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Monday, January 31, 2011

III year Second Semister 1st Mid Important Questions

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Saturday, January 8, 2011

Testing PCB Components: Transistors and Fuses

Printed circuit boards (PCBs) function based on a complex array of devices that must transmit electricity without interfering with other devices’ signals. Because the components of PCB are closely placed, testing each device independently is difficult to conduct without picking up surrounding signals. However, it is essential to ensure that the PCB works properly by testing each component individually. There are several ways to test individual devices, depending on the exact component—diodes, for example, must be handled differently than resistors. Below, we explore processes for testing fuses and transistors within PCB set-ups.

Transistors
 
A transistor’s basic purpose is to amplify current, although they also switch electronic signals. They are semiconductors with three terminals—the additional terminal provides them with greater flexibility when transmitting electricity. As voltage is applied to one set of terminals, the voltage is altered between all pairs of terminals.
 
There are two general types of transistors: NPN and PNP. The letters refer to the main semiconductor material from which the transistor is made—an NPN transistor features two terminals made from N-type material, which is negatively charged, and one terminal made from P-type material, which is positively charged. A PNP, on the other hand, features the opposite configuration: two terminals feature P-type material and one terminal features N-type material. (Typically, P and N type materials are made from doped silicon. Silicon, in its natural state is an insulator. When mixed with certain impurities it becomes semi-conductive, and either assumes a positive or negative charge depending on the nature of the impurity added.)
 
Testing transistors in PCB circuits first requires identifying the type of transistor. Every transistor, regardless of type, has three leads: C, B and E, which stand for collector, base, and emitter. Once the type and the various leads have been determined, the transistor can be tested. When testing a transistor in a PCB circuit, disconnect the base lead from the circuit before attempting to measure current. Emitter current is typically between 0.5 milliampere to 3 milliamperes, where as collector voltage ranges from 3 to 15 volts. An ohmmeter can also be used to check emitter, collector and base leads.
 
Fuses
 
A fuse in a PCB guards against current overload—when too much current is present, a small metal wire begins to melt, which inhibits current from flowing from the wire to the next circuit. Each fuse features two leads or pads: in a PCB, the leads on a fuse will be radial or axial. In other applications, fuses can feature solder pads in place of leads; in semi-enclosed fuses, the fuse wire can be replaced if damaged by operation.
 
Typically, fuse wires are made of aluminum, coated nickel or coated copper. The fuse body is often made of glass and cylindrical in shape, featuring fuse leads or pads on each end. Current range can widely vary, from as low as a few one hundredths of an ampere to several hundred amperes. Fuses can be fast acting or slow acting—fast acting fuses opens quickly when current is exceeded, whereas slow blow fuses are slower to respond, and may not be an adequate choice for high current applications because they may not prevent damage in time.
 
Testing a fuse in a PCB is done with a multimeter. After the PCB is turned off and the fuse is removed, connect the probed of the ohmmeter to both leads, and make sure the multimeter range is as low as possible. A fuse that hasn’t blown will register zero ohms; a blown fuse will register infinity.
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Different types of sensors and their uses in instrumentation

Pressure Sensors

Pressure measurements can be taken to determine a range of different values depending on whether the pressure is relative to atmosphere, vacuum conditions, or other measuring factors. Pressure sensors are instruments that can be designed and configured to detect pressure across these variables. Absolute pressure sensors are intended to measure pressure relative to a vacuum and they are designed with a reference vacuum enclosed within the sensor itself. These sensors can also measure atmospheric pressure. Similarly, a gauge pressure sensor detects values relative to atmospheric pressure, and part of the device is usually exposed to ambient conditions. This device may be employed for blood pressure measurements.

Aneroid Barometer Sensors
 
An aneroid barometer device is composed of a hollow metal casing that has flexible surfaces on its top and bottom. Atmospheric pressure changes cause this metal casing to change shape, with mechanical levers augmenting the deformation in order to provide more noticeable results. The level of deformation can also be enhanced by manufacturing the sensor in a bellows design. The levers are usually attached to a pointer dial that translates pressurized deformation into scaled measurements or to a barograph that records pressure change over time. Aneroid barometer sensors are compact and durable, employing no liquid in their operations. However, the mass of the sensing element limits the device’s response rate, making it less effective for dynamic pressure sensing projects.
 
Manometer Sensors
 
A manometer provides a relatively simple design structure and an accuracy level greater than that afforded by most aneroid barometers. It takes measurements by recording the effect of pressure on a column of liquid. The most common form of manometer is the U-shaped model in which pressure is applied to one side of a tube, displacing liquid and causing a drop in fluid level at one end and a correlating rise at the other. The pressure level is indicated by the difference in height between the two ends of the tube, and measurement is taken according to a scale built into the device.
 
The precision of a reading can be increased by tilting one of the manometer’s legs. A fluid reservoir can also be attached to render the height decreases in one of the legs insignificant. Manometers can be effective as gauge sensors if one leg of the U-shaped tube vents into the atmosphere, and they can function as differential sensors when pressure is applied to both legs. However, they are only effective within a specific pressure range and, like aneroid barometers, have a slow response rate that is inadequate for dynamic pressure sensing.
 
Bourdon Tubes
 
Although they function according to the same essential principles as aneroid barometers, bourdon tubes employ a helical or C-shaped sensing element instead of a hollow capsule. One end of the bourdon tube is fixed into connection with the pressure, while the other end is closed. Each tube has an elliptical cross-section that causes the tube to straighten as more pressure is applied. The instrument will continue to straighten until fluid pressure is matched by the elastic resistance of the tube. For this reason, different tube materials are associated with different pressure ranges. A gear assembly is attached to the closed end of the tube and moves a pointer along a graduated dial to provide readings. Bourdon tube devices are commonly used as gauge pressure sensors and as differential sensors when two tubes are connected to a single pointer. Generally, the helical tube is more compact and offers more reliable performance than the C-shaped sensing element.
 
Vacuum Sensors
 
Vacuum pressure is below atmospheric pressure levels, and it can be challenging to detect through mechanical methods. Pirani sensors are commonly used for measurements in the low vacuum range. These sensors rely on a heated wire with electrical resistance correlating to temperature. When vacuum pressure increases, convection is reduced and wire temperature rises. Electrical resistance rises proportionally and is calibrated against pressure in order to provide an effective measurement of the vacuum.
 
Ion or cold cathode sensors are commonly used for higher vacuum range applications. These instruments rely on a filament that generates electron emissions. The electrons pass onto a grid where they may collide with gas molecules, thereby causing them to be ionized. A charged collection device attracts the charged ions, and the number of ions it accumulates directly corresponds to the amount of molecules within the vacuum, thus providing an accurate reading of the vacuum pressure.

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Sunday, January 2, 2011

INSTRUMENTATION JOBS

jobs.gecareers.com
jobs.gecareers.com
GE - India
jobs.gecareers.com
Afcons Infrastructure Ltd (SP Group) Carry out instrumentation design engineering - Mumbai, Maharashtra
www.careerbuilder.co.in
Saipem India Projects Ltd. BE/B.Tech in Instrumentation Engineering with 10 - Mumbai, Maharashtra
www.careerbuilder.co.in
TCE Consulting Engineers Ltd Graduate / Post Graduate In Instrumentation - Mumbai, Maharashtra
www.careerbuilder.co.in
L&T Chiyoda Ltd Diploma in Engineering having experience in Instrumentation - Mumbai, Maharashtra
www.careerbuilder.co.in
India - Saudi Arabia
www.timesjobs.com
G. GHEEWALA HUMAN RESOURCES CONSULTANTS - India - Saudi Arabia
www.timesjobs.com - January 2
Larsen Toubro - Chennai, Tamil Nadu
www.timesjobs.com
India - Saudi Arabia
www.timesjobs.com
SAMSUNG HEAVY INDUSTRIES INDIA PVT. LTD. - Delhi - Noida, Uttar Pradesh
www.timesjobs.com
ZUARI INDUSTRIES LIMITED - Gurgaon, Haryana - Bangalore, Karnataka
www.timesjobs.com 
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L&T-Chiyoda Limited (LTC), an engineering consultancy JV between two engineering giants LARSEN & TOUBRO LIMITED, India and CHIYODA CORPORATION, Japan, offers the complete range of process plant engineering and design solutions to both domestic & international clients in sectors critical to economic growth – Petroleum refining, Petrochemicals, Chemicals, Fertilizers, Oil & Gas and LNG & LPG.
Designation : Fresh Instrumentation Engineer 
Experience : 0 – 1 Years

Education : B.Tech/B.E. – Instrumentation
Desired : Fresh Passout BE / B.TEch (2008 batch) in Instrumentation Engineering.
Final year / semester students in B.Tech / B.E. (Instrumentation) course (Students passing out in June 2008).
First class in SSC / X Std & HSC / XII Std.
Candidate should have passed both the examinations in first attempt.
60% aggregate marks or 6.0 CPI in the Engg. Degree course (aggr. of semesters completed and for which results declared).
Not more than 1 (One) ATKT permitted.
Age : Not more than 24 years as on 1st July 2008.

Location : Mumbai, Vadodara/Baroda

Contact Mr.Maharshi Mehta

Telephone 0265-6691108
Apply:
If you have registered your CV with Naukri.com, click here to apply
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