Saturday, January 8, 2011

Testing PCB Components: Transistors and Fuses

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

Atransistor’s basic purpose is to amplify current, although they alsoswitch electronic signals. They are semiconductors with threeterminals—the additional terminal provides them with greater flexibilitywhen transmitting electricity. As voltage is applied to one set ofterminals, 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 fromwhich the transistor is made—an NPN transistor features two terminalsmade from N-type material, which is negatively charged, and one terminalmade from P-type material, which is positively charged. A PNP, on theother hand, features the opposite configuration: two terminals featureP-type material and one terminal features N-type material. (Typically, Pand N type materials are made from doped silicon. Silicon, in itsnatural state is an insulator. When mixed with certain impurities itbecomes semi-conductive, and either assumes a positive or negativecharge depending on the nature of the impurity added.)
Testingtransistors in PCB circuits first requires identifying the type oftransistor. Every transistor, regardless of type, has three leads: C, Band E, which stand for collector, base, and emitter. Once the type andthe various leads have been determined, the transistor can be tested.When testing a transistor in a PCB circuit, disconnect the base leadfrom 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 canalso be used to check emitter, collector and base leads.
Afuse in a PCB guards against current overload—when too much current ispresent, a small metal wire begins to melt, which inhibits current fromflowing from the wire to the next circuit. Each fuse features two leadsor pads: in a PCB, the leads on a fuse will be radial or axial. In otherapplications, fuses can feature solder pads in place of leads; insemi-enclosed fuses, the fuse wire can be replaced if damaged byoperation.
Typically,fuse wires are made of aluminum, coated nickel or coated copper. Thefuse body is often made of glass and cylindrical in shape, featuringfuse leads or pads on each end. Current range can widely vary, from aslow as a few one hundredths of an ampere to several hundred amperes.Fuses can be fast acting or slow acting—fast acting fuses opens quicklywhen current is exceeded, whereas slow blow fuses are slower torespond, and may not be an adequate choice for high current applicationsbecause they may not prevent damage in time.
Testinga fuse in a PCB is done with a multimeter. After the PCB is turned offand the fuse is removed, connect the probed of the ohmmeter to bothleads, and make sure the multimeter range is as low as possible. A fusethat hasn’t blown will register zero ohms; a blown fuse will registerinfinity.

Different types of sensors and their uses in instrumentation


Pressuremeasurements can be taken to determine a range of different valuesdepending on whether the pressure is relative to atmosphere, vacuumconditions, or other measuring factors. Pressure sensorsare instruments that can be designed and configured to detect pressureacross these variables. Absolute pressure sensors are intended tomeasure pressure relative to a vacuum and they are designed with areference vacuum enclosed within the sensor itself. These sensors canalso measure atmospheric pressure. Similarly, a gauge pressure sensordetects values relative to atmospheric pressure, and part of the deviceis usually exposed to ambient conditions. This device may be employedfor blood pressure measurements.

Aneroid Barometer Sensors
Ananeroid barometer device is composed of a hollow metal casing that hasflexible surfaces on its top and bottom. Atmospheric pressure changescause this metal casing to change shape, with mechanical leversaugmenting the deformation in order to provide more noticeable results.The level of deformation can also be enhanced by manufacturing thesensor in a bellows design. The levers are usually attached to a pointerdial that translates pressurized deformation into scaled measurementsor to a barograph that records pressure change over time. Aneroidbarometer sensors are compact and durable, employing no liquid in theiroperations. However, the mass of the sensing element limits the device’sresponse rate, making it less effective for dynamic pressure sensingprojects.
Manometer Sensors
Amanometer provides a relatively simple design structure and an accuracylevel greater than that afforded by most aneroid barometers. It takesmeasurements by recording the effect of pressure on a column of liquid.The most common form of manometer is the U-shaped model in whichpressure is applied to one side of a tube, displacing liquid and causinga 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 thetwo ends of the tube, and measurement is taken according to a scalebuilt into the device.
Theprecision of a reading can be increased by tilting one of themanometer’s legs. A fluid reservoir can also be attached to render theheight decreases in one of the legs insignificant. Manometers can beeffective as gauge sensors if one leg of the U-shaped tube vents intothe atmosphere, and they can function as differential sensors whenpressure is applied to both legs. However, they are only effectivewithin a specific pressure range and, like aneroid barometers, have aslow response rate that is inadequate for dynamic pressure sensing.
Bourdon Tubes
Althoughthey function according to the same essential principles as aneroidbarometers, bourdon tubes employ a helical or C-shaped sensing elementinstead of a hollow capsule. One end of the bourdon tube is fixed intoconnection with the pressure, while the other end is closed. Each tubehas an elliptical cross-section that causes the tube to straighten asmore pressure is applied. The instrument will continue to straightenuntil fluid pressure is matched by the elastic resistance of the tube.For this reason, different tube materials are associated with differentpressure ranges. A gear assembly is attached to the closed end of thetube and moves a pointer along a graduated dial to provide readings.Bourdon tube devices are commonly used as gauge pressure sensorsand as differential sensors when two tubes are connected to a singlepointer. Generally, the helical tube is more compact and offers morereliable performance than the C-shaped sensing element.
Vacuum Sensors
Vacuumpressure is below atmospheric pressure levels, and it can bechallenging to detect through mechanical methods. Pirani sensors arecommonly used for measurements in the low vacuum range. These sensorsrely on a heated wire with electrical resistance correlating totemperature. When vacuum pressure increases, convection is reduced andwire temperature rises. Electrical resistance rises proportionally andis calibrated against pressure in order to provide an effectivemeasurement of the vacuum.
Ionor cold cathode sensors are commonly used for higher vacuum rangeapplications. These instruments rely on a filament that generateselectron emissions. The electrons pass onto a grid where they maycollide with gas molecules, thereby causing them to be ionized. Acharged collection device attracts the charged ions, and the number ofions it accumulates directly corresponds to the amount of moleculeswithin the vacuum, thus providing an accurate reading of the vacuumpressure.

Sunday, January 2, 2011


GE - India
Afcons Infrastructure Ltd (SP Group) Carry out instrumentation design engineering - Mumbai, Maharashtra
Saipem India Projects Ltd. BE/B.Tech in Instrumentation Engineering with 10 - Mumbai, Maharashtra
TCE Consulting Engineers Ltd Graduate / Post Graduate In Instrumentation - Mumbai, Maharashtra
L&T Chiyoda Ltd Diploma in Engineering having experience in Instrumentation - Mumbai, Maharashtra
India - Saudi Arabia
www.timesjobs.com - January 2
Larsen Toubro - Chennai, Tamil Nadu
India - Saudi Arabia
ZUARI INDUSTRIES LIMITED - Gurgaon, Haryana - Bangalore, Karnataka
L&T-Chiyoda Limited (LTC), an engineeringconsultancy JV between two engineering giants LARSEN & TOUBROLIMITED, India and CHIYODA CORPORATION, Japan, offers the complete rangeof 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

Mr.Maharshi Mehta

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

Wednesday, November 10, 2010

IBM Hiring freshersintervew on : 13-Nov-10

IBM is Hiring Freshers (2010 pass outs with all the marks sheets and
Provisional Degree certificates also) and Experienced candidates.

...Graduates & Post Graduates from any discipline (BSc, BCom, BSc, BE,B.Tech MCA, MBA)

Freshers 2010, 2009,2008,2007 pass outs

Also, Diploma holders (10+2+3)

Experience : 0 to 18 months


Good communication skills are requisite

Knowledge of Computer Hardware and Networking concepts

Willingness to work in shifts

Please do refer your friends for this opening

Venue Details:

IBM India Pvt Ltd,

3B,Mindspace, Hi-Tech City

Hyderabad, 500081


Date - 13-Nov-10

Timings: 10AM to 1PM


*Walk-in candidates are requested to carry Resume, one photo-id,
* You can directly walkin onthe date mentioned

* Selection of candidates is at the sole discretion of the IBM Recruitment Team

Sunday, November 7, 2010