May 2017 – APSEEE

Month: May 2017



A diac belongs to thyristor family. Once it is triggered, it starts conducting in both directions. It is similar to diode but there is a main difference between diode and diac.  The diode conducts in one direction, when it is forward biased. On the other hand, diac conducts in both directions or we can say that diac conducts both half cycle of the alternating current. That is why the diac is also known as a bidirectional avalanche diode. It is known as bidirectional avalanche diode because it is conducts only when applied voltage becomes equal or more the forward breakover voltage.

Basically, diac is a two terminal bidirectional semiconductor device that can conduct alternating current.

Di means (two terminal device)

Ac means conduct alternating current.

Construction of Diac

The material used for manufacturing of diac is silicon like thyristor. Silicon materials have many advantages.

A diac is a four layer (PNPN), two junctions (J1, J2) and two terminal (MT1 and MT2) semiconductor device.

MT1 is known as Main Terminal One.

MT2 is known as Main Terminal Two.

There is no control terminal in this device.


Symbol of Diac

symbol of diac

Working of Diac

AC supply is given to the input of circuit. Diac is connected in series with load. The current does not flow through the load because the voltage applied to the diac is less than breakover voltage and at this stage  diac remains in non-conducting state.

When input voltage exceeds the breakover voltage, the diac starts conduct and current start flowing through the load.

working of diac

I-V Characteristics of Diac

The device can be triggered by either positive or negative half of an ac cycle. When the supply voltage is less than the breakover voltage, a very small amount of current called the leakage current flows through the device as shown in figure.

i v characteristics of diac

Leakage current is not sufficient to conduct the device. The device remains practically in non-conducting mode.

When the voltage level reaches the breakover voltage, the device start conducting. The current increased exponentially at this stage.

For positive half cycle the characteristics obtained in the first quadrant.

Negative half cycle the characteristics obtained in the third quadrant.

Diac does not give controlled output like Triac.

Applications of Diac

Diac are basically used to triggering of triac. Triac and diac pairs are available in market.


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Underground Cables

Underground Cables

Underground Cables are used for transmission and electric poser in congested areas (Such as in cities, towns etc) at comparatively low at medium voltages. Usually, the underground cables used in congested cities or towns, substations, railway crossings and where safety is very important. However, recent improvements in the design and manufacture have led to the development of cables suitable for use at high voltage. In this article, we will discuss about definition of underground cable and construction of underground cables.

Define Cables

A conductor covered with a suitable insulation and protecting layer is called cable.

Requirement of Underground Cables

  • The conductor used in underground cable should be standard in order to provide flexibility. The conductor should have high conductivity so that current flows easily through them.
  • The area of cross section should be such that the cable carries the desired load current without overheating.
  • The voltage drop in conductor should be within permissible limits.
  • The cable must have proper thickness of insulation.
  • The cable must be provided with suitable mechanical protection.
  • The cable do not react with chemicals.

Construction of Cables

The cable have various parts. There are given below.


The cable consists of one or more than on core. It depends upon the typ0es of service for which it is intented. The conductors are made of tinned copper or aluminium . The stranded conductor are used in order to increase flexibility.

underground cables


Each conductor or core covered with proper thickness of insulation. Basically thickness of insulation depends upon the voltage level. The commonly used insulating materials are impregnated paper, varnished cambric etc.

Metallic Sheath

A metallic sheath is provided over the insulation in order to protect the cable from moisture, other liquids that are present in soil and atmosphere. The commonly used material for metallic sheath is lead or aluminium.


Over the metallic sheath bedding layer is provided which consists of a fibrous metarial like jute or hessian tape. Bedding protects the metallic sheath against corrosion and from mechanical injury due to armovring.


Armouring is provided over the bedding which consists of one or two layers of galvanized steel wire. It protect the cable from mechanical injury.


In order to protect armouring from atmospheric conditions, a layer of fibrous material provided over the armouring. This layer is called serving.



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Electrical Transducer

Electrical Transducer

There are number of physical or non-electrical quantities which can be measured with the help of electrical or electronic instruments. Transducer plays very important role in industries.  Basically, an instrumentation system consists of three main parts, namely input device, a signal conditioning device and an output device. The output device may be a simple indicating meter, display etc. An Electrical Transducer used in industries.

electrical transducer

The Input device is usually  known as transducer. In this article we will discuss about transducer, advantages of transducer classification of transducer.

What is Electrical Transducer?

An electrical transducer is a device that converts non-electrical or physical quantity into electrical signal or quantity.

For most of the instrumentation systems, the input quantities are non-electrical, but these quantities are required to convert into electrical. Because electrical signal or quantity can be easily measured and processed.


  • Electrical signals can be easily processed as per need.
  • The electrical signals can be converted any desired form.
  • The power required for electrical or electronic system is very small.
  • These signals can easily transmitted and processed for the purpose of measurement.

Classification of Transducers

The transducer are classified on the basis of power utilized them for operations, there are two basic types of transducers (i) Active transducer (ii) Passive transducer.

Active Transducer

Active transducers are those transducers that does not require external power for their operation. Active transducers, also called self generating transducer. In these transducers, a voltage or current produced proportional to the quantity under measurement. The example of active transducer is piezoelectric crystal. When external force is applied to piezoelectric crystal it converts mechanical energy into desired output.

Passive Transducer

Electrical transducers are those transducers that require external power for their operation. In these transducers, some parameters is varied by the measurand which in turn varies the electrical quantity such as voltage or current.

The example of such type of transducers are resistive, inductive and capacitive. These are the basic types of transducer and further classified into many types.

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Buchholz Relay

Buchholz Relay

Buchholz relay is a protective and gas actuated relay . It is used in that transformer whose winding is  immersed in oil and rating having more than 500KVA. Buchholz relay is not used in small transformer because it increase the cost. It is used to protect transformer against internal or incipient faults only. Basically, it is used for detection of winding faults.

Buchholz relay is invented by Buchholz in 1921.

Buchholz Relay Construction

Buchholz relay is installed in the pipe connecting the conservator to the main tank. There are two mercury switches used in the relay. One of the mercury switches is attached to the upper float. This switch send signal to the alarm circuit when relay operates. Whereas the other switch is mounted on lower hinged type flap. This switch send signal to the trip circuit. A release cock is mounted top of the chamber.

Buchholz Relay


Operation or Working of Buchholz Relay

When a minor fault occurs in a transformer, heat is produced, oil gets heat up. Hydrogen gas is produce in transformer. The gases being light, try to go into conservator tank but gas is trapped in Buchholz-Relay which is connected between main tank and conservator tank. It is collected in top of the chamber while passing to the conservator tank. When the predetermined amount of gas is accumulate in the upper part of the relay, oil level falls due to gas pressure. This tilt the upper float downward and closes the alarm circuit contacts through mercury switch attached to the float arm. The alarm rings bell and gives the warning signal.

In case of series fault within the transformer, large amount of gas in evolved in the main tank. In this time, lower mercury switch of the buchholz relay operates and sends the signal to the trip circuit. This completes the trip coil circuit of the circuit breaker. Thereafter, the transformer is disconnected automatically.

A release cock is provided at the top of the relay chamber so that after operation, gas pressure is released through this cock.

‘Buchholz Relay’ indicates internal faults or incipient faults in the transformer, such as inter turns fault, insulation failure. It does not respond to external faults.