
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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.
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T-109 relay tester
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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 relay types.
➤ Siemens R-109 relay tester
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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]:
The model number is always prefixed with 'T.rls.' which stands for
Telegrafenrelais (telegraph relay). It is followed by the
model number (in this case '63') and a suffix (e.g. 'a'). The model
number (e.g. '63') consists of two parts: a series identification (e.g.'6')
and a type (e.g. '3'). In addition to the model number, there is also a version number.
It is prefixed with 'T.Bv.' which stands for Telegrafen-Bauvorschrift
(telegraph assembly specification). It consists of two parts (here: '3302' and
'36) separated by a '/'. The part after the '/' specifies the coil configuration.
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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.
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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.
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| | | | Connector | Maximum switch ratings |
| Suffix | Contacts | Armature | Pins | Type | Vs | Is | Ps |
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| 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 |
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| 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 |
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Fits the old 8-pin banana socket of 4-series relays like the T.rls.43a.
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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).
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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.
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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:
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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.
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| Relay | D | Ic | Coil resistance in Ω | Examples |
| T.rls. | T.Bv. | mm | mA | I | II | III | IV | V | VI | VII | VIII | Remark |
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| 39b2 | 4/155 | ? | ? | ? | ? | | | | | | | T-36 |
| 39b2 | 4/266 | ? | ? | ? | ? | | | | | | | T-36 |
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| 43a | 4/302 | 0.1 | ? | ? | ? | |
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| 43a | 4/308 | 0.1 | ? | 800 | 800 | | | | | | | T-52 |
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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.
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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
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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.
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| | 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) |
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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).
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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.
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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.
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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.
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| Relay | D | Ic | Coil resistance in Ω | Examples |
| T.rls. | T.Bv. | mm | mA | I | II | III | IV | V | VI | VII | VIII | Remark |
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| 63a | 3302/1 | 0.11 | 12 | 120 | 120 | 120 | 120 | 26 | 26 | 2k | |
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| 63a | 3302/4 | 0.11 | 5 | 720 | 3k8 | 780 | |
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| 63a | 3302/5 | 0.11 | 1.36 | 9k | 9k | |
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| 63a | 3302/11 | 0.11 | 1.77 | 6k2 | 6k2 | 400 | |
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| 63a | 3302/18 | 0.11 | 15 | 105 | 105 | 105 | 105 | |
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| 63a | 3302/19 | 0.11 | 7.5 | 210 | 210 | |
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| 63a | 3302/21 | 0.11 | 4.68 | 1k | 1k | 3k | |
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| 63a | 3302/28 | 0.11 | 12 | 77 | 1k6 | |
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| 63a | 3302/30 | 0.11 | 17.4 | 46 | 46 | |
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| 63a | 3302/33 | 0.11 | 1.82 | 4k9 | 4k9 | |
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| 63a | 3302/36 | 0.11 | 15 | 70 | 70 | | | | | | | T-56 |
| 63a | 3302/37 | ? | ? | ? | ? | |
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| 63a | 3302/50 | 0.11 | 34.3 | 46 | 46 | |
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| 63a | 3302/55 | 0.11 | 6.25 | 900 | 900 | 900 | 900 | 900 | 900 | |
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| 63a | 3302/57 | 0.11 | 12 | 110 | 110 | 110 | 110 | 1k9 | |
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| 63a | 3302/60 | 0.11 | 15 | 70 | 70 | 50 | 50 | |
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| 63a | 3302/63 | 0.11 | 15 | 70 | 70 | 600 | |
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| 63a | 3319/63 | 0.05 | 15 | 70 | 70 | 600 | |
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| 63a | 3302/81 | 0.11 | 15 | 70 | 70 | 380 | |
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| 63a | 3302/89 | 0.11 | 6 | 470 | 740 | 1k6 | |
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| 63a | 3302/139 | 0.11 | 7.5 | 70 | 70 | |
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| 63a | 3314/11 | 0.08 | 1.77 | 6k2 | 6k2 | 400 | |
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| 63ag | 3307/20 | ? | ? | 3.8 | 315 | |
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| 63ag | 3302/55 | ? | ? | 900 | 900 | 900 | 900 | 900 | 900 | |
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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.
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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
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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.
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| | 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) |
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Equipment in which the Trls-63a is used
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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).
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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.
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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.
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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.
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| Relay | D | Ic | Coil resistance in Ω | Examples |
| T.rls. | T.Bv. | mm | mA | I | II | III | IV | V | VI | VII | VIII | Remark |
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| 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 | |
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| 64a | 3402/5 | 0.04 | 1.32 | 9k | 9k | |
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| 64a | 3402/7 | 0.04 | 1.52 | 670 | 4k9 | |
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| 64a | 3422/7 | 0.04 | 1.52 | 670 | 4k9 | | | | | | | T-109 |
| 64a | 3402/9 | 0.04 | 1.4 | 475 | 475 | |
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| 64a | 3402/14 | ? | ? | |
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| 64a | 3402/16 | 0.04 | 1.21 | 5k8 | 5k8 | 5k8 | |
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| 64a | 3402/21 | 0.04 | 1.1 | 1k | 1k | 3k | |
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| 64a | 3402/24 | ? | ? | |
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| 64a | 3402/35 | 0.04 | 0.46 | 3k | 2k5 | |
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| 64a | 3402/39 | 0.04 | 5.22 | 15k | 800 | 140 | |
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| 64a | 3402/40 | 0.04 | 2.8 | 110 | 110 | 110 | 110 | 1k9 | |
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| 64a | 3402/46 | 0.04 | 0.4 | 6k5 | 6k5 | 230 | 240 | |
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| 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 | |
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| 64a | 3402/65 | 0.04 | 1.1 | 60 | 900 | 1k8 | |
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| 64a | 3402/67 | 0.04 | 2.8 | 120 | 120 | 120 | 120 | 55 | 55 | 2k2 | |
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| 64a | 3402/73 | 0.04 | 1.1 | 1k2 | 1k2 | 3k1 | |
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| 64a | 3402/89 | 0.04 | 1.4 | 470 | 740 | 1k6 | |
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| 64ad | 3428/1 | ? | ? | 120 | 120 | 120 | 120 | 26 | 26 | 2k | |
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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.
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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
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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.
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| | 1 | Coil (with 4) | w1a |
| | 2 | ? | |
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| | 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) |
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Equipment in which the Trls-64a is used
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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.
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| Relay | D | Ic | Coil resistance in Ω | Examples |
| T.rls. | T.Bv. | mm | mA | I | II | III | IV | V | VI | VII | VIII | Remark |
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| 66a | 3602/54 | ? | ? | 250 | 15k | | | | | | | 1 |
| 66a | 3603/54 | ? | ? | 250 | 15k | | | | | | | 1 |
| 67a | 3704/96 | ? | ? | |
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| 68c | 3807/114 | ? | ? | 7k | 7k | | | | | | | 2 |
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| 92b | ? | ? | |
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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.
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Double-Pole Double-Throw switch (DPDT) (i.e. double switch).
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This relay has a neutral centre position.
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- Dr.-Ing. Fritz Schiweck, Fernschreibtechnik
Lehrbücher der Feinwerktechnik, Band 9.
CFW Leipzig (Germany), 1942.
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© Crypto Museum. Created: Thursday 12 September 2024. Last changed: Tuesday, 08 October 2024 - 07:01 CET.
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