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Relays can be categorized according to the magnetic system and operation.
Neutral Relays
This is the most elementary type of relay. The neutral relays have a magnetic
coil, which operates the relay at a specified current, regardless of the polarity
of the voltage applied.
Biased Relays
Biased relays have a permanent magnet above the armature. The relay operates
if the current through the coil winding establishes a magneto-motive force that
opposes the flux by the permanent magnet. If the fluxes are in the same
direction, the relay will not operate, even for a greater current through the
coil.
Polarized Relays
Like the biased relays, the polarized relays operate only when the current through
the coil in one direction. But there the principle is different. The relay coil
has a diode connected in series with it. This blocks the current in the reverse
direction.
The major difference between biased relays and polarized relays is that the
former allows the current to pass through in the reverse direction, but does the
not operate the relay and the later blocks the current in reverse
direction. You can imagine how critical these properties when relays are
connected in series to form logic circuits.
Magnetic Stick Relays or Permopolarized Relays
These relays have a magnetic circuit with high remanence. Two coils,
one to operate (pick up) and one to release (drop) are present. The relay
is activated by a current in the operate coil. On the interruption of the
current the armature remains in picked up position by the residual magnetism.
The relay is released by a current through the release coil.
Slow Release Relays
These relays have a capacitor connected in parallel to their coil. When the
operating current is interrupted the release of relay is delayed by the stored
charge in the capacitor. The relay releases as the capacitor discharges through
the coil.
Relays for AC
These are neutral relays and picked up for a.c. current through
their coil. These are very fast in action and used on power circuits of the
point motors, where high current flows through the contacts. A normal
relay would be slow and make sparks which in turn may weld the contacts
together.
All relays have two operating values (voltages), one pick-up and the
other other drop away. The pick-up value is higher than the drop away
value.
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