How Scr Works Analysis Essay Example

How Scr Works Analysis Essay How an SCR works? -Principle of Operation SCR Working Principle The SCR is a four-layer, three-junction and a three-terminal device and is shown in fig. a. The end P-region is the anode, the end N-region is the cathode and the inner P-region is the gate. The anode to cathode is connected in series with the load circuit. Essentially the device is a switch. Ideally it remains off (voltage blocking state), or appears to have an infinite impedance until both the anode and gate terminals have suitable positive voltages with respect to the cathode terminal. The thyristor then switches on and current flows and continues to conduct without further gate signals. Ideally the thyristor has zero impedance in conduction state. For switching off or reverting to the blocking state, there must be no gate signal and the anode current must be reduced to zero. Current can flow only in one direction. In absence of external bias voltages, the majority carrier in each layer diffuses until there is a built-in voltage that retards further diffusion. Some majority carriers have enough energy to cross the barrier caused by the retarding electric field at each junction. These carriers then become minority carriers and can recombine with majority carriers. Minority carriers in each layer can be accelerated across each junction by the fixed field, but because of absence of external circuit in this case the sum of majority and minority carrier currents must be zero. A voltage bias, as shown in figure, and an external circuit to carry current allow internal currents which include the follow ¬ing terms: The current Ix is due to †¢Majority carriers (holes) crossing junction J1 †¢Minority carriers crossing junction J1 Holes injected at junction J2 diffusing through the N-region and crossing junc ¬tion J1 and †¢Minority carriers from junction J2 diffusing through the N-region and crossing junction J1. Similarly I2 is due to six terms and I3 is due to four terms. The two simple analogues to explain the basic action for the thyristor are those of the diode and the two transistor models. 1. 1. Diode Model. The thyristor is similar to three d iodes in series as there are three P-N junctions. Without gate bias, there is always at least ne reverse biased junction to prevent conduction irrespective of the polarity of an applied voltage between anode and cathode. If the anode is made positive and the gate is also biased positively with respect to cathode, the P-layer at the gate is flooded by the electrons from the cathode and loses its identity as a P-layer. Accordingly the thyristor becomes equivalent to a conducting diode. Scr working 1. 2. Two Transistor Model. Imagine the SCR cut along the dotted line, as shown in fig. a. Then we can have two devices, as shown in fig. b. We will write a custom essay sample on How Scr Works Analysis specifically for you for only $16.38 $13.9/page Order now We will write a custom essay sample on How Scr Works Analysis specifically for you FOR ONLY $16.38 $13.9/page Hire Writer We will write a custom essay sample on How Scr Works Analysis specifically for you FOR ONLY $16.38 $13.9/page Hire Writer These two devices can be recognized as two transistors. The upper left one is P-N-P transistor and the lower right N-P-N type. Further it can be recognized that the base of the P-N-P transistor is joined to the collector of the N-P-N transistor while the collector of P-N-P is joined to the base of N-P-N transistor, as illustrated in fig. c. The gate terminal is brought out from the base of the N-P-N material. This construction has been conceived merely to explain the working of SCR, otherwise in physical shape the SCR has four solid layers of P-N-P-N type only. Now we can see that the two transistors are connected in such a manner that the collector of Q1 is connected to the base of Q2 i. e. the output collector current of Qt becomes the base current for Q2. In the similar way the collector of Q2 is joined to the base of Q1 which shows that the output collector current of Q2 is fed to Q1 as input base current. These are back to back connections of transistors in such a way that the output of one goes into as input of other transistor and vice-versa. This gives net gain of loop circuit as ? 1 x ? 2 where ? 1 and ? are current gains of two transistors respectively. When the gate current is zero or the gate terminal is open, the only current in circulation is the leakage current, which is very small in case of silicon device specially and the total current is a little higher than sum of individual leakage currents. Under these conditions P-N-P-N device is said to be in its forward blocking or high impedance ‘off state. As soon as a small amount of gate current is given to the base of transistor Q2 by applying forward bias to its base-emitter junction, it generates the collector current as ? times the base current. This collector current of Q2 is fed as input base current to Q: which is further multiplied by ? 1 times as ICl which forms input base current of Q2 and undergoes further amplification. In this way both transistors feedback each other and the collector current of each goes on multiplying. This process is very quick and soon both the transistors drive each other to saturation. Now the device is said to be in. on-state. The current through the on-state SCR is controlled by external impedance only. How Scr Works Analysis Essay Example How Scr Works Analysis Essay How an SCR works? -Principle of Operation SCR Working Principle The SCR is a four-layer, three-junction and a three-terminal device and is shown in fig. a. The end P-region is the anode, the end N-region is the cathode and the inner P-region is the gate. The anode to cathode is connected in series with the load circuit. Essentially the device is a switch. Ideally it remains off (voltage blocking state), or appears to have an infinite impedance until both the anode and gate terminals have suitable positive voltages with respect to the cathode terminal. The thyristor then switches on and current flows and continues to conduct without further gate signals. Ideally the thyristor has zero impedance in conduction state. For switching off or reverting to the blocking state, there must be no gate signal and the anode current must be reduced to zero. Current can flow only in one direction. In absence of external bias voltages, the majority carrier in each layer diffuses until there is a built-in voltage that retards further diffusion. Some majority carriers have enough energy to cross the barrier caused by the retarding electric field at each junction. These carriers then become minority carriers and can recombine with majority carriers. Minority carriers in each layer can be accelerated across each junction by the fixed field, but because of absence of external circuit in this case the sum of majority and minority carrier currents must be zero. A voltage bias, as shown in figure, and an external circuit to carry current allow internal currents which include the follow ¬ing terms: The current Ix is due to †¢Majority carriers (holes) crossing junction J1 †¢Minority carriers crossing junction J1 Holes injected at junction J2 diffusing through the N-region and crossing junc ¬tion J1 and †¢Minority carriers from junction J2 diffusing through the N-region and crossing junction J1. Similarly I2 is due to six terms and I3 is due to four terms. The two simple analogues to explain the basic action for the thyristor are those of the diode and the two transistor models. 1. 1. Diode Model. The thyristor is similar to three d iodes in series as there are three P-N junctions. Without gate bias, there is always at least ne reverse biased junction to prevent conduction irrespective of the polarity of an applied voltage between anode and cathode. If the anode is made positive and the gate is also biased positively with respect to cathode, the P-layer at the gate is flooded by the electrons from the cathode and loses its identity as a P-layer. Accordingly the thyristor becomes equivalent to a conducting diode. Scr working 1. 2. Two Transistor Model. Imagine the SCR cut along the dotted line, as shown in fig. a. Then we can have two devices, as shown in fig. b. We will write a custom essay sample on How Scr Works Analysis specifically for you for only $16.38 $13.9/page Order now We will write a custom essay sample on How Scr Works Analysis specifically for you FOR ONLY $16.38 $13.9/page Hire Writer We will write a custom essay sample on How Scr Works Analysis specifically for you FOR ONLY $16.38 $13.9/page Hire Writer These two devices can be recognized as two transistors. The upper left one is P-N-P transistor and the lower right N-P-N type. Further it can be recognized that the base of the P-N-P transistor is joined to the collector of the N-P-N transistor while the collector of P-N-P is joined to the base of N-P-N transistor, as illustrated in fig. c. The gate terminal is brought out from the base of the N-P-N material. This construction has been conceived merely to explain the working of SCR, otherwise in physical shape the SCR has four solid layers of P-N-P-N type only. Now we can see that the two transistors are connected in such a manner that the collector of Q1 is connected to the base of Q2 i. e. the output collector current of Qt becomes the base current for Q2. In the similar way the collector of Q2 is joined to the base of Q1 which shows that the output collector current of Q2 is fed to Q1 as input base current. These are back to back connections of transistors in such a way that the output of one goes into as input of other transistor and vice-versa. This gives net gain of loop circuit as ? 1 x ? 2 where ? 1 and ? are current gains of two transistors respectively. When the gate current is zero or the gate terminal is open, the only current in circulation is the leakage current, which is very small in case of silicon device specially and the total current is a little higher than sum of individual leakage currents. Under these conditions P-N-P-N device is said to be in its forward blocking or high impedance ‘off state. As soon as a small amount of gate current is given to the base of transistor Q2 by applying forward bias to its base-emitter junction, it generates the collector current as ? times the base current. This collector current of Q2 is fed as input base current to Q: which is further multiplied by ? 1 times as ICl which forms input base current of Q2 and undergoes further amplification. In this way both transistors feedback each other and the collector current of each goes on multiplying. This process is very quick and soon both the transistors drive each other to saturation. Now the device is said to be in. on-state. The current through the on-state SCR is controlled by external impedance only.

The Health Care System Essays - Health Economics, Free Essays

The Health Care System Essays - Health Economics, Free Essays The Health Care System More Nobel Prizes in physiology and medicine have been won by doctors or scientists working in the United States than the rest of the world. It is widely accepted that the best training and education is available in the United States in the field of medicine. Despite the fact that over $750 billion is spent on health care in the United States, more than 30 million Americans have no medical coverage and over 100 million are reportedly underinsured according to Nancy Watzman, of the Washington Monthly. We spend 14 percent of our Gross National Product (GNP) on health care each year, while our neighbors to the north, the Canadians, spend only nine percent of their GNP on the same but cover 100 percent of their citizens. If the Canadians can manage to cover every one of their citizens, then we, The People, must find a way to do the same. We have one of two options; either we can spend billions of dollars inventing a new health care system for the United States, or we can be frugal and efficient by adopting the Canadian model of national health care and merely customizing it to fit our own needs. And, although critics may decry the prudence of implementing a system based on socialized medicine, there would be minimal to no sacrifice in coverage, cost, and quality of health care as compared to what little health coverage we have today. If we model our system after the Canadians then this would mean all people share the same waiting rooms, the same doctors, the same equipment, and receive the same medical care. This also alleviates the problem of some Americans receiving the best medical attention available in the world while others receive miserable attention or none at all. This idea of universal coverage sounds wonderful if you have no medical insurance currently, but it is unthinkable if you are accustomed to private rooms in hospitals and hospital meals prepared by gourmet chefs. But if this is what you are accustomed to, you belong to a small minority of Americans. Only ten percent of Americans approve of our health care system, while 56 percent of Canadians approve of theirs (Goodman 35). Perhaps the disgruntled 90 percent majority could also vote the right people into congress to change the health care system to provide universal coverage here in the United States. We could not only provide universal coverage, but we could cover everyone at a lower cost for health care than we are currently paying. It actually costs less to supply medical coverage for the average Canadian than it does for the average American. A Canadian who earns the equivalent of $26,000 US dollars pays about $1,300 a year for medical coverage, while an American earning the same amount pays $2,500 per year according to Mark Kelly of the Chicago Tribune. And its not only the citizens who would save money. The General Accounting Office estimates conservatively that at least $68 billion could be saved in paperwork costs alone if the United States switched to a national health plan. That amount itself would be enough to pay for all the uninsured citizens in the United States with some money left over for lollipops for the kids. It may be argued that these cost savings will come at the price of quality of patient care, but that is just another myth propagated by the American Medical Association, which is easily debunked by facts. If we take the $68 billion in administrative savings and subtract the estimated $12 billion it would cost to extend the same quality of care for the uninsured as the currently insured are receiving, we are left with $56 billion to do with as we wish. Furthermore, how can we even begin to talk about a decline in patient care when the numbers of the uninsured are so staggering? If over 130 million U.S. citizens, or over half of the United States population, are currently uninsured or underinsured and we provide them some kind of medical coverage, this in itself is a significant improvement in quality of care for these people. The fault lies with the doctors who are used to providing state of the art care for the economically advantaged

Collaborative learning in the virtual Workplace Article

Collaborative learning in the virtual Workplace - Article Example As a learning facilitator my goal and therefore ultimate expectation is for students to collaboratively get the course content and concepts. I will therefore expect the design of an online learning program to facilitate participant engagement, concentration and lively participation. The technology should allow me a facilitator and to monitor and track every individual student’s activity respond to their learning concerns and therefore meet their expectations. The program should therefore encourage me as the facilitator to pull more are as opposed to pushing as this is the only way to gauge whether the learners are progressing (Brindly and Walti, 2009). My other expectation of an online learning program is the technology should be user friendly and offer a platform for groups to build social networks in that the entire experience is anchored on experimentation with ideas that assist the organisation. The program should also create an enabling environment for continuous improvement by spreading the events over time. The program should also put into considerations the unique situations such as differences in time zones, work schedules and availability so that no learner is left behind. Virtual instructor-led trainings will define the future of learning and therefore the goal should to identify technologies that enable improved collaboration and interactive