Name: MARMOL, Frank C.
Date: August 5, 2025
Subject: ECE 312-EE Industrial Electronics
Section: B
Professor: Engr. Victor Solito Isaac
Assignment (Thursday, August 7, 2025)
SILICON-CONTROLLED RECTIFIERS
Introduction
The SCR is a silicon unilateral three-terminal thyristor. It is the most commonly used and
highest power rated thyristor currently available. The SCR is available in current ratings from
around 1.0 A up to values in excess of 1000 A, and voltage ratings up to 5 kV.[1]
The device performs in much the same manner as a p–n diode; that is, it will allow a current to
flow in one direction, and it will block current in the other direction. The major difference is that
forward conduction can be controlled in the SCR. Conduction is controlled by passing current
through the gate terminal.[1]
The SCR is a four-layer silicon device, the layers being alternately p- and n-type semiconductor
materials. This structure is referred to as p–n–p–n. Three semiconductor junctions are therefore
formed in the device.[2]
When the device itself is forward biased, that is, anode positive with respect to the cathode,
two of the junctions will be forward biased, while the third is reverse biased. It is this reverse
biased junction that allows the SCR to block anode current until gate current flows. The actual
size of the silicon wafer will be varied at the time of manufacture to achieve required on state
voltage and current ratings. The higher the power handling capabilities of the SCR, the larger is
the wafer.[3]
For the SCR to turn off, the anode current must be reduced to near zero. If the anode current falls
below a value known as the ‘holding current’ it will relax back to the off state. The processes
involved in reducing the anode current to this value are discussed in Section 10.2.3.[3]
The holding and latching currents for a particular SCR are always very small values when
compared with the anode current rating. The latching current is slightly higher than the holding
current. For example, a C122E SCR has the following current ratings:[3]
anode current—8.0 A
latching current—25 mA
holding current—20 mA.
The reverse operation of an SCR is identical to that of a p–n diode. It will block current until
breakdown occurs. This is caused by the reverse voltage exceeding the peak reverse voltage
(PRV) rating of the device.[3]
Characteristics of SCR
I.
Reverse Blocking Mode of SCR
In this mode, the SCR is reverse biased by connecting its anode terminal (A) to negative end and
the cathode terminal (K) to the positive end of the battery. This leads to the reverse biasing of the
junctions J1 and J3, which in turn prohibits the flow of current through the device, in spite of the
fact that the junction J2 remains in forward biased condition.[4]
In this state, the SCR behaves as a typical diode. In this reverse biased condition, only reverse
saturation current flows through the device as in the case of the reverse biased diode which is
shown in the characteristic curve by blue line. The device also exhibits the reverse breakdown
phenomenon beyond a reverse safe voltage limit just like a diode.[4]
II.
Forward Blocking Mode of SCR
Here a positive bias is applied to the SCR by connecting anode terminal (A) to the positive and
cathode terminal (K) to the negative terminal of the battery, as shown in the figure below. Under
this condition, the junction J1 and J3 get forward biased while junction J2 gets reverse biased.[5]
FORWARD CHARACTERISTICS
i.
When anode is +vew.r.t cathode the curve between V &I is called Forward
characteristics. OABC is the forward characteristics of the SCR at Ig =0. if the
suppliedvoltage is increased from zero point A is reached .SCR starts conducting
voltage across SCR suddenly drops (dotted curve AB) most of supply voltage appears
across RL.
REVERSE CHARACTERISTICS
ii.
When anode is –ve w.r.t cathode the curve b/w V&I is known as reverse characteristics
reverse voltage come across SCR when it is operated with ac supply reverse voltage is
increased anode current remains small avalanche breakdown occurs and SCR starts
conducting heavily is known as reverse breakdown voltage.
REFERENCES:
[1] https://electricalacademia.com/electronics/silicon-controlled-rectifier-scr/
[2] https://www.electrical4u.com/thyristor-silicon-controlled-rectifier-scr/
[3] https://www.rcet.org.in/uploads/academics/rohini_96440522279.pdf. Rohini (PDF), Rohini
College of Engineering and Technology, [Online]. Accessed: Aug. 6, 2025.
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