RELAYS

Electronic relays for generally used as switches to control circuits. These are widely used where several circuits need to be controlled by one signal. These are generally low voltage-driven devices that control high powered circuits. They are used in cases to close two circuits still electrically not connected.

Basic design and working:

The basic parts of a relay are:

• Electromagnet
• Movable armature
• Switch point contact

 

The above shown is an example of a single pole double throw relay. When the switch closes the current passes through the coil which in turn generates the magnetic field. The generated magnetic field attracts the pole and closes the circuit.In this case, when the relay is not energized the contact is open. When the relay gets energized due to the magnetic field the circuit closes thus it is a NORMALLY OPEN TYPE(NO TYPE).There’s another type called NORMALLY CLOSED TYPE( NC TYPE) Where the contact is closed even when the relay is not energized.

Types of relays based and the number of poles and throws:

Generally, we say there are 4 types of relays based on the number of poles and throws. They are:

• Single pole single throw (SPST)
• Single pole double throw(SPDT)
• Double pole single throw(DPST)
• Double pole double throw(DPDT)

SPST

A single pole single throw relay consists of a single contact point as shown. There are a total of 4 terminals for an SPST relay.

SPDT:

A single pole double throw relay consists of two contact points as shown. It has 5 terminals in total as shown.

DPST:

A double pole single throw relay is that which has two poles and each of them is single thrown as shown. It can be defined as a combination of 2 single pole single throw relays. It contains 6 terminals in total.     

DPDT:

A double pole double throw relay is that containing two double-throw relays. It, in total, consists of 8 terminal.

All these come under electromagnetic relays. In the above-shown types of poles and throws are replaced by another number, we obtain another different type of a relay.

Types of relays:

There are many kinds of relays. Most important among them are electromagnetic relays, solid-state relays, hybrid relays, reed relays, thermal relays.

SOLID STATE RELAYS(SSR):

In electromechanical relays, we find moving parts like an armature. Whereas in SSR we don’t find moving parts, instead we find the semiconductor devices which switch between on and off states. It is much more efficient than the conventional electromagnetic relays since it hasn’t got any moving parts. They work with both AC and DC.

Main parts of SSRs:

1.Coupling circuit:

This provides a channel between input and output. But does not connect them electrically. Couplingcircuits are generally optical couplers. Optical coupling provides electrical insulation. Optical couplers have high sensitivity, highest response speed, good insulation. The input drive section is generally a light-emitting diode that is an LED. A low voltage signal generally greater than 3  volts energizes this ledwhich emits the light. Also, a photosensitive transistor is placed at its output which absorbs the emitted light.

2.Triggering circuit:

 It generates the desired trigger signal to drive the switching circuit.

3.Switching circuit:

Here we employ a circuit for a purpose which is called zero-crossing control circuit. Zero crossings refer to that solid-state relay is in its on state after applying the control signal and AC voltage crossing zero after the control signal is cut off solid state relay does not switch to its off state until AC is at zero potential. this circuit prevents the interference of higher harmonics.

4.Snubber circuit:

It comprises a resistor and capacitor network which suppresses sudden rise in voltage applied to triac. The switching and snubber circuits were employed for AC solid-state relays.

Advantages:

• Fast response
• No moving parts so highly efficient
• Less power consumption
• Higher lifetime.

Disadvantages:

• Leakage current at the output.
• Much costlier.

REED RELAYS:

These generally employed where faster switching actions are needed. Also, they are small in size. They require even low voltage levels as inputs than that of conventional relays. The main principle of Reed relay working is controlling of a magnetically controlled contact under a magnetic field.

 

Reed relays consist of Reed switch that consists of reed contacts. These contacts or spaces are 0.05 to 1 mm apart from each other. This small space enables the fastest switching speeds. the materials with which these are made must be also taken care of so that they withstand High voltages. This entire assembly is placed in a glass tube with an inert gas atmosphere.

 

Principle:

When they are placed in a magnetic field the contact develops north pole and other the opposite. As the strength of the field at contact increases gradually the magnetization strength also increases. at a particular point when it starts to overcome the spring force in the contacts it starts attracting. When the magnetic field is removed contact is opened. To create a magnetic field we generally use a coil. this assembly of the about discusses the parts with the coil completes the construction of Reed relay.

Advantages:

• Small in size
• Faster switching.

Disadvantages:

• Not suitable for a large amount of current.
• Not as fast as solid-state relays.