October 1963 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Silicon controlled rectifiers (SCR) have been around for half
a century and are still workhorses in power control and switching
circuits. The SCR's usefulness comes both from being a diode
with a settable forward conduction point ('breakover voltage')
and its property of continuing to conduct below that threshold
voltage once it has been reached. It then stays 'on,' acting
like a conventional bipolar junction diode until p-n junction
is no longer forward biased. At that point the diode is 'off'
again until the breakover voltage is once again reached. When
a sinewave is applied, as in a power supply design, this action
allows the SCR to be turned on for less than half a cycle as
a standard diode would do. Yes, you could design a biasing circuit
to prevent a standard diode from conduction until a voltage
is reached that is higher than the p-n junction barrier voltage,
but then it would also turn off once the applied voltage dropped
back below that level, whereas an SCR keeps conducting all the
way down to the p-n junction barrier voltage. Figure 6 in the
article illustrates the behavior.
See Lothar Stern's article titled "Some
'Technical Terms' Aren't," in the June 1969 issue of Electronics
Silicon Controlled Rectifiers - New Applications in the
By Lothar Stern
Motorola Semiconductor Products Inc.
Already used in industry for power-control
applications, recent drastic price cuts make the SCR attractive
for use in electrical appliances and lighting circuits for the
Editor's Note: The operating principles of SCR's, as described
here, are the same whether these semiconductor devices are used
to control large amounts of current in an industrial application
or smaller current in a home appliance. Because of recent price
reductions, the technician can expect to see more of these devices,
not only in industrial plants, but also in high-volume-produced
electrical appliances for home use. Although some of the highest
current SCR's used in industry may cost several hundred dollars
each, the 18-amp. units discussed below are priced as low as
$1.80, in quantities of 5000.
All too often there is a substantial lag between the development
of a new device and its actual commercial use in applications
for which it is obviously well suited. Such has been the case
with transistors in television applications where, until recently,
the cost of a TV transistor complement has been considered too
high in comparison with vacuum tubes to offset the apparent
advantages of transistorization. It has also been the case with
the silicon controlled rectifier (SCR) which has found widespread
use for power-control applications in industrial equipment,
but whose cost has been too high for the consumer mass market
despite the operating improvements and flexibility it offers
for both large and small home appliances.
Now that SCR prices have been suddenly and drastically reduced,
at least for original equipment manufacturing purposes, there
has been a dramatic increase in interest in such devices for
the electrical appliance market. By the end of the year, a number
of manufacturers are expected to introduce SCR-controlled appliances
in what may well prove to be a new and major breakthrough of
electronic applications in the home.
Basically an SCR is a four-layer n-p-n-p device (Fig. 1)
whose primary application is in electronic switching and power-control
Fig. 1 - Simplified cross-sectional drawing
showing the internal structure of the all-diffused silicon controlled
As a rectifier the SCR will conduct current in only one direction.
But, unlike a conventional rectifier, which begins to conduct
almost the instant its anode becomes even slightly positive
with respect to its cathode, the SCR will remain nonconductive,
even in a forward direction, until the anode voltage exceeds
a certain minimum value called the "forward breakover voltage"
(VBO) . Moreover, the value of VBO can
be varied through the injection of a signal to the third or
gate element of the device which governs the amplitude of the
anode voltage needed to cause conduction or firing. It is this
characteristic which makes the SCR an ideal switch or power-control
device, especially in high-power circuits.
In electronic switching, the SCR can successfully replace thyratrons,
vacuum tubes, and power transistors. In electromechanical equipment,
it replaces switches, relays, variable autotransformers, rheostats,
and timers. As safety devices they can take the place of fuses
and circuit breakers. Moreover, they can duplicate the functions
of magnetic amplifiers and saturable reactors and can serve
as high-speed protective devices and as lightweight, compact
power controls. It is in the area of power control that they
are likely to make their greatest impact on the appliance market.
With SCR control it is possible to continuously vary the
amount of current supplied to an electrical appliance, thereby
providing a precise degree of control over the output of light
and heat and over the speed of universal motors. While it may
appear that similar control can be provided by a conventional
rheostat, the SCR can accomplish this without the power wasting
effect of rheostats and, in higher power devices, it can be
less expensive and is much smaller and lighter than an equivalent
rheostat would be in a similar application.
How It Works
For an over-all indication of how an SCR operates, consider
the voltage-current relationship of the device as illustrated
in Fig. 2. In this diagram, the gate terminal is considered
to be open-circuited, or shorted to the cathode, and external
voltage is applied only to the anode-cathode terminals. Under
these conditions, it is evident that the reverse-bias voltage-current
relationship (anode negative with respect to cathode) is identical
to that of a reverse-biased conventional rectifier. As the reverse
voltage is increased beyond the breakdown level, the semiconductor
junction goes into avalanche and is usually destroyed because
of the excessive junction temperature created by the relatively
high power dissipation (voltage-current product).
Fig. 2 - Curve showing the anode-cathode
characteristics of controlled rectifier with gate open or shorted
Under forward-bias conditions, however, the characteristics
curve is entirely different. As forward bias is increased, in
the region from A to B, there is virtually no current flow through
the device (except for a small leakage current similar to the
reverse leakage current).
At point B, the forward breakover voltage, an avalanche action
takes place and current tends to rise very rapidly. But, if
the external load resistance is low enough to permit a rise
in current to point C, an unusual "switchback" effect takes
place. At the breakover-current value, point C, the voltage
across the rectifier suddenly drops to a very low value, and
the device acts very much like a conventional rectifier. The
internal resistance of the device becomes very low and the current
is limited primarily by the applied voltage and the external
Note here the difference between the reverse and forward
characteristics of the SCR. In the reverse direction, when the
avalanche breakdown voltage (X) is exceeded, the reverse current
rises rapidly but the voltage across the device itself remains
essentially at the breakdown value. The power dissipated in
the rectifier, therefore, is extremely high and the device is
usually damaged irreparably. For this reason, a rectifier or
SCR is never operated beyond its reverse-breakdown point.
In the forward direction, due to the switchback phenomenon,
the internal resistance of the SCR suddenly switches from a
very high to a very low value. Thus, in a circuit containing
an SCR in series with a load resistor, the voltage across the
SCR in a conductive state is negligibly small and current through
the device can reach extremely high levels before rated junction
temperature is exceeded.
From the foregoing discussion it can be appreciated that
the SCR, with the gate open or shorted, acts very much like
a voltage-operated switch (provided the operating voltage is
in the forward direction). At voltage levels below the breakover
point, the switch is open, and beyond the breakover point the
switch is closed. To cause a change in the switch position from
"off" to "on," it is merely necessary to increase the source
voltage, say, from zero to the breakover-voltage value.
To cause a change in the switch position from "on" to "off,"
however, is quite another matter. In the "on" condition, the
voltage drop across the SCR is extremely low and almost the
total applied voltage appears across the external load resistor.
Reducing the applied voltage does not materially change the
voltage drop across the SCR. It does, however, reduce the current
through the load resistance and, equally, through the series-connected
SCR. Hence, as the voltage is reduced, current in the circuit
decreases until a value is reached which is not sufficient to
sustain the avalanche condition within the SCR. Below this current
value, called "holding current," the SCR again reverts to its
high-resistance condition and the switch is shut off.
Using the Gate Terminal
At this point one might logically ask, "What is the value
of this type of performance?" It must be admitted that applications
for this characteristic are indeed limited. There are some possible
uses, as voltage-operated safety devices, for example, but the
SCR's function for even these uses can be greatly improved by
using its third or gate terminal.
Consider now the theoretical static characteristics of the
SCR which are often used to explain its operation with various
current levels injected into the gate terminal, as shown in
Fig. 3. With no gate current applied, the anode voltage must
reach point A before breakover occurs. Now, if a small amount
of voltage is applied to the gate so that the gate terminal
is positive with respect to the cathode, gate current flows
and the forward breakover voltage of the rectifier anode is
reduced to point B. If the gate current is increased further,
anode breakover occurs at point C and, for still higher levels
of gate currents, the SCR characteristics approach those of
a conventional rectifier, point D.
Fig. 3 - Static characteristics with various
The word "theoretical" has been emphasized in the above paragraph
because this type of explanation can lead to erroneous assumptions
regarding actual applications for the devices. It leads to the
assumption, for example, that the SCR could be held at just
below the breakover point for a certain anode potential by the
application of a specified value of d.c. gate current. In actual
practice, however, this is not the case. While the phenomenon
of Fig. 3 can be readily observed, the gate-current range over
which anode breakover is reduced from its open-gate value to
virtually zero is extremely small. Moreover, this gate-current
range varies from one device to another so that no accurate
specifications of this type can be developed.
Therefore, the conventional method of operating SCR's is
to supply a gate signal of sufficient amplitude to assure firing
of all devices. A plot as illustrated in Fig. 4 for Type MCR-808
devices, clearly shows the magnitude of gate voltage and current
needed for reliable triggering. For these units it is seen that
a gate signal of 3.5 volts and 0.1 ampere will trigger all devices
of this type, although triggering for most devices can be achieved
with much lower gate signal values.
Fig. 4 - Gate firing characteristics of type
An important point here is that the amount of gate current
required to change the VBO point from its zero-gate-current
value to almost zero is very small - on the order of milliamps.
And, since the SCR in the breakover or "on" condition can handle
many amperes of current, the current gain of the device is quite
high. In this respect the device acts very much like a sensitive
relay where a small amount of current through the relay coil
can control a much larger current in the relay contact circuit.
There is, however, a major and important difference between
the operation of a relay and an SCR. With a relay, the contacts
will close as soon as an activating current is applied to the
relay coil and they will remain closed only as long as the activating
coil current is present. With an SCR, the "contacts" will close
(the resistance between cathode and anode is reduced to a very
low value) as soon as the required gate current is applied,
but they will remain closed (the SCR will remain in a breakover
condition) even if the gate current is removed. Once fired,
the gate loses all control and the "switch" will remain in a
"closed" or latched state irrespective of any current or voltage
applied to the gate terminal.
The only way to turn off an SCR that is in the "on" state
is to reduce the anode current below the level of the holing
current needed to sustain anode conduction. This, in a d.c.
circuit, can be accomplished in a number of ways, such as mechanically
interrupting the load current, reversing the voltage polarity
from anode to cathode, shunting the major portion of the load
current around the SCR, or by means of commutating capacitors
or the use of LC circuits in the load circuit.
For a.c. circuits, which are of primary interest in the home
appliance field, the SCR is turned off at the end of each positive-going
half-cycle of applied anode voltage.
Ratings & Packages
Today's SCR's are available with maximum forward-cur4ent
ratings ranging from approximately 1 amp up to as much as 300
amps and with reverse breakdown voltage ratings from 25 to 1500
volts. Forward breakover voltages are normally much higher than
reverse breakdown ratings so that a device that will break down
under relatively low reverse voltages may be able to successfully
block forward voltages of several hundred volts. Since the cost
of SCR's increases with increasing reverse-voltage ratings,
it is often desirable to design circuits in which the reverse
voltage is prevented from appearing across the SCR anode-to-cathode
terminals. This can be done by shunting the SCR with a conventional
diode, connected in such a way that the diode conducts when
the voltage across the SCR tends to reverse direction. In this
way, the maximum reverse voltage across the SCR will be equal
to the forward-voltage drop of the diode - on the order of a
fraction of a volt - and, in many instances, the cost of the
SCR-diode combination will be less than the cost of an SCR with
a high reverse-voltage rating.
While high-current SCR's are required for many industrial
applications, devices with current ratings in the 10- to 25-ampere
range are most likely to meet the need of the appliance industry.
Units of this type are available in three basic packages with
a variety of mounting possibilities, including the single-hole-mount
stud package, the popular diamond package, and the highly versatile
press-fit package. It is the press-fit package, designed specifically
for high-volume, low-cost production, that is largely responsible
for the SCR price reductions that have recently been announced.
Typical case configurations and their respective internal connections
are illustrated in Fig. 5.
Fig. 5 - Typical specifications for various
SCR packages. These types are suitable for use in home appliances.
Principles of SCR Power Control
To understand how the electrical characteristics of silicon
controlled rectifiers are generally employed for power-control
purposes, consider the simplified schematic of Fig. 6. Here
the SCR is connected in series with a load resistance and an
a.c. power source. A separate pulse circuit supplies positive-going
trigger pulses to the SCR gate.
Fig. 6 - A simplified circuit which shows
The SCR is selected to have a VBO rating that
is higher than the peak value of the applied a.c. anode voltage.
This means that under conditions with no signal applied to the
gate, the SCR will remain in the off condition at all times
and no current will flow through the load (except for some slight
forward and reverse leakage currents).
If a gate trigger pulse of sufficient amplitude is applied
at the beginning of the positive-going anode cycles, the breakover
voltage of the SCR can be reduced to the point where breakover
will occur almost at the beginning of the anode cycle. The SCR,
therefore, is turned on and will remain in the on condition
for the remainder of the positive half of the anode cycle even
though the gate trigger pulse is removed. Load current will
follow the positive-going anode voltage, being limited principally
by the value of the load resistance. During the negative portion
of the anode voltage, load current will be cut off entirely,
irrespective of any gate signal.
If the gate trigger pulse is delayed so that it occurs, for
example, at the peak of the positive anode cycle, the SCR will
conduct for only a quarter of a cycle. By introducing a variable
phase shift between anode and gate signals, complete control
can be achieved over the positive half cycle of anode voltage.
For equipment whose output depends on the average value of the
load current (such as the light from an incandescent bulb, the
heat from a heating element, or the speed of a universal motor),
this provides the means for controlling the output from zero
to some maximum value.
Of course, for a simple half-wave circuit the maximum output
will not be as great as if both halves of the anode cycle were
utilized. Maximum control can thus be obtained by using SCR's
in full-wave or bridge circuits.
A simple SCR control circuit, in this case a motor-speed
control for electrical appliances, is shown in Fig. 7.
Fig. 7 - A simple motor speed control circuit.
In this circuit, the anode-cathode terminals of the SCR are
connected in series with the motor field and armature across
the 117-volt a.c. line. Resistors R1 and R2 in series with potentiometer
R3 represent a voltage divider from which the gate signal is
Values for the resistive divider are calculated so that,
with the variable arm of R3 in position A, the amount of gate
current is not great enough for SCR triggering even at the maximum
instantaneous anode potential. With the control advanced to
point B, enough gate current would flow at the peak of the cycle
to trigger the device. At that point, field current would flow
during 90° of the applied voltage. At point C, the firing
potential would be reached sooner so that load current would
flow for perhaps 130° or more of the applied voltage. This
circuit offers control over almost half of the positive-going
portion of the applied voltage. During the negative half cycle,
SCR current is cut off.
Diode D1 is inserted in the gate circuit to block the application
of excessive reverse current to the gate electrode which could
result in damage.
A more elaborate circuit, one that permits the control of
both halves of the applied voltage cycle, is shown in Fig. 8.
Here, a full-wave rectifier bridge is employed in such a way
that the voltage applied to the divider and SCR circuit is pulsating
d.c. comprising both halves of the input cycle. Operation otherwise
is similar to the previous circuit. The utilization of the entire
input cycle in this bridge circuit permits higher maximum motor
speeds and smoother operation than obtainable in the half-wave
Fig. 8. Circuit that permits the control
of both alternations. Diode across motor field protects circuit
from reverse voltages.
One advantage of SCR control is that the circuits often can
be designed to accomplish additional functions. This is illustrated
in the above designs, patented by Momberg and Taylor of Singer
Mfg. Co., where the SCR is connected between the motor field
and armature. In this type of connection, for all but the maximum-speed
setting, voltage feedback from the motor tends to keep the motor
speed constant under varying loads - an advantage that is of
considerable importance in the power tool field.
In each of these circuits, of course, the load may be a heat-
or light-producing appliance provided that the current rating
of the SCR is high enough to handle the required full-load current.
Since the minimum conduction angle in the above circuits
is 90°, these do not offer continuous control from zero
to maximum. There are other configurations, however, that do
provide this feature. One such circuit is shown in Fig. 10.
With this type of connection, changing the setting of potentiometer
R varies the phase angle between the SCR anode and gate voltages
from zero to 180°, thus controlling the firing point of
the SCR's over an entire half cycle.
Fig. 10 - Circuit permitting control over
entire half cycle of applied voltage.
In this circuit, the value of R must be at least ten times
the reactance of C at the operating frequency. With a minimum-resistance
setting of R, the gate circuits are connected across the lower
half of the center-tapped transformer winding and the voltage
applied to the gates of the SCR's is in phase with the anode
voltage. The SCR's conduct for nearly 180° on alternate
half cycles. With maximum setting of R, the reactance of C
can be considered negligible and the gate circuits are effectively
connected across the opposite half of the transformer winding.
The voltage applied to the anodes and gates are nearly 180°
out of phase and virtually no conduction takes place. With intermediate
settings of R, conduction angles ranging from near zero to almost
180° can be achieved.
Unijunction transistors and four-layer diodes (similar to
SCR's but without the gate-trigger provisions) may be used to
provide turn-on pulses for SCR circuits. Such devices are normally
employed in relaxation oscillator circuits with variable pulse
spacing so that triggering may occur at any point of the SCR
anode cycle. A typical circuit using unijunction transistor
triggering is shown in Fig. 9.
Fig. 9 - SCR controlled light-dimmer circuit.
Although much simpler circuits can be used for this purpose,
including the circuits previously shown for motor-speed control,
this circuit provides a full range of light-brightness control.
This arrangement employs a full-wave bridge rectifier, a zener
diode (D1) to clip and regulate the voltage applied to unijunction
transistor Q1, and an SCR. By varying the value of R2, the charging
rate of C can be controlled so that the trigger pulse across
R4 can appear at any point of each applied half-cycle. This
results in complete control over the SCR.
With SCR control, every electric light switch in the home
becomes a potential light dimmer that provides continuously
variable operation from full off to full on. A high-power bulb
in a child's nursery may be adjusted to give plenty of light
during play hours, but it can be turned down to just a glimmer
for night-light purposes. In living and dining rooms, light
dimmers can provide just the right degree of illumination to
fit any mood and, for amateur puppeteers, the basement rumpus
room can be converted into a theater, complete with theater
as well as stage light dimming equipment.
SCR control can increase the functions and conveniences of
kitchen electrical appliances. Electric ranges with SCR control
can match the infinite heat selection of gas equipment, electric
toasters can become more efficient and far more reliable, electric
mixers and blenders, automatic refrigerators and freezers, even
dishwashers, can benefit from the variable current capabilities
of SCR circuits.
In the workshop, SCR circuits in electric power tools can
convert a particular implement into a multi-purpose device.
Drills and saws can be adjusted for just the right speed for
virtually all types of materials and, with feedback circuitry,
can provide constant torque irrespective of load. Soldering
irons with heat control can be used for a variety of purposes
other than soldering and every piece of electrical equipment
that runs too fast, gets too hot, or burns too bright for a
particular application can benefit.
These applications, of course, are in addition to those where
SCR's can replace relays or contactors in equipment where the
reliability and ruggedness of semiconductor devices have decided
advantages. While these advantages, with yesterday's high-priced
SCR's, seemed rather vague, today they take on a new significance.
And, for the electronics engineer and technician, the widespread
application of SCR's in volume-produced electrical equipment
promises another, as yet unexplored, field of operation with
new opportunities for all.
Posted April 6, 2015