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Polar Standards
Solid-state replacement for polar telegraph relay Trls-43a
Crypto Museum, Marc Simons, December 2024
This article describes a fully electronic replacement for the
Trls-43a electromechanical polar relays
of the Siemens T-52 Geheimschreiber.
Five such relays are used in the T-52 to latch the last decrypted
(5-bit) character prior to printing.
These relays are a the most significant cause of problems with the T-52,
as their operation relies on pre-magnetisation of the cores of their coils.
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In the T-52, five polar relays are used to
latch the last received (5-bit) teletype character. They are driven by
short pulses from five capacitors that act as a memory.
The timing of a polar relay is extremely critical. If
it reacts too slow, or too quick, the charge that is held in the
capacitor (i.e. one of the 5 bit-values) may not be sufficient
to alter the state of the internal SPDT switch.
The timing of the Trls-43a relay depends on the distance between the
arm that controls the SPDT switch and the pre-magnetised core
at either side, and also on the level of pre-magnetisation.
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Although the timings are well-documented and the contact distances
can easily be adjusted, the level of pre-magnetisation has deteriorated
over the (more than 80) years since they were manufactured.
This makes the Trls-43a unreliable at best. Even when they are
re-adjusted, they are likely to fail again within days or even hours.
Although replacement Trls-43a relays can be found from new-old-stock (NOS)
surplus, they will be equally old and, hence, equally unreliable.
This problem can be overcome, by replacing the old deteriorated Trls-43a
relays with modern electronics. This provides a guaranteed timing and has
no contact and/or pre-magnetisation issues. A suitable solution, that can
be fitted inside the existing bakelite enclosure of the relay, is described
below. It requires a small (reversible) modification to the wiring of the
T-52 — to provide the power supply for the electronic circuit — but is
otherwise fully transparent.
The same solution can also be used for the restoration of a
T-36 teletypewriter, which uses similar relays.
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UNDER DEVELOPMENT —
Please note that this page describes a project that is currently in
progress.
A polar relay is also known as a bistable relay. It can be set to one
of two positions – denoted as (A) and (B) – and retains that position
when the relay core is no longer powered. In most cases it consists
of two electromagnetic coils with a pre-magnetised core, that activate
a SPDT switch.
By applying power to one of the coils — A or B — the SPDT switch can be
placed in either position (a or b). This situation is shown in the leftmost
diagram below. In most cases the coil will be activated with a short pulse
(rather than a continuous voltage). This places the contact in the desired
position. Because the core of the corresponding coil is pre-magnetised,
the contact will be held in that position, even when the pulse has meanwhile
dissappreared. Applying a pulse to the other core, places the contact in the
alternative position, whilst its pre-magnetised core keeps it there.
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Two individually driven coils
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Two series connected coils
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The two coils inside the polar relay can be driven individually, as shown
in the leftmost diagram above. In that case, one side of the coils is commonly
connected to the 0V rail (ground). In many cases however, the coils are connected
in series, so that the same current flows through both coils, allowing them
to assist each other. This results in a stronger 'push-pull' response.
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The diagram below shows the internal circuit of the Trls-43a polar relay
used in the T-52. At the left is the single-pole double-throw (SPDT)
switch, which has three terminals: the common contact (c), the contact
corresponding to coil A (a) and the contact corresponding to coil B (b).
At the right are the two coils, which are wired individually to the terminals
of the relay (1,5 and 2,6).
Each coil is specified for a nominal voltage of 30V. In the T-52 they are
connected in series, so that they can be driven with -60V or +60V. This is
done by means of a shorting bridge between pins 1 and 2 of the relay socket.
The shorting bridge is part of the internal wiring of the T-52.
In the original documentation of the T-52 the two states of the SPDT
switch inside the polar relay – here denoted as (a) and (b) – are called (+)
and (-), as they correspond to the +60V and -60V voltages of the
telegraphy line.
They represent the logical states (1) and (0) respectively.
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Below is the block diagram of the
Siemens T-36 teletypewriter, as it was
introduced in 1931. The upper half shows the transmitter path, whilst the
lower half shows the receiver. When receiving a character, the serial data
signal is first converted into individual bits by means of a rotating
commutator. As the bits are received sequentially (rather than in
parallel), they are first stored in a 5-bit memory that consists of five
capacitors. As the energy that is stored in the capacitors is not enough
to drive the printing mechanism, the bits are first latched in five
polar relays (R1-R5).
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Block diagram of T-36 teletypewriter
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The five polar relays are located in the red circle.
The energy from the capacitors create a short pulse, just enough to
activate the relay. As the charge in the capacitor can be positive (+60V)
or negative (-60V), the relay will be put in the (a) or (b) position
respectively.
In this single-ended configuration,
all five polar relays share a common line which
is connected to the 0V rail (GND).
➤ More about the T-36
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Use in T-52 Geheimschreiber
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Below is the block diagram of the
Siemens T-52 cipher machine (Geheimschreiber).
Although the T-52 is much more complicated than the
T-36, the basic elements are the same,
as the device is built around a T-36.
In the diagram below, the five polar relays that form the latch, are located
in the red circle.
In this situation, the polar relays do not share a common line.
Each relay is driven differentially (on both wires).
This is also known as a double-ended or balanced configuration.
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Block diagram of T-52d shown in cipher mode
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The diagram below shows how the solder pads on the PCB are wired to
the relay plug. The diagram shows the component side of the PCB and
the solder side of the 8-pin relay plug.
The specified colours are for reference only. They match the colours
in the photographs.
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+ Black Power +6 to 14V DC - Red Power 0V (ground)
Rb white Relay contact (b) Rc bl/wh Relay contact (c) Ra blue Relay contact (a)
RA pink Coil A RB grey Coil B
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The diagram below shows a practical setup for testing these polar relays —
both the original T-rls-43a and the electronic replacement.Use a signal generator as the source, and set it up to produce a 70Hz square wave with
a peak-to-peak value of 10Vpp. This signal is then fed – via a 50Ω
resistor – to the secondary side of a 220V/10V transformer. The primary
side of the transformer is then connected to the coil of the relay.
The voltage over the coil should be around 60Vpp.
Note that the pinout of the polar relay is as seen from the top (i.e. when
looking into the socket).
Connect the two contacts of the switch – (1) and (2) – to the inputs of
a 2-channel oscilloscope, and connect the source (T) to the trigger input
of the oscilloscope. The two images below show the result of the above
measurement for an original and an electronic variant of the T-rls-43a
respectively. The yellow line represents the leftmost contact (1).
Green is the rightmost contact (2).
The line at the top (purple) shows the signal source which is used as the
trigger signal (T).
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The image below gives an impression of our small T-rls-43a factory.
For this projects we used 50 complete in-house assembled PCBs,
which were then mounted onto the metal frame of an original
– disassembled – T-rls-43a relay. The latter are in short supply.
We are still looking for more.
At the rear is the oscillosope on which
the signals of the relay currently under test are shown.
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Relay socket before modification
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RC Coil common (loop to 2) RC Coil common (loop to 1) - not connected Rc Relay contact (common) RA Coil A RB Coil B Rb Relay contact (b) Ra Relay contact (a)
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Relay socket after modification
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0V Ground (0V power) 0V Ground (0V power) +V Power +6 to +15V DC Rc Relay contact (common) RA Coil A RB Coil B Rb Relay contact (b) Ra Relay contact (a)
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© Crypto Museum. Created: Wednesday 07 August 2024. Last changed: Monday, 23 June 2025 - 16:25 CET.
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