# Solid State Relays (Static Relays)

- Link: https://studyelectrical.com/2016/10/solid-state-relays-static-relays.html
- Published: 2016-10-06T18:34:00+00:00
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## What is a Static Relay?

There are two basic classifications of relays – **Electromechanical Relays and Solid
State Relays (Static relay)**. One main difference between them is electromechanical
relays have moving parts, whereas solid-state relays have no moving parts. 

**Solid State Relay (Static Relay) is an [electrical relay](https://studyelectrical.com/2016/03/types-classification-of-electrical-relays.html)
in which the response is developed by electronic/magnetic/optical or other components,
without mechanical motion of components.**

![Solid State Relay](https://studyelectrical.com/wp-content/uploads/2016/10/Solid-
State-Relay.jpg)

40 A Solid State Relay

A solid state relay is composed of both static and electromechanical units in which
the response is accomplished by the static units. Therefore solid state relays are
also called as static relays.

In static relays, the measurement is performed by electronic/magnetic/optical or
other components _without mechanical motion_. However additional electromechanical
relay units may be used in output stage as auxiliary relays. _A protective system
is formed by static relays and electromechanical auxiliary relays._

### **Working of Solid State Relay**

The figure shows the essential components in a static relay. The output of Current
Transformers or Potential Transformers or transducers is rectified in the rectifier.

![Components of Static Relay](https://3.bp.blogspot.com/-gdSB0XiD7iI/V_aIQqGJhhI/
AAAAAAAACCM/K1yJxQpCTGgX7fun2FnEcO9PX0K330dIACLcB/s1600/zzz.jpg "Components of Static
Relay")

Components of Static Relay

The rectified output is fed to the measuring unit. The measuring unit comprises 
comparators, level detectors, ﬁlters, logic circuits. The output is initiated when
the input reaches the threshold value.

The output of the measuring unit is ampliﬁed by Ampliﬁer. The ampliﬁed output is
given to the output unit which energizes the trip coil only when the relay operates.

In conventional electromagnetic, the measurement is carried out by comparing operating
torque/force with restraining torque/force. The electro-mechanical relay operates
when operating torque/fence exceeds the restraining torque/force. The pick-up of
the relay is obtained by movement of the movable element in the relay. In a static
relay, the measurement is performed by static councils.

A simpliﬁed block diagram of a single input static relay is given in the above figure.
In individual relays, there is a wide variation. The quantities: voltage, current.
etc. is rectified and measured. When the quantity to be measured reaches certain
well-deﬁned value, the output device is triggered. Thereby current ﬂows in the trip
circuit of the circuit-breaker.

The figure below gives a block diagram of a microprocessor-based digital, programmable
static relay.

[⌊Microprocessor based digital static relay⌉⌊Microprocessor based digital static
relay⌉[

Microprocessor-based digital static relay

Static relays can be arranged to respond to electrical inputs. The other forms of
inputs such as heat, light, magnetic field, traveling waves, etc. can be suitably
converted into equivalent analog or digital signals and then fed to the static relay.

The multi-input static relay can accept several inputs. The logic circuit in the
multi-input digital static relay can determine the conditions for relay response
and sequence of events in the response.

A programmable protection and control system has a microprocessor or microcomputer
in its circuit.

With the help of the logic circuits and the microprocessor, the integrated protection
and control system can perform several functions of data acquisition, data processing,
data transmission, protection, and control. Earlier, for each of these functions,
separate electromechanical or static units were used along with complex wiring.

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