28-11-2012, 02:47 PM
Silicon Controlled Rectifier
[attachment=41496]
Definition
The Silicon Controlled Rectifier (SCR) is a semiconductor device that is a member
of a family of control devices known as Thyristors. The SCR has become the
workhorse of the industrial control industry. Its evolution over the years has
yielded a device that is less expensive, more reliable, and smaller in size than ever
before. Typical applications include : DC motor control, generator field regulation,
Variable Frequency Drive (VFD) DC Bus voltage control, Solid State Relays
and lighting system control.
· The SCR is a three-lead device with an anode and a cathode (as with a standard
diode) plus a third control lead or gate. As the name implies, it is a rectifier
which can be controlled - or more correctly - one that can be triggered to
the “ON” state by applying a small positive voltage ( VTM ) to the gate lead.
· Once gated ON, the trigger signal may be removed and the SCR will remain
conducting as long as current flows through the device.
· The load to be controlled by the SCR is normally placed in the anode
circuit. See drawing below.
Volt-Ampere Characteristics
Figure One below illustrates the volt-ampere characteristics curve of an SCR. The vertical axis + I represents the
device current, and the horizontal axis +V is the voltage applied across the device anode to cathode. The parameter
IF defines the RMS forward current that the SCR can carry in the ON state, while VR defines the amount of voltage
the unit can block in the OFF state.
Biasing
The application of an external voltage to a semiconductor is referred to as a bias.
Forward Bias Operation
· A forward bias, shown below as +V, will result when a positive potential is applied to the anode and negative to
the cathode.
· Even after the application of a forward bias, the device remains non-conducting until the positive gate trigger
voltage is applied.
· After the device is triggered ON it reverts to a low impedance state and current flows through the unit. The unit
will remain conducting after the gate voltage has been removed. In the ON state ( represented by +I), the current
must be limited by the load, or damage to the SCR will result.
Reverse Bias Operation
· The reverse bias condition is represented by -V. A reverse bias exists when the potential applied across the
SCR results in the cathode being more positive than the anode.
· In this condition the SCR is non-conducting and the application of a trigger voltage will have no effect on the
device. In the reverse bias mode, the knee of the curve is known as the Peak Inverse Voltage PIV (or Peak Reverse
Voltage - PRV) and this value cannot be exceeded or the device will break-down and be destroyed. A
good Rule-of -Thumb is to select a device with a PIV of at least three times the RMS value of the applied voltage.
SCR Protection
The SCR, like a conventional diode, has a very high one-cycle surge rating. Typically, the device will carry from
eight to ten time its continuous current rating for a period of one electrical cycle. It is extremely important that the
proper high-speed, current-limiting, rectifier fuses recommended by the manufacturer be employed - never substitute
with another type fuse. Current limiting fuses are designed to sense a fault in a quarter-cycle and clear the
fault in one-half of a cycle, thereby protecting the SCR from damage due to short circuits.
Switching spikes and transients, which may exceed the device PIV rating, are also an enemy of any semiconductor.
Surge suppressors, such as the GE Metal-Oxide-Varistor (MOV), are extremely effective in absorbing these shortterm
transients. High voltage capacitors are also often employed as a means of absorbing these destructive spikes
and provide a degree of electrical noise suppression as well.
Computing the Required Firing Angle
For accurate SCR gating, the Firing Circuit must be synchronized with the AC line voltage being applied anodeto-
cathode across the device. Without synchronization, the SCR firing would be random in nature and the system
response erratic.
In closed-loop systems, such as motor control, an Error Detector Circuit computes the required firing angle based
on the system setpoint and the actual system output.
The firing circuit is able to sense the start of the cycle, and, based on an input from the Error Detector, delay the
firing pulse until the proper time in the cycle to provide the desired output voltage. An analogy of a firing circuit
would be an automobile distributor which advances or retards the spark plug firing based on the action of the vacuum
advance mechanism.
Testing the SCR
Shorted SCRs can usually be detected with an ohmmeter check (SCRs usually fail shorted rather than open).
Measure the anode-to-cathode resistance in both the forward and reverse direction; a good SCR should measure
near infinity in both directions.
Small and medium-size SCRs can also be gated ON with an ohmmeter (on a digital meter use the Diode Check
Function). Forward bias the SCR with the ohmmeter by connecting the red ( + ) lead to the anode and the black
( - ) lead to the cathode. Momentarily touch the gate lead to the anode; this will provide a small positive turn-on
voltage to the gate and the cathode-to-anode resistance reading will drop to a low value. Even after removing the
gate voltage, the SCR will stay conducting. Disconnecting the meter leads from the anode or cathode will cause
the SCR to revert to its non-conducting state.
When conducting the above test, the meter impedance acts as the SCR load. On larger SCRs, the unit may not
latch ON because the test current is not above the SCR holding current. Special testers are required for larger
SCRs in order to provide an adequate value of gate voltage and load the SCR sufficiently to latch ON.
[attachment=41496]
Definition
The Silicon Controlled Rectifier (SCR) is a semiconductor device that is a member
of a family of control devices known as Thyristors. The SCR has become the
workhorse of the industrial control industry. Its evolution over the years has
yielded a device that is less expensive, more reliable, and smaller in size than ever
before. Typical applications include : DC motor control, generator field regulation,
Variable Frequency Drive (VFD) DC Bus voltage control, Solid State Relays
and lighting system control.
· The SCR is a three-lead device with an anode and a cathode (as with a standard
diode) plus a third control lead or gate. As the name implies, it is a rectifier
which can be controlled - or more correctly - one that can be triggered to
the “ON” state by applying a small positive voltage ( VTM ) to the gate lead.
· Once gated ON, the trigger signal may be removed and the SCR will remain
conducting as long as current flows through the device.
· The load to be controlled by the SCR is normally placed in the anode
circuit. See drawing below.
Volt-Ampere Characteristics
Figure One below illustrates the volt-ampere characteristics curve of an SCR. The vertical axis + I represents the
device current, and the horizontal axis +V is the voltage applied across the device anode to cathode. The parameter
IF defines the RMS forward current that the SCR can carry in the ON state, while VR defines the amount of voltage
the unit can block in the OFF state.
Biasing
The application of an external voltage to a semiconductor is referred to as a bias.
Forward Bias Operation
· A forward bias, shown below as +V, will result when a positive potential is applied to the anode and negative to
the cathode.
· Even after the application of a forward bias, the device remains non-conducting until the positive gate trigger
voltage is applied.
· After the device is triggered ON it reverts to a low impedance state and current flows through the unit. The unit
will remain conducting after the gate voltage has been removed. In the ON state ( represented by +I), the current
must be limited by the load, or damage to the SCR will result.
Reverse Bias Operation
· The reverse bias condition is represented by -V. A reverse bias exists when the potential applied across the
SCR results in the cathode being more positive than the anode.
· In this condition the SCR is non-conducting and the application of a trigger voltage will have no effect on the
device. In the reverse bias mode, the knee of the curve is known as the Peak Inverse Voltage PIV (or Peak Reverse
Voltage - PRV) and this value cannot be exceeded or the device will break-down and be destroyed. A
good Rule-of -Thumb is to select a device with a PIV of at least three times the RMS value of the applied voltage.
SCR Protection
The SCR, like a conventional diode, has a very high one-cycle surge rating. Typically, the device will carry from
eight to ten time its continuous current rating for a period of one electrical cycle. It is extremely important that the
proper high-speed, current-limiting, rectifier fuses recommended by the manufacturer be employed - never substitute
with another type fuse. Current limiting fuses are designed to sense a fault in a quarter-cycle and clear the
fault in one-half of a cycle, thereby protecting the SCR from damage due to short circuits.
Switching spikes and transients, which may exceed the device PIV rating, are also an enemy of any semiconductor.
Surge suppressors, such as the GE Metal-Oxide-Varistor (MOV), are extremely effective in absorbing these shortterm
transients. High voltage capacitors are also often employed as a means of absorbing these destructive spikes
and provide a degree of electrical noise suppression as well.
Computing the Required Firing Angle
For accurate SCR gating, the Firing Circuit must be synchronized with the AC line voltage being applied anodeto-
cathode across the device. Without synchronization, the SCR firing would be random in nature and the system
response erratic.
In closed-loop systems, such as motor control, an Error Detector Circuit computes the required firing angle based
on the system setpoint and the actual system output.
The firing circuit is able to sense the start of the cycle, and, based on an input from the Error Detector, delay the
firing pulse until the proper time in the cycle to provide the desired output voltage. An analogy of a firing circuit
would be an automobile distributor which advances or retards the spark plug firing based on the action of the vacuum
advance mechanism.
Testing the SCR
Shorted SCRs can usually be detected with an ohmmeter check (SCRs usually fail shorted rather than open).
Measure the anode-to-cathode resistance in both the forward and reverse direction; a good SCR should measure
near infinity in both directions.
Small and medium-size SCRs can also be gated ON with an ohmmeter (on a digital meter use the Diode Check
Function). Forward bias the SCR with the ohmmeter by connecting the red ( + ) lead to the anode and the black
( - ) lead to the cathode. Momentarily touch the gate lead to the anode; this will provide a small positive turn-on
voltage to the gate and the cathode-to-anode resistance reading will drop to a low value. Even after removing the
gate voltage, the SCR will stay conducting. Disconnecting the meter leads from the anode or cathode will cause
the SCR to revert to its non-conducting state.
When conducting the above test, the meter impedance acts as the SCR load. On larger SCRs, the unit may not
latch ON because the test current is not above the SCR holding current. Special testers are required for larger
SCRs in order to provide an adequate value of gate voltage and load the SCR sufficiently to latch ON.