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3.1 Types of Relays
  
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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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R. Jayanthan
Last updated on 01 January, 2002