[GreenKeys] Teletype Model 15 : polar relay, pulling magnet, holdi ng magnet, loop wiring

[Harold Hallikainen]

I'll comment on pulling vs holding magnets.

With pulling magnets, the selector magnets need to pull the armature to
the pole pieces on a space to mark transition. It takes more magnetic
force to pull the armature in than to hold the armature in place. With
pulling magnets, the coils are wired in series and run with 60 mA loop
current.

With holding magnets, a cam pushes the armature to the pole pieces just
before each bit is to be sampled. Once past that cam, the armature falls
away from the pole pieces if there is no current (space) and stays if
there is current (mark). Since it takes less current to hold the armature
than to pull it, series wired coils can run at 20 mA instead of the 60 mA
required for pulling. The coils can be wired in parallel for operation at
60 mA (30 mA in each coil).

Since the inductance of the parallel coils is substantially less than
series coils, the loop voltage can be lower. I recently heard that holding
magnets were introduced to allow use of a 48 volt loop supply making it
safe for cord switchboards (since this is the same voltage used for Plain
Old Telephone Service).

I still run a high voltage 60 mA loop for everything since all machines
can be wired for a 60 mA loop.

Harold
https://w6iwi.org


On Mon, October 20, 2025 12:34 pm, til128 at juno.com wrote:
> https://www.navy-radio.com/manuals/tty/wd/m15-wiring-anon-2112.pdf
>
>
> ---------- Original Message ----------
> From: NOWTEL <nowtel at gmail.com>
> To: greenkeys at mailman.qth.net
> Subject: [GreenKeys] Teletype Model 15 : polar relay, pulling magnet,
> holding magnet, loop wiring Date: Mon, 20 Oct 2025 15:07:07 -0400
>
>
> Since I don't know anything yet, please provide detailed assistance.
>
>
> For the Teletype Model 15 that I have, how can I find information
> about the polar relay, the pulling magnet, and the holding magnet?
>
> Also, regarding the loop wiring, how can I determine if the wires are
> connected to the printer and keyboard loops separately?
>
> Thank you in advance for your help.
>
>
> 73,
>
>
> Youngluck Sheen
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>>>> Jordan Spencer Cunningham's GreenKeys Search Tool:
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>>>>
>
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>>>> Jordan Spencer Cunningham's GreenKeys Search Tool:
>>>> https://teletype.net/gksearch
>>>> 2002-to-present greenkeys archive:
>>>> http://mailman.qth.net/pipermail/greenkeys/
>>>> 1998-to-2001 greenkeys archive:
>>>> http://mailman.qth.net/archive/greenkeys/greenkeys.html
>>>> Randy Guttery's 2001-to-2009 GreenKeys Search Tool:
>>>> http://comcents.com/tty/greenkeyssearch.html
>>>>
>
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