relay logic

[Jim Beacon]

From: "Chuck Guzis" <cclist at sydex.com>



> Relay logic can depend on more than simple make/break on
> coil-current-present types.
>
> Stepping relays were used in telephone and logic systems.  I used to have
a
> telephone relay with two windings on the same electromagnet.  I'm not
sure,
> but I suspect the two might have been used as sort of a logical "AND"
> function.  Latching relays use multiple coils also.
>
There are several reasons for multiple relay windings, especially on
telephone relays:

two windings, where the sum magnetic field of both is required to operate
the relay - often used in off hook detect circuits (a and b wires both go
through relay, inductance of coils prevents speech from entering the
exchange power supply).

two windings, where one will operate the relay in a normal manner, the other
provides enough flux to hold the relay in - used in some decoders.

three windings, two operate the contacts, the third is used to couple tones
(NU, dial, engaged) to the speech pair.

there are lots of other configurations - these are the most common!

slow operate relays have a copper "slug" on the core (basically a large
shorted turn which slows the build up of flux).

slow release can either use a slug, or a resistor in parallel with the coil
(slows the collapse of the magnetic field). a longer delay can be obtained
using a diode in a similar way.

If you want the theory of relays, try Atkinson's "Telephony" (long out of
print, but many libraries have copies still).

Jim.