19-03-2012, 04:00 PM
To study the VI characteristic of SCR.
Experiment2 power ele.docx (Size: 15.45 KB / Downloads: 72)
THEORY:
The silicon controlled rectifier is a four layered PN bipolar semiconductor device with three terminal ,SCRS belong to the thyrister family of electronics device which operate on the principle of current conduction when the break doen voltage is reached ,these devise may also have a control element such as a gate which can trigger the device in to conduction below the breakdown voltage point .An SCR has as anode, cathode and gate terminal .it operate similar to a normal biased condition but SCR must triggerd in to conduction by gate terminal .once gate terminal triggered the SCR in to conduction .it also acts like a latched switch.
Operation :
The anode is connected through a series limiting resistor RL to a positive cathode is connected to ground. In this junction 1 and 3 are forward biased but junction 2 is reverse biased which prevent any appreciable current from following through the SCR when s1 is moved up to the bottom side of RA ,a small gate current flows towards the supply voltage this introduce holes in to P type gate region which allow a large current to flow from cathode to anode through the SCR in turn I2 is now forward biased when the gate is set back to ground via silicon the large current flowing through the SCR tend to keep I2 internally forward biased and the SCR is on or latched.
Current voltage characteristic of an SCR:
The SCR operates similar to normal diodes when in the reverse biased condition the SCR exhibit a very high internal impedence with perhaps a slight reverse blocking if the reverse current rapidly increase to a large value and may restore the SCR.
Application:
(1) High voltage direct current system
(2) Machine tool control
(3) Aircraft power supplier
(4) Motor control
(5) Battery charging and welding system
CONCLUSION:
When the SCR is turned on, voltage drops and current increases sharply. When we increase the gate current, VBO decrease as we can see from the characteristic and practical.