The Zener Diode

Electronic Devices plays an important role in sensors and controllers which are the two eyes of instrumentation and control engineering. So i am slowly going to discuss about the important electronic devices in our blog.

Zener diode is a specially designed PN junction diode. a reverse biased, heavily doped PN junction diode which is operated in the breakdown region is known as zener diode. It is also called a voltage regulator diode or breakdown diode.

Construction of Zener Diode:

zener-diode-symbol

zener-diode-constrctionFigure 1 shows the symbol of a zener diode. it is similar to the symbol of an ordinary PN junction diode except that its bar is just turned into Z shape. Figure 2 shows a practical equivalent circuit of a zener diode. this circuit shows that a zener diode is equivalent to a battery with voltage (Vz) called zener voltage in series with a resistance (rz) called zener resistance.

Working Principle of Zener Diode:


We will see the working of zener diode as two sections like forward bias and reverse bias.

Forward bias:

zener diode forward bias

Figure 3 shows the arrangement for forward bias. The positive terminal of the battery is connected to the anode (A) and the negative terminal of the battery is connected to the cathode (K). When applied voltage is zero no current flows through the zener diode. when the forward biasing voltage is increased the potential barrier is reduced and the current starts flowing in the circuit.

Reverse Bias:

zener diode reverse bias

Figure 4 shows the arrangement for reverse bias. The negative terminal of a battery is connected to the anode (A) and positive terminal of the battery is connected to cathode (K). the reverse bias operation is explained in VI characteristic.

V-I characteristic of zener diode.

v-i-characteristics-of-zener-diode

The forward and reverse V-I characteristic shown in the figure 5. The forward current increases slowly upto the knee voltage. Beyond this voltage the current increases sharply with increase in applied voltage. Thus under forward bias condition zener diode acts like an ordinary PN junction diode.

Under reverse bias condition a small reverse current flows through the zener diode. When a reverse voltage across a zener diode is increased, a critical voltage called breakdown voltage is reached at which the reverse current increases sharply as shown by the curve PQ in the figure 5. This breakdown voltage is called zener breakdown voltage or simply zener voltage. This voltage is almost constant over the operating region. This ability of a diode is called regulating ability and is an important feature of a zener diode. It maintains an essentially a constant voltage across its terminal over a specified range of zener current values.

Vz – Zener breakdown voltage.
Iz (min) – A minimum value of zener current called break over current.
Iz (max) – A maximum value of zener current above which the zener diode may be damaged.

Zener break-down


Zener breakdown takes when both sides of the junction are very heavily doped and the depletion layer is thin. When a small reverse voltage is applied a very strong electric field is set up across the thin depletion layer. This electric field is enough to break the covalent bonds. Now extremely large number of free charge carriers are produced which constitute the zener current. This process is known as zener breakdown. In this process the junction is not damaged. The junction regains its original position when the reverse voltage is removed.
Zener+break-down+%2526+Avalanche+Break-down

Avalanche breakdown


The avalanche breakdown takes place when both sides of the junction are lightly doped and the depletion layer is large. When the reverse bias voltage is increased, the accelerated free electron collides with the semiconductor atoms in the depletion region. Due to the collision with valence electrons, covalent bonds are broken and electron-hole pairs are generated. These new charge carriers so produced acquire energy from applied potential and in turn produced additional carriers. This forms avalanche multiplication. This avalanche multiplication causes the reverse current to increase rapidly. This leads to avalanche breakdown. Once this breakdown occurs the junction cannot regain its original position. This breakdown occurs at higher reverse voltage as shown in the figure 6.

Applications of zener diode.


  • It can be used as a voltage regulator.
  • It can be used as a limiter in wave shaping circuits.
  • It can be used as a fixed reference voltage in transistor biasing circuits.
  • It is used for meter protection against damage from accidental over voltage.
  • It can be used as a fixed reference voltage in a network for calibrating voltmeters.
Please use the comments box to clarify your doubts.

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Torque Equation for PMMC

This post is a continuation of my previous post on Permanent magnet moving coil instruments . This post is written since a is request made by the face book user Shivany Sweet


Torque Equation for PMMC

The equation for the delevoped torque of the PMMC can be obtained from the basic law of electromagnetic torque. The deflecting torque is given by,

                                                   Td = NBAI

Where,
Td = deflecting torque in N-m
B = flux density in air gap, Wb/m2
N = Number of turns of the coils
A = effective area of coil m2
I = current in the moving coil, amperes

Therefore, Td = GI
Where, G = NBA = constant

The controlling torque is provided by the springs and is proportional to the angular deflection of the pointer.
                                               Tc = KØ

Where,
Tc = Controlling Torque
K = Spring Constant Nm/rad or Nm/deg
Ø = angular deflection
For the final steady state position,
                        Td = Tc
Therefore GI = KØ
So,                      Ø = (G/K)I          or            I = (K/G) Ø

Thus the deflection is directly proportional to the current passing through the coil. The pointer deflection can therefore be used to measure current.

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Distributed control system:

Definition:

A collection of processing elements, which are inter connected both logically and physically with decentralized system, wide control of resources for co-operative execution of application programs.

A DCS consists of a number of microprocessor – based modules that work together to control and monitor a plant’s operations. The modules are distributed graphically.

Distributed processing is useful in process control for the following reasons:

  1. DCS gives increased performance through resource sharing.
  2. Increased reliability.
  3. Modularity and expandability.
  4. Reduced cabling cost (i.e., field wiring and installation)
  5. It also reduces risk by distributing the control function throughout a number of small modules rather than concentrating it in one large module.

Disadvantages of Distributed control system:

Increased software development cost, more complex failure diagnosis and dependence on communication technology. These have been over come to a great extent by the development of high – level languages and structured – programming concept, communication technology (e.g. fiber optics) and fault – diagnosis methods.

In my next post, I will explain about DCS Architecture.

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ABB wins Eskom upgrade contract - Instrumentation News

POWER and automation technology group ABB has won an order from Eskom for the provision of new equipment for the utility’s Arnot power station in Mpumalanga.

ABB yesterday said its control and instrumentation system would integrate Arnot’s water treatment plant, condensate plant regeneration, condensate polishing plant, and ammonia and sedimentation plant into one common plant network.

The scope of the project, which started this month, includes design, production engineering, factory testing, delivery, installation, commissioning, operational acceptance testing, plant documentation and training, ABB said.

The project would be implemented over 20 months, ABB said.

"ABB experts have engaged with the customer to develop operation and control philosophies for implementing on our control system. This will fully automate and allow remote central operation of the plant and provide a safer working environment for plant workers," Carlos Pone, country manager of ABB SA, said yesterday.

The need to upgrade Arnot arose because of the current manual operation with dangerous chemicals and the labour-intensive working environment where operators might be prone to errors, ABB said.

Eskom is investigating the circumstances that led to a blast in its Duvha power station earlier this year. The reasons for the accident, which has taken out 600MW from the national electricity system, are still unknown.

Eskom CE Brian Dames last month said that the unit, which was damaged in the accident, could be returned to service by next June.

ABB said it had recently completed the Kendal power station water treatment plant, which uses the same technology platform as offered for Arnot.

"ABB has an experienced engineering team with water treatment plant process knowledge and because safety is important not just for us at ABB but for our customers , we are pleased to provide automation solutions that address the plant safety issues," Mr Pone said.

This is the latest of several contracts that ABB has won from Eskom. These include a two-year contract for the expansion and replacement of high-voltage products for the transmission network in the country.

ABB said that power utilities and municipalities in Southern Africa and sub-Saharan Africa are investing in improving infrastructure to ensure reliable power supply.

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E&I Engineer Wanted in UK

Our client has a strong focus on the growing deepwater subsea umbilical, riser and flowline markets in Africa, Asia Pacific, Brazil and the Gulf of Mexico as well as a leading role in the North Sea. Employing over 5,000 personnel onshore and offshore they are one of the world's leading subsea engineering and construction companies servicing the Oil & Gas industry.

As E&I Engineer your main responsibilities will include:

Contribute to creating a positive HSEQ culture within the company.

Actively contribute to position the Offshore Resources Technical & Engineering Department as a centre of electrical engineering expertise within the company.

Support Offshore Resources Marine Superintendents in identifying and proposing CAPEX related to shipboard electrical systems .

Prepare detailed budgets (with the assistance of OR cost controllers) required to support the CAPEX approval process (new equipments, upgrades, conversions, refits) including engineering phase, procurement and certification, transportation, installation & commissioning.

Perform concept engineering works for marine power generation and electrical distribution systems during refit projects, conversions or new buildings.

Perform preliminary electrical engineering works required to support installations of new marine & construction equipments or modifications and/or improvements planned for existing equipments .

Prepare detailed engineering scopes of work when required, technical specifications, participate in the sourcing of electrical engineering subcontractors & equipments manufacturers with Offshore Resources SCM Department.

Manage or undertake site surveys on board vessels as required during the engineering phase .
Manage commissioning activities as per the approved quality plan and set off all technical claims if required with the relevant subcontractors.

Ensure proper handovers to Marine Superintendents and ship crew will be performed in an orderly and timely manner .

Ensure for all new equipments delivered on board vessels that manuals, drawings, lift plans and maintenance information are part of the deliverables to be handed over to the Marine Superintendents.

This is a permanent position

If you feel that you are well suited to the above opportunity and would like to find out more then please contact Orion Group for more information or apply by forwarding your current CV quoting reference 77218

Status: Staff
Required Skills/Experience: E&I Engineer
Location: Aberdeen, UNITED KINGDOM
Advert Published: 4 Sep 2011
Expiry date: 4 Oct 2011
Orion Group Ref. No.:
77218
OilCareers Ref. No.: J560255
Work Permit: Requirements: EUROPEAN UNION
(Applications will only be considered from people who are authorised to work in this location by being a national of that country or region, or by holding a valid work permit.)

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