SCR – Silicon Controlled Rectifier
SCR – Silicon Controlled Rectifier
• Lighting, motor speed control, electric welding
and electric heating all demand the delivery of a
variable and controlled amount of electrical
power
• Tradition delivery methods employed using
variable transformers or rheostats in series with
load
• These solutions are usually bulky and expensive
in terms of power wastage
SCR – Silicon Controlled Rectifier
• Silicon controlled-rectifier (SCR) is part of the
thyristor family that are used in high-power
electronic switching circuits
• SCR has been used for power control with none
of the above faults
• Advantages: small, inexpensive, needs no
maintenance, and wastes very little power
• Some modern SCRs can control currents of
several hundred amperes and voltages higher
than 1000 V
SCR – Silicon Controlled Rectifier
The PNPN junction Schematic symbol for a silicon-controlled
rectifier
• The silicon controlled-rectifier (SCR) is often
referred to as a PNPN junction because of its
composition
• The schematic symbol is the same as the diode
except for the added gate lead
Silicon Controlled-Rectifier
• The anode is composed of P-type material
• The cathode is composed of N-type material
• The gate is made of P-type substrate
• Below is the transistor NPN and PNP equivalent
circuit
Gate
Silicon Controlled-Rectifier
• The SCR is a latching device that requires a
small value of positive gate current for a brief
instant to switch on
• The amount of gate gate current required to
switch on an SCR is called the gate-trigger
current, IGT
• Once the SCR has been switched on, it will
remain on as long as a minimum value of current,
called the holding current, flows between the
anode and the cathode
Silicon Controlled-Rectifier
• When a positive current is applied to the base of
transistor Q2, Q2 turns on
• The collector of Q2 drives the base of Q1
negative, forward biasing the emitter-base
junction of Q1
Silicon Controlled-Rectifier
• Current then flows at the base of Q1 and Q1
conducts
• If the gate is now disconnected, current will
continue to flow at the base of Q2 through the
collector of Q1
• Therefore the two transistors hold each other in
the conducting state, allowing a continuous flow
of current between the anode and cathode
SCR Characteristic Curve
• Initially as VF is increased, the SCR conducts
only a small amount of leakage current
• As VF increases, eventually the breakover voltage
is reached and latches into the conducting state
SCR Characteristic Curve
• The forward breakover voltage represents the
point where the SCR switches from a non-
conducting state to a conducting state with no
triggering gate signal
• The SCR is usually triggered at a value below
the breakover voltage
• The reverse breakdown voltage is equivalent to
the avalanche region of a diode
• Under normal operation, this voltage should not
be exceeded
SCR Characteristics
• Forward Operating Region - This is the ON state or the forward
conduction region between the anode and the cathode. There is very
little opposition to the flow of current.
• Forward-Breakover Voltage VBR(F) - Once this voltage is
exceeded, the SCR is forward biased and enters the forward operating
region. The value of VBR(F) is maximum when the gate current is
zero (IG = 0). With a gate current, the value of VBR(F) decreases as
the value of gate current increases.
• Forward Blocking Region - This is the OFF condition of the SCR.
The forward current from the anode to cathode is blocked due to an
effective open circuit of the SCR.
SCR Characteristics
• Reverse Blocking Voltage - This is the OFF condition of the SCR
during reverse bias conditions.
• Reverse Breakdown Region VBR(R) - This is the same as the
Zener or avalanche region of a semi-conductor diode.
• Holding Current (IH) - Once the anode current drops below this
level, the SCR is no longer in the forward operating region, but is in
the OFF or forward blocking region.
• Gate Trigger Current (IGT) - The value of gate current that
switches the SCR from the forward blocking region to the forward
operating region.
SCR – Application Example
