What is an SCR _ Thyristor, Silicon Controlled Rectifier _ Radio-Electronics

15/06/2015 WhatisanSCR|Thyristor,SiliconControlledRectifier|RadioElectronics.com

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What is an SCR Silicon Controlled Rectifier orThyristor- the thyristor or silicon controlled rectifier, SCR is a device that is widely used forcontrolling or switching power and often high voltage AC or DC circuits.

IN THIS SECTION

Thyristors or silicon controlled rectifiers, SCR are find many uses in electronics, and in particular for powercontrol. These devices have even been called the workhorse of high power electronics.

Thyristors are able to switch large levels of power are accordingly they used in a wide variety of differentapplications. Thyristors even finds uses in low power electronics where they are used in many circuits from lightdimmers to power supply over voltage protection.

The term SCR or silicon controlled rectifier is often used synonymously with that of thyristor - the SCR or siliconcontrolled rectifier is actually a trade name used by General Electric.

SCR discovery



The idea for the thyristor was first described by Shockley in 1950. It was referred to as a bipolar transistor with ap-n hook-collector. The mechanism for the operation was analysed further in 1952 by Ebers.

Then in 1956 Moll investigated the switching mechanism of the thyristor. Development continued and more waslearned about the device such that the first silicon controlled rectifiers became available in the early 1960s whereit started to gain a significant level of popularity for power switching.

Thyristor applicationsThyristors, or silicon controleld rectifiers, SCRs are used in many areas of electronics where they find uses in avariety of different applications. Some of the more common applications for them are outlined below:

AC power control (including lights, motors,etc).

Overvoltage protection crowbar for power supplies.

AC power switching.

Control elements in phase angle triggered controllers.

Within photographic flash lights where they act as the switch to discharge a stored voltage through theflash lamp, and then cut it off at the required time.

Thyristors are able to switch high voltages and withstand reverse voltages making them ideal for switchingapplications, especially within AC scenarios.

Thyristor basicsThe thyristor or silicon controlled rectifier, SCR is a device that has a number of unusual characteristics. It hasthree terminals: Anode, cathode and gate, reflecting thermionic valve / vacuum tube technology. As might beexpected the gate is the control terminal while the main current flows between the anode and cathode.

As can be imagined from its circuit symbol shown below, the device is a "one way device" giving rise to the GEname for it the silicon controlled rectifier. Therefore when the device is used with AC, it will only conduct for amaximum of half the cycle.

In operation, the thyristor or SCR will not conduct initially. It requires a certain level of current to flow in the gateto "fire" it. Once fired, the thyristor will remain in conduction until the voltage across the anode and cathode isremoved - this obviously happens at the end of the half cycle over which the thyristor conducts. The next halfcycle will be blocked as a result of the rectifier action. It will then require current in the gate circuit to fire the SCRagain.

Thyristor symbolThe silicon controlled rectifier, SCR or thyristor symbol used for circuit diagrams or circuit seeks to emphasis itsrectifier characteristics while also showing the control gate. As a result the thyristor symbol shows the traditionaldiode symbol with a control gate entering near the junction.



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SCR / Thyristor symbol for circuit diagrams and schematics

Other types of thyristor or SCRThere is a number of different types thyristor - these are variants of the basic component, but they offer differentcapabilities that can be used in various instances and may be useful for certain circuits.

Reverse conducting thyristor, RCT: Although thyristors normally block current in the reverse direction,there is one form called a reverse conducting thyristor which has an integrated reverse diode to provideconduction in the reverse direction, although there is no control in this direction.

Within a reverse conducting thyristor, the device itself and the diode do not conduct at the same time. Thismeans that they do not produce heat simultaneously. As a result they can be integrated and cooledtogether.

The RCT can be used where a reverse or freewheel diode would otherwise be needed. Reverse conductingthyristors are often used in frequency changers and inverters.

Gate Assisted Turn-Off Thyristor, GATT: The GATT is used in circumstances where a fast turn-off isneeded. To assist in this process a negative gate voltage can sometimes be applied. In addition to reducingthe anode cathode voltage. This reverse gate voltage helps in draining the minority carriers stored on then-type base region and it ensures that the gate-cathode junction is not forward biased.

The structure of the GATT is similar to that of the standard thyristor, except that the narrow cathode stripsare often used to enable the gate to have more control because it is closer to the centre of the cathode.

Gate Turn-Off Thyristor, GTO: The GTO is sometimes also referred to as the gate turn off switch. Thisdevice is unusual in the thyristor family because it can be turned off by simply applying a negative voltageto the gate - there is no requirement to remove the anode cathode voltage. See further page in this seriesmore fully describing the GTO.

Asymmetric Thyristor: This device is used in circuits where the thyristor does not see a reverse voltageand therefore the rectifier capability is not needed. As a result it is possible to make the second junction,often referred to as J2 (see page on the device structure) can be made much thinner. The resulting n-baseregion provides a reduced V as well as improved turn on time and turn off time.

By Ian Poole (https://plus.google.com/104687638164370436625?rel=author)

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What is an SCR _ Thyristor, Silicon Controlled Rectifier _ Radio-Electronics