Monday, September 26, 2011

Pin description and working of IC: NE555



Working of timer IC: NE555


This IC can be used in many ways like timer, multivibrator(in astable mode), as a flip-flop(in bistable mode). It was designed by Hans R. Camenzind in 1970 and brought to market in 1971 by Signetics (later acquired by Philips). The original name was the SE555 (in metal can) and NE555(in plastic DIP) and the part was described as The IC Time Machine . It is believed that the 555 gets its name from the three 5 kO resistors used in typical early implementations, but Hans Camenzind has stated that the number was arbitrary.The part is still in wide use, due to its easy use, good stability and low cost.
An standard 555 package available in markets contains over 20 transistors, 2 diodes, and 15 resistors on a silicon chip all in a single 8-pin mini package(IC). now a number of different form od NE555 is present in market depending upon the number of pins and number of timers present in them some of which can be listed as below:

Pin desrciption


1) 556 : It has 14 pins, combination of two 555 having Vcc and ground connections commen.
2) 558 : It has 16 pins, combination of four 555 timer ICs with DIS, THR and power connections in common.
Functions of all 8 pins of NE555:
1) pin 1 (GND) : It is always grounded i.e. connected to negative terminal of battery.
2) pin 2 (TRIG) : This pin is use for triggring the timer on when an negative pulse is applied at it by means of an push button(remember only a input negative pulse is needed to triggger the timer and not a continuos negative potential supply)
3) pin 3 (OUT) : The output supply pulse is obtained from this pin of a specific width and time periode which is decided by the resistor connected at THR and DIS.
4) pin 4 (RESET) : When an negative pulse is given to this pin it resets the timer and begins it from starting, so it is generally connected to positive(when not in use) of the battery to avoide any miss-functioning of the circuit.
5) pin 5 (CTRL) : This pin is generally not used and remains unconnected because it controls voltage, allows access to the internal voltage divider (2/3 VCC).
6) pin 6 (THR) : This is the pin which dicides the potential at OUT pin i.e. when potential at pin 6 is atleast 2/3 Vcc it gives an lower potential at OUT othewise if potential at pin 6 is less than 2/3 Vcc higher potential appears at pin OUT.
7) pin 7 (DIS) : Potential at this pin is also decided by THR pin i.e. when potential at THR is atleast 2/3 Vcc then is pontential reduced to Zero(i.e. it is grounded), and provides a discharge path for capacitor C.
8) pin 8 (Vcc) : This pin is always connected with positive input supply which must be between 3-15v for working of IC safly and reliably.

Working of the IC as a time in monostable mode:


For the IC to work as a timer pins 8 and 4 are connected to positive input supply which is generally 5-9V, pin 1 is grounded, pin 3 is connected to output circuit(if any), pin 7 and 6 are connected togather and given a positive supply from the source through a resistor R and also connected to ground throuth a capacitor C such that R and C form a potential divider and give a specific voltage at pin 7 and 6. As capacitor C charger with time i.e. its effective resistance increases wiht time so by the potential divider rule potential at pins 7 and 6 increases with time with a specific time rate which depends upon values of R and C.
Pin 2 is initially connected to positive terminal through a resistor of about 5 Kohm. As soon as, pin connected to negative potential of battery, for a while, timer starts working i.e. capacitor C starts discharching through R and potential at pin 6 increases. At the same time a higher potential appesres at pin 3 and it remains high as long as the potentia at pin 6 reaches 2/3 Vcc i.e. when the potential reaches 2/3 Vcc pin 3 is shifted to lower potential i.e. no output is obtained at pin 3 after thet time. At the same time potential at pin 7 reduces to Zero(i.e. connencted to ground), so, capacitor discharges through pin 6, instantaneously, and resets the timer for next time. When, again, pin 2 is connected to negative potential consanguineously the same functioning repeats itself.

Principle and Working of SCR (silicon controlled rectifier)

This article mentions the detailed working of SCR(silicon controlled rectifier ) with application, characteristics and parameters affecting it.

SCR, It is actually an thyristor which is used to control the high voltage supply as compared to the conventional transistors. It is an four layered P-N-P-N semiconductor three terminal device, such that all the four segment arranged in a single array. it has an anode, cathode and an gate terminal. The upper P-type semiconductor is termed as anode ,usually given an positive voltage, the lower N-type semiconductor segment is termed as cathode and usually given an negative voltage while p-type semiconductor near cathod is gate which supplied with an positive pulse to trigger the device.



Modes of operation of an SCR


There are three modes of opearation for an SCR depending upon the biasing given to it:

a) Forward blocking mode(off state)
b) Forward conduction mode(on state)
c) Reverse blocking mode(off state)


a) Forward blocking mode: In this mode of operation anode is given an positive potential while cathode is given negative voltage keeping gate at zero potential i.e. disconnected. In this case junction J1 and J3 are forward biased while J2 is reversed biased due to which only a small leakage current flows from anode to cathode till applied voltage reaches it breakover value at which J2 undergoes avalanche breakdown and at this breakover voltage it stars conducting but below breakover voltage it offers very high resistance to the flow of current through to it and said to be in off state .
b) Forward conduction mode: SCR can be brought from blocking mode to conduction mode in two way either by increasing the voltage across anode to cathode beyond breakover voltage or by application of positive pulse at gate. Once it stars conducting no more gate voltage is required to maintain it in on state. Now there are two way to turn it off i.e.
* Reduce the current flowing through it below a minimum value called holding current .
*Apply an negative pulse at gate which will bring it in off state instantaneously.
c)Reverse blocking mode: In this mode anode it supplied with negative and cahtode with positive voltage which reverse bias J1 and J3 and no current passes through it.


Application of SCR


It can be used in many ways dependinding upon its operation in the circuit.
1) As the name suggests it can be used as an rectifier which offers low resistance in direction while high resistance in opposite direction just like an ordinary diode but difference is that for the conduction in one direction it also requires an positive gate pulse. But while using it as an rectifier does'n prove to very useful because the output wave form obtained is distorted.

Working as an rectifier


As per shown in figure:1 the input ac power obtained from the transformer in fed between anode and cathode while gate is supplied with an positve voltage by the application of an diode upon it though the resistor, both togather givev only a small positive voltage to the gate. Now, when positive half half cycle appeares across the transformer it produces positive potential to anode and negative potential at cathode and at the same time small voltage also appear across gate which triggers the SCR and current conducts from anode to cathode while when negative half cycle appears across transformer it make anode negative, cathode positive and gate is given negative pulse which completely revers bias the SCR and no current flows though it. Thus, it produces half wave rectified output. it cannot be used for rectification of samall voltages(2-6V) because in conduction state an voltage appears across it which is around 1-2V so a major drop in output waveform will be seen if used for small voltages.

 2) The second major application is as an switching device, in which when an positive pulse is given at gate the SCR stars to conduct current from anode to cathode. But here it can be used only for DC power source.

V-I characteristics of SCR

It has very different characteristics in forward and verese biases as is seen in figure:2 . in forward bias it has

  •   Forward blocking region: In which SCR offers very high resistance.
  •   breakover voltage: At this voltage it changes its state form off state to on state.
  •   Holding current: This is the minimum current required to maintain it in on state.
  •   Latching region: In this region it remains in conduction mode and a voltage of 1-2V appears across it.
In reverse biasing its characteristics are are just like an diode in reverse biased state.
Note One very important parameter affecting working of an SCR is its dV/dt value that rate of chance of anode to cathode voltage with respect to time. Very high value of dV/dt causes the SCR trigerred before its breakover voltage and can result in undesired triggering of the device there is always a maximum possible value of dV/dt for avoiding any undesired triggering of SCR and it can be different for defferent SCRs.

 

Tuesday, September 13, 2011

What is the functioning, use, construction and working of Electronic multimeter?


It is a very versatile meter in laboratories because it can be used for many purposes. Some of its functions are DC and AC current and voltage measurement and resistance of resistors used in electronic circuits. The solid state version is described here in oncoming discussion. Following components are constituent parts of a multi-meter.
·         A balance bridge DC amplifier and PMMC indicator
·         A range switch which is an attenuator to limit the value of input voltage to the desired one.
·         A rectifier on order to convert AC to  equivalent DC value
·         An internal battery and additional circuitry for the measurement of resistance.
·         A function Switch to reset various measurement functions for the instrument like current, voltage and resistance.

Along with this all the instruments has a built in power supply for the purpose of performing operation on power mains and many cases one or more than one batteries for operation as portable test instrument. Here is figure that is capable of showing all the part that has been discussed about in our discussion so far.
Balanced DC amplifier
 
This figure shows the schematic diagram of a balanced bridge DC amplifier that uses two field effect transistors (FETs). These two FETs are reasonably matched with each other on the basis of current gain this done to ensure thermal stability in the circuitry. The upper arm of the bridge is formed with the help of two FETs the source resistors R1 and R2 together with zero adjustment of resistor R3 form the lower bridge. The PMMC instrument is connected between the source terminals of FETs. These two FETs represent two opposite end of the bridge. In case of no input signal applied to the gate terminal of FETs they remain to be at the zero potential or ground potential. In this case in ideal situation there should be no current flowing through the meter. But due to presence of some miss matches between the FETs and also due to some differences in values of resistor a current does flows. This unwanted current can be reduced to zero with the help of a grounded variable resistor.

Friday, September 2, 2011

Data conversion system with time division multiplexing

What is time division multiplexing?

Time division multiplexing means that each input channel is sequentially connected to the multiplexer for a certain time.(the input signals are not applied to the multiplexer continuously but are connected in turns to the multiplexer thereby sharing time in between them). The time of sharing the output is controlled by the unit called control unit. This control unit controls the sampling and hold circuits.

Importance of a D.C. system

D.C. systems are generally used for common resistance transducers such as potentiometer and resistance strain gauges. A.C. systems have to be used for variable reactance transducer and systems where signals have to be transmitted via long cables to connect the transducers to the signal conditioning equipment. After this physical quantities like temperature, pressure upon the system, strain in it, acceleration of the system during operation and many more have been transduced into their analogue electrical forms and amplified to a sufficient level of current and voltages. These levels can be generalized as 1 V to 10 V and they are further processed by electronic circuits. In some indicating or recording or may be control instruments. But many applications involves further processing of the signals which involves the linear and non linear operations, these have been mentioned earlier.

The input signal is applied to sampling and hold circuits which has been shown in the following figure with the help of circuit diagram and block diagram.

This may be fed to an analogue multiplexer and and analogue to digital convertor (called A/D convertor). Here it is to be mentioned that a multiplexer is and electronic device which make it possible to transmit a number of signal through a signal transmission line by the property of time sharing but under the control of control lines. There can mainly two types of multiplexer i.e. analogue and digital multiplexer as the name suggests analogue multiplexer multiplexes an analogue signal while in case of digital multiplexer the multiplexing is done with digital data i.e. a stream of 0s and 1s.

If the signal is in digital form it can be applied to a variety of digital systems this involves computers, digital controller, digital data logger or a digital data transmitter.

The figure which has been mentioned earlier is of an ideal sampling and hold circuit which samples the different inputs at a specific time and then hold the voltage levels at their output while analogue multiplexer performs the time division multiplexing (called TDM) operation between different data inputs.

What is word length of a computer or microprocessor?



The word length of microprocessor leads to the n-bit, where n may be 8, 16,32 or so on. The similarity in them is that this number n should be multiple of 2. A binary digit 0 or 1 is called a bit. An 8-bit microprocessor can process 9-bit of data at a time. If data consists of more than 8 bit the processor or the microprocessor of computer system then computer first process the 8-bit data and then go to the next 8-bit of data for its processing. The result thus obtained in then sent to the memory for its storage or to the display devices. Its ALU is designed to process 8-bit of data at a time. Its general purpose registers which hold the data for processing are 8-bit long registers. Similarly, when we take an case of an 16-bit microprocessor based computer system then the functioning will still remain the same but the only difference that now the processing of data is performed in packets of 16-bit i.e. the data having more than 16-bit of capacity is processed such that first the 16 bits are processed followed by the processing of remaining bits and so on.

This leads to the accomplishment of more complex calculations done with limited and present capabilities. This procedure goes on for all the other types of processor.
This is called the word length of the processor that how many bits of data can it process at a time without break in between them. This leads to the word length of the computer system because it is actually the microprocessor that works in it. A microprocessor of larger word length is more powerful that the others. This is because the computer having more word length can work faster and efficiently.