andy chuksA thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased (that is, as long as the voltage across the device has not reversed).
Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to switch off automaticall; referred to as Zero Cross operation. The device can be said to operate synchronously as, once the device is open, it conducts current in phase with the voltage applied over its cathode to anode junction with no further gate modulation being required to replicate; the device is biased fully on. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature.
Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers.
They can also be found in power supplies for digital circuits, where they are used as a sort of "circuit breaker" or "crowbar" to prevent a failure in the power supply from damaging downstream components. A thyristor is used in conjunction with a zener diode attached to its gate, and when the output voltage of the supply rises above the zener voltage, the thyristor will conduct, then short-circuit the power supply output to ground (and in general blowing an upstream fuse).
The first large scale application of thyristors, with associated triggering diac, in consumer products related to stabilized power supplies within color television receivers in the early 1970s. The stabilized high voltage DC supply for the receiver was obtained by moving the switching point of the thyristor device up and down the falling slope of the positive going half of the AC supply input (if the rising slope was used the output voltage would always rise towards the peak input voltage when the device was triggered and thus defeat the aim of regulation). The precise switching point was determined by the load on the output DC supply as well fluctuations on the input AC supply. They proved to be unpopular with the AC grid power supplier companies because the simultaneous switching of many television receivers, all at approximately the same time, introduced asymmetry into the supply waveform and, as a consequence injected DC back into the grid with a tendency towards saturation of transformer cores and overheating. Thyristors were largely phased out in this kind of application by the end of the decade.[dubious â€“ discuss]
Thyristors have been used for decades as lighting dimmers in television, motion pictures, and theater, where they replaced inferior technologies such as autotransformers and rheostats. They have also been used in photography as a critical part of flashes (strobes). 0014 November 2010
adetoro oluwasegun adetolaTHANKS SO MUCH,RILI APPRECIATE IT 0020 December 2010
adetoro oluwasegun adetolaPLS, how and why is SCR differ from every other rectifier. 0020 December 2010
andy chuksSilicon Controlled Rectifier SCR Functions as a semiconductor, controls small and very large amounts of electrical power by functioning as a switch. This switch function causes the electric current to be rapidly turned off and on by the SCR. This provides overload protection for digital circuits protecting against too much voltage.
SCRs contain three terminals, or components: an anode, a cathode and a gate, none of which are moving parts that could wear out. The gate will always contain a positive charge. For current to flow through the SCR the anode must be more positive than the cathode. When the cathode becomes more positive than the anode no current will be conducted, even with a positive gate. When the gate has a lower voltage than the cathode then the SCR is off but when the gate has higher voltage the SCR turns on and the current runs from the anode to the cathode.
A SCR becomes a valuable tool in high power devices, such as phase control, power switching, battery charging, high voltage conversion and inverters. It can function in the presence of 2,500 volts and 3,000 amps.
SCRs possess the ability to turn off and on rapidly. In fact, a SCR can switch off and on up to 25,000 times a second. The SCR can switch in microseconds (millionths of a second). This rapid switching regulates the flow of power, or alternating current, through the switch. 0020 December 2010