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SUNILMANJERI. MOB:9895 34 56 16

SUNILMANJERI. MOB:9895 34 56 16

SUNILMANJERI MOB:9895 34 56 16

Wednesday, May 16, 2012

SCS-Silicon Controlled Switch


Silicon Controlled Switch
Silicon Controlled Switch
Silicon controlled switch (SCS), like the SCR, is a unilateral, four layer three junction P-N-P-N silicon device with four electrodes namely cathode C, cathode gate Gx, anode gate G2 and the anode A, as shown in figure. Infact, the SCS is a low power device compared with the SCR. It handles currents in milli amperes rather than amperes. SCS differs from an SCR in the following aspects. It has an additional gate—the anode gate.It is physically smaller than SCR.It has smaller leakage and holding currents than SCR.It needs small triggering signals. It gives more uniform triggering characteristics from sample to sample.
The basic structure and schematic symbol of SCS are shown in the figures. It may be fabricated by using either the grown junction technique or the planar technique.
SCS Symbol
SCS Symbol

Operation of a Silicon Controlled Switch

The easiest way to understand how it operates is to realize it to be formed of two transistors Q1 and Q2 placed back-to-back, as shown in figure.b
In a two-transistor equivalent circuit shown in figure.c, it is seen that a negative pulse at the anode gate G2 causes transistor Q1 to switch on. Transistor Q1 supplies base current to transistor Q2, and both transistors switch-on. Similarly, a positive pulse at the cath­ode gate G1 can switch the device on. Since only small currents are involved, the SCS may be switched off by an appropriate polarity pulse at one of the gates. At the cathode gate a negative pulse is required for switching-off while at the anode gate a positive pulse is needed.
Silicon Controlled Switch Characteristics
Silicon Controlled Switch Characteristics

Volt-Ampere Characteristic of SCS

The volt-ampere characteristic of an SCS is similar to that of an SCR and is shown in figure. With the increase in applied voltage, the current first increases slowly upto point A and then rapidly in the region AB, as shown in the figure. At point B, the product β1β2 exceeds unity and the device is suddenly switched on. In the on-state, the current increases enormously and is limited by the external series resistor. SCS also exhibits negative differential resistance in the on region similar to SCR. SCS gets switched on accidentally if the anode voltage gets applied suddenly. This is known as rate effect, which is caused by inter-electrode capacitance between elec­trodes G1 and G2, known as transition capacitance.
Advantages and Applications of SCS
An advantage of SCS over an SCR is the reduced turn-off time, typically within the range of 1 to 10 micro seconds for the SCS and 5 to 30 micro seconds for the SCR. Other advantages of the SCS over SCR are increased control and triggering sensitivity and a more predictable firing situation. However, the SCS is limited to low power, current, and voltage ratings (typical maximum anode currents range from 100 mA to 300 mA with dissipation rating of 100 to 500 mW).
A few of the more common areas of application of SCS include a variety of computer circuits (such as counters, registers, and timing circuits) voltage sensors, pulse generators, oscillators etc.

Tuesday, May 15, 2012

Error Detecting Codes

When data is transmitted from one point to another, like in wireless transmission, or it is just stored, like in hard disks and memories, there are chances that data may get corrupted. To detect these data errors, we use special codes, which are error detection codes.

Error-Correcting Codes

Error-correcting codes not only detect errors, but also correct them. This is used normally in Satellite communication, where turn-around delay is very high as is the probability of data getting corrupt.


ECC (Error correcting codes) are used also in memories, networking, Hard disk, CDROM, DVD etc. Normally in networking chips (ASIC), we have 2 Error detection bits and 1 Error correction bit.

Alphanumeric Codes

The binary codes that can be used to represent all the letters of the alphabet, numbers and mathematical symbols, punctuation marks, are known as alphanumeric codes or character codes. These codes enable us to interface the input-output devices like the keyboard, printers, video displays with the computer.

EBCDIC stands for Extended Binary Coded Decimal Interchange.

EBCDIC stands for Extended Binary Coded Decimal Interchange. It is mainly used with large computer systems like mainframes. EBCDIC is an 8-bit code and thus accomodates up to 256 characters. An EBCDIC code is divided into two portions: 4 zone bits (on the left) and 4 numeric bits (on the right).

ASCII stands for American Standard Code for Information Interchange

ASCII stands for American Standard Code for Information Interchange. It has become a world standard alphanumeric code for microcomputers and computers. It is a 7-bit code representing 27 = 128 different characters. These characters represent 26 upper case letters (A to Z), 26 lowercase letters (a to z), 10 numbers (0 to 9), 33 special characters and symbols and 33 control characters.

The 7-bit code is divided into two portions, The leftmost 3 bits portion is called zone bits and the 4-bit portion on the right is called numeric bits.

An 8-bit version of ASCII code is known as USACC-II 8 or ASCII-8. The 8-bit version can represent a maximum of 256 characters.