Different types of solid state relays (SSR) and their working

Classification of Solid State Relays (SSR)

SSR Definition: A solid state relay may be defined as an electrical switch that changes its state when an external voltage is applied to it. Solid state relay has a capability to control a larger load current or voltage.

There are mainly three types of Solid State Relays:

  1. Reed-Relay-Coupled SSR
  2. Transformer-Coupled SSR
  3. Photo-coupled SSR

Let us discuss these solid state relays one by one.

1)      Reed-Relay-Coupled SSR’s

Reed Relay Coupled SSR
Reed Relay Coupled SSR

Above circuit shows the circuit diagram for Reed Relay coupled SSR. In this type of SSR control signal is applied directly or using preamplifier (if control signal is weak) to the coil of a reed relay. This relay causes the operation of the trigger circuit which is used to trigger the power transistor or TRIAC.

2)      Transformer Coupled SSR’s

Transformer Coupled SSR
Transformer Coupled SSR

Second type of solid state relay is transformer coupled SSR. It uses a transformer for sending the control signal from primary windings to the secondary winding. Secondary winding is connected to the trigger circuit which is used for the triggering of thyristor switch. It also uses DC-AC converter if the input control signal is DC because transformer can not work with DC.

3)      Photo coupled SSR:

Photo Coupled SSR
Photo Coupled SSR

Another type of solid state relay is photo coupled SSR. Above figure shows the circuit diagram of the photo coupled SSR.In this type of SSR control signal is applied to the infrared source or light source like LED. When it emits light, a photo sensitive element like photosensitive diode, photo-sensitive transistor or a photo-sensitive thyristor will sense that light and operates the load current through trigger circuit. It is obvious that the input and output circuit are completely isolated from each other since only light is used between them. Therefore sometimes this SSR is also called as “optically coupled” or “photo-isolated” SSR.

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Synchro pair as an Error Detector

Synchro pair as an error detector

In the last article we have seen the application of Synchro as a position transducer.


Synchro pair
Synchro pair

We can also use the pair of Synchro as an error detector. Here error means the output voltage which depends upon the difference between the angular positions of two rotors of synchro pair.

As the name indicates, it uses two synchros. First synchro is called as synchro generator (or transmitter) and second synchro is called as control transformer (or receiver).

Synchro generator has dumb-bell shaped (salient pole) rotor where as control transformer contains umbrella shaped rotor. For easy of understanding consider that initially two rotors as perpendicular to each other.

Synchro pair as an error detector
Synchro pair as an error detector

When single phase AC supply is applied to the rotor of synchro generator an alternating flux is generated in the rotor and the empty space between rotor and stator. Stator has three windings, one end of each winding is connected in the star connection and ends are connected to the three ends of stator of control transformer. Remaining three ends of the control transformer are also connected in star fashion.

As alternating flux is generated in rotor of generator it produces statically induced emf in the stator windings. As these windings are connected to the three windings of the stator of the control transformer same current flows through it.

Initially the flux axis of both the rotors are perpendicular so that the output voltage (E0) will be zero because it depends upon cos (θ-α). Where θ is angular position of first rotor and α is angular position of the second rotor.

As the angle between two rotor changes output voltage also changes which is given by,

E0 = Eom cos(θ-α) sin(wt-β)


  • θ = angular position of the rotor of the generator,
  • α = angular position of the rotor of the control transformer,
  • β = phase lag due to resistances and inductances of the windings,

let us define a new angle for ease of calculations as , δ = α + 90, → α = δ – 90

so the output voltage becomes,

Eo = Eom cos[θ-(δ-90)] sin (wt-β)

Eo = Eom cos[90+(θ-δ)] sin (wt-β)

Eo = Eom [-sin(θ-δ)] sin (wt-β)              (since cos(90+A) = -sin(A))

Eo= Eom sin (δ-θ) sin (wt-β)

For small value of (δ-θ),  sin (δ-θ) ≈ (δ-θ)


E = Eom (δ-θ) sin (wt-β)

Above equation gives the value of error voltage. Thus the synchro pair can be used as error detector.

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Synchro Position Transducer Working Principle

We know that Syncro is an inductive device which works on the principle of rotating transformer. Here the term rotating transformer means the primary to secondary coupling can be changed by physically changing the relative orientation of the winding. So based on this working principle of syncro we can use it as position transducer.

Synchro pair
Synchro pair

Construction of Position Transducer:

Position transducer is one of the basic application of the Synchro. It uses dumb-bell shaped rotor. Single phase ac supply is given to the rotor of the Synchro. This rotor is mechanically coupled with the shaft of rotating element whose angular position is to be determined.

synchro as position transducer
synchro as position transducer

Position Transducer Working Principle:

We know that the stator of the synchro has three windings. These three winding of the stator are connected in star connection. Remaining ends of each winding are taken out to connect them with the voltmeter as shown in the figure. When the angle of the rotor changes the output voltage i.e. the stator voltages of each winding is given by,

E1 = Eom cosθ sin wt = instantaneous voltage for stator windings S1.

E1 = Eom cos(θ+120) sin wt = instantaneous voltage for stator windings S2.

E1 = Eom cos(θ+240) sin wt = instantaneous voltage for stator windings S3.


  • θ= angular position of the rotor
  • Eom = peak value of voltage of each winding
  • w= 2πf
  • f= frequency of the rotor
  • t = time in seconds.

All instantaneous voltages are sinusoidal in nature. But they give different values of voltages at different position of rotor.

Thus using these three values of stator voltages we can easily measure the position of the rotor. Hence Synchro can be used as a position transducer.

Applications of Position Transducer:

1) For measuring the angle of the rotating machine like antenna platform.
2) Position transducer can be used as ratary position sensor for aircraft control surfaces

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