What Is Silicon Controlled Rectifier (SCR?

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5. Silicon Controlled Rectifier (SCR):
A silicon-controlled rectifier or semiconductor-controlled rectifier is a four-layer solid state current controlling device. It has three junctions, labeled J1, J2 and J3 and three terminals. The anode terminal of an SCR is connected to the P-Type material of a PNPN structure, and the cathode terminal is connected to the N-Type layer, while the gate of the Silicon Control Rectifier SCR is connected to the P-Type material nearest to the cathode.
To turn-on of an SCR, a positive gate voltage between gate and cathode. This gives rise to a gate current where charges are injected into the inner p layer of the device. This effectively reduces the voltage at which forward break-over occurs. It can be gathered …show more content…

Holding current: When the gate terminal of SCR is kept open, then the maximum value of anode current at which Silicon Controlled Rectifier is turned OFF from conducting condition, is known as Holding Current.
Requirement of the R2 resistor:
The second resistor, R2 is the gate cathode resistor. This is sometimes denoted as RGK and it is included to prevent spurious triggering the action of the resistor can be seen with respect to the two transistor analogy of the SCR. It shows that a low external resistance between the gate and cathode bypasses some current around the gate junction. Accordingly a higher anode current is required to initiate and maintain conduction. It is particularly found that low current high sensitivity SCRs are triggered at very low current levels and therefore an external gate-cathode resistance is required to prevent triggering by thermally generated leakage current in the gate region.

CIRCUIT …show more content…

So voltage drop across resistor R4 varies accordingly. This is applied to the gate of SCR to control its angle of firing. Increase in voltage across resistor R4 increases the firing angle of SCR and makes the bulb glow brighter .the opposite happens if T1 conducts less. In this way, the light intensity of a 100W bulb gets varied from minimum to maximum. So it can be used as a 15W or lower wattage bulb, while going to bed and a 60W bulb or so while reading.

Remote control of variation on light intensity:

Here the circuit brings a advantage for the disabled persons. A remote sensing is used here to control the light intensity. Here the signal is transmitted and received which are commonly used television remotes. This receiver provides negative voltage across across RS1. Thus the current across RS2 flows opposite to the supply current. The summation of the supply current and negative current through drain terminal produces a decrease in total current acrcoss the resistor Rd.
Thus the flow of current is varied by the remote sensing by the disabled persons. Once the remote is turned on the switch closes the

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