jim at g1jbg.co.uk
Wed Jan 4 09:39:52 CST 2006
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
> telephone relay with two windings on the same electromagnet. I'm not
> 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
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).
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