What is Miniature Relay & its features?

The Miniature relay is an electro-mechanical switch, consisting of a coil, an armature, and contacts. 

A current passes through the coil creating a magnetic field that attracts the armature, causing the contacts to move, either making or breaking a connection. The following contacts are known by how they can be thrown:

Normally open (NO)

When the relay is activated, NO contacts connect the circuit and will disconnect the circuit when it is inactive.

Normally closed (NC)

NC contacts disconnect the circuit when the relay is activated and connect when inactive.

Change over (CO)

The contacts have a common terminal and control two circuits. One is a NO contact and the other is an NC contact. It is also known as a ‘double-throw’ (DT).

The following contact designations are also commonly encountered 

Miniature relay contacts designations

Single Pole Single-Throw (SPST)

These have two terminals that can be connected or disconnected (11-12 or 13-14) and two terminals for the coil. The terminology’SPNO’and ‘SPNC’ is sometimes used to indicate the contacts’ de-energized state (Figure 3.4a and b).

Double Pole Single-Throw (DPST)

The relay has four terminals that can be connected or disconnected. This is equivalent to two SPST switches actuated by a single coil and has two terminals for the coil (Figure 3.4c).

Single Pole Double-Throw (SPDT)

With a total of five terminals, two for the coil, a further terminal connects to two others as a CO contact (Figure 3.4d).

Double Pole Double-Throw (DPDT)

This has eight terminals with two rows of CO contacts. It is equivalent to two SPDT relays actuated by a single coil (Figure 3.4e).

Miniature relays are also subdivided by their operation:

Non-Latching (stable type)

A relay is activated when the coil is energized and turns off when de-energized. Non-latching relays are used in control applications when the switch must return to a neutral state if power is lost.

Latching type

Latching relays are used when power consumption and dissipation are limited. For example, after the initial actuation of the relay, no further power is needed to maintain the state.

  1. One Coil Latching Type
    This relay uses a pulse input to a single coil and a latching mechanism to maintain the contact as either on or off. By applying signals of opposite polarities the relay is set and reset.
  2. Two Coil Latching Type
    This relay uses a latching construction and two coils, one to set and another to reset. Setting and resetting is achieved by applying pulse signals of the same polarity.

Miniature relays are designed so that the casing does not become detached under normal use, as performance could be reduced. Consequently, the DIN rail mounting system allows simple replacement of malfunctioned relays. A PCB (Printed Circuit Board) miniature relay is illustrated in Figure 3.5a, socket mounted ‘ice cube miniature relays and their terminal designation are illustrated in Figures 3.5b and c.

figure 3.5a

Miniature relays are assembled into control cabinets with their sockets, which can in general be subdivided into two basic types:

  • Screw terminals sockets
  • screwless terminals sockets (for fast wiring). All sockets are either panel- or 35 mm rail-mounted (EN 60715). Socket’s terminals, as well as relays’ contact pins, are assigned with reference numbers, which should be used when troubleshooting to verify the contact’s condition in an energized and de-energized state.

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