Bulldozer, pala cargadora y retroexcavadora

Relays cannot switch rapidly (except reed relays), transistors can switch many t imes per second. Relays use more power due to the current flowing through their coil. Relays require more current than many chips can provide, so a low power tr ansistor may be needed to switch the current for the relay s coil.

• •

5.6 LCD DISPLAY:

Liquid crystal displays (LCDs) have materials which combine the properties of bo th liquids and crystals. Rather than having a melting point, they have a tempera ture range within which the molecules are almost as mobile as they would be in a liquid, but are grouped together in an ordered form similar to a crystal. An LC D consists of two glass panels, with the liquid crystal material sand witched in between them. The inner surface of the glass plates are coated with transparent electrodes which define the character, symbols or patterns to be displayed poly meric layers are present in between the electrodes and the liquid crystal, which makes the liquid crystal molecules to maintain a defined orientation angle. One each polarizer’s are pasted outside the two glass panels. This polarizer’s would ro tate the light rays passing through them to a definite angle, in a particular di rection. When the LCD is in the off state, light rays are rotated by the two pol arizers and the liquid crystal, such that the light rays come out of the LCD wit hout any orientation, and hence the LCD appears transparent. When sufficient vol tage is applied to the electrodes, the liquid crystal molecules would be aligned in a specific direction. The light rays passing through

the LCD would be rotated by the polarizer’s, which would result in activating / hi ghlighting the desired characters. The LCD’s are lightweight with only a few milli meters thickness. Since the LCD’s consume less power, they are compatible with low power electronic circuits, and can be powered for long durations. The LCD does don’t generate light and so light is needed to read the display. By using backligh ting, reading is possible in the dark. The LCD’s have long life and a wide operati ng temperature range. Changing the display size or the layout size is relatively simple which makes the LCD’s more customer friendly. The LCDs used exclusively in watches, calculators and measuring instruments are the simple seven-segment dis plays, having a limited amount of numeric data. The recent advances in technolog y have resulted in better legibility, more information displaying capability and a wider temperature range. These have resulted in the LCDs being extensively us ed in telecommunications and entertainment electronics. The LCDs have even start ed replacing the cathode ray tubes (CRTs) used for the display of text and graph ics, and also in small TV applications. Crystalonics dot–matrix (alphanumeric) liq uid crystal displays are available in TN, STN types, with or without backlight.

The use of C-MOS LCD controller and driver ICs result in low power consumption.

These modules can be interfaced with a 4-bit or 8-bit microprocessor /Micro cont roller.

5.6.1 Features of LCD

• The built-in controller IC has the following features: • Correspond to high speed MPU interface (2MHz) • 80 x 8 bit display RAM (80 Characters max)

• 9,920-bit character generator ROM for a total of 240 character fonts. 208 charac ter fonts (5 x 8 dots) 32 character fonts (5 x 10 dots) • 64 x 8 bit character gen erator RAM 8 character generator RAM 8 character fonts (5 x 8 dots) 4 characters

fonts (5 x 10 dots) • Programmable duty cycles • 1/8 – for one line of 5 x 8 dots wit h cursor • 1/11 – for one line of 5 x 10 dots with cursor • 1/16 – for one line of 5 x 8 dots with cursor • Wide range of instruction functions display clear, cursor home

, display on/off, cursor on/off, display character blink, cursor shift, display shift. • Automatic reset circuit, which initializes the controller / driver ICs af ter power on.

5.6.2 Applications:

Personal computers, word processors, facsimiles, telephones, etc.

5.7 KEYPAD

Keypad is used to enter the predefined values of the power transformer. Keypad w ith four keys is employed. The operations of the keys are to increment and decre ment the values to be set.

CHAPTER 6 CONCLUSION

Transformers are static devices, totally enclosed and generally oil immersed. Th erefore chances of faults occurring on them are very rare. However the consequen ces of even a rare fault may be very serious unless the transformer is quickly d isconnected from the system. This necessitates to provide adequate automatic pro tection for transformers against possible faults. The major faults on transforme rs occur due to short circuits in the transformers or in their connections. The basic system used for protection against these faults is the differential relay scheme. Protection of power transformer is a big challenge nowadays. By the help of microcontroller-based relay, protection of transformer is performed very qui ckly and accurately. This system provides a better and safer protection than the other methods which are currently in use. The advantages of this system over th e current methods in use are fast response, better isolation and accurate detect ion of the fault. This system overcomes the other drawbacks in the existing syst ems such as maintenance and response time.

REFERENCES

1. Guzman, A., S. Zocholl and G.Benmouyal, 2000. “Performance

analysis of traditional and improved transformer differential protective relays.

Hector J. Altue (Universidad Autonoma de Nuevo Leon) SEL. 2. Guzman, A., S. Zoc holl, G.Benmouyal, and H.J.Altuve 2001. “A

current based solution for differential protection IEEE Trans., Power Deliv., 3.

Mao, P.L. and R.K. Aggarwal, 2001. A novel approach to the

classification of transient phenomena in power transformers using combined wavel et transform and neural network. IEEE Trans. Power Deliv., 16:4 4. Sidhu, T.S., M.S. Sachdev and M. Hfuda, 1996 Computer simulation

of protective relay design for evaluation their performance. Power System Resear ch Group University of Saskatchewan, Canada. 5. Sachdev M.S., T.S. Sidhu and H.C . Wood, 1989. A digital relaying

algorithm for detecting transformer winding faults. IEEE Trans. Power Deliv., 43 :1638-1648.