February 1966 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.
Prior to the invention
of the silicon-controlled rectifier (SCR), most semiconductor devices were only
able to handle powers in milliwatt range, with some reaching into the realm of a
few watts or tens of watts. Voltage handling got up to around 1 kV (and 500 A)
for diodes and 200 V for transistors. The higher power and voltage devices
were exceedingly expensive and were used primarily in special applications. When
this article appeared in a 1966 issue of Electronics World magazine and
the SCR had been introduced less than a decade earlier, the price of an SCR had
dropped from more than $300 to less that 50¢ apiece. Due to the construction
of the SCR, it is more robust and can handle high power switching applications (it
is generally not used as a signal amplifier). Unlike a triac, which can be controlled
with a signal to turn the device both on and off while voltage is present across
the anode and cathode, the SCR can only be used to switch off current, and then
to turn back on the bias needs to be removed. At that point voltage can be restored
and the SCR will conduct again until the triggering signal cuts it off once more.
The SCR Revolution
A drastic price decline has opened up many new applications for
this semiconductor switching device. By the end of this year, SCR's may represent
a $40 million market. This article shows a large number of basic static switching
and control circuits.
By J. E. Mungenast
Semiconductor Products Dept.
General Electric Co.
In 1957 electronics took a giant step into the broad world of electric control.
In that year, the first practical silicon controlled rectifier was fabricated by
Gordon Hall of General Electric. This was the first semiconductor that could economically
switch kilowatts. The SCR has since grown into a wide family of devices with a potential
$40 million market by the end of 1966. Let's examine why it has grown so swiftly
and what opportunities it offers to the electronics industry as a whole.
The SCR's and their close relative the bidirectional a.c. switches (known variously
as Triacs, symmetrical switches, or five-layer switches) are logical extensions
of the solid-state rectifier and transistor family. Fig. 1 shows these four basic
devices along with some of their characteristics and applications. When the SCR
and its thyratron gas-tube counterpart is compared with the transistor and its receiving-tube
counterpart, we see that the prime difference is not only the power-handling capability
but also the fact that the SCR works in the switching mode only. This is the key
to its power-handling capability - but it can cause some inconvenience on direct-current
circuits, as we will see later. The second valuable SCR attribute is its extraordinary
sensitivity. Again, like the thyratron, it offers a degree of power amplification
that makes possible inexpensive, simple control circuitry triggered by a number
of sensors, such as photocells and thermistors.
Fig. 1 - Four basic semiconductor devices along with their main
characteristics and their applications.
Three members of the SCR family showing the very wide range of
currents and voltages. The large assembly is a water-cooled a.c. switch that will
handle up to 1200 amps at 1800 volts peak. The small unit at the left is the new,
inexpensive plastic-encapsulated SCR with a current rating of 2 amps at 200 volts
peak. The small unit at the right is a bi-directional switch (Triac) with a 10-amp,
400-volt peak rating. Additional SCR's are shown in the cover photograph.
Fig. 2 - Simple static-switch SCR circuit.
Fig. 3 - A popular variable speed control circuit using SCR.
Fig. 4 - High-gain limited-range light control circuit with unijunction
transistor. For applications in which control of the load below the half-power point
is not required, a rectifier can supply a half cycle uncontrolled, and an SCR can
provide regulation by phase control during the other half cycle.
Fig. 5 - Time-delay relay circuit for enlarger, patio light,
or garage light. Circuit can be used to turn off lights after an interval of up
to about 1 minute. The SCR supplies enough current to energize a low-cost relay
coil. The SCR is triggered by only a few microamps of current from the RC network.
Note: All Resistors 1/2 Watt
Fig. 6 - Full-wave control circuits using SCR's and Triacs.
Fig. 7 - SCR inverter circuit changes d.c. to higher voltage
This golf cart employs SCR's for speed control. The entire electronic
package with the throttle rod coming out of the top of it has been removed and placed
on the golf-cart seat.
Fig. 8 - "D.c. transformer" or "chopper" battery motor drive.
A bank of 500 SCR inverter modules, each producing kilowatts
of power, are connected in parallel to produce almost a mega-watt of ultrasonic
power for a particular military application.
Fig. 9 - A 15-amp SCR-based battery-charging regulator which
cuts down the high charging rate to a trickle charge at a preset level.
Thus, the SCR will play an important role in automotive applications, as for
example in electronic ignition systems or in electric windshield wiper controls.
It will be used in the appliance field for varying the speed of motors and for varying
the brightness of lamps. It will be found in the toy/hobby market where its sensitivity
will make possible unique control schemes such as for model trains.
The cover photograph, as well as the one in this article, show some members of
the SCR family. Note the wide range of size and operating characteristics. Also
note particularly the tiny new plastic-encapsulated SCR. This device has an instantaneous
power gain of over one million, yet it costs only 35 to 50 cents in large quantities.
Like most semiconductors, the SCR has been characterized by a precipitous price
decline. The first units in 1957 cost about $325 each. Today a comparable unit costs
less than $2.00.
The SCR is in itself of little use; auxiliary circuitry is necessary to accomplish
the desired function. The main circuits that form the basis for many of the thousands
of SCR applications of today and tomorrow are discussed below.
The Static Switch
Fig. 2 shows the simplest form of the half-wave static switch using a magnetic
reed switch for energizing the gate circuit of an SCR. Note that the SCR fires,
or latches into conduction, whenever gate current is allowed to flow through the
reed switch. This can be actuated either by a permanent magnet, making a proximity
switch, or by passing current through an electromagnet coil. The wide range of uses
includes simple explosion-proof switches (there is no arcing mechanism and no fumes
are entrapped, as the semiconductor and reed switch are mounted in a plastic block),
relay replacement where highly repetitive operations are needed, and sensitive relay
replacement where an SCR amplifier in conjunction with a rugged, inexpensive power
relay does a more reliable, economic job than a delicate sensitive relay.
But the most promising family of applications for the SCR's involves phase control,
whereby the load is switched on and off at certain times in the applied a.c. cycle,
thereby metering the power to the load. The basic principles of operation have been
described many times in past issues of this publication, so they will not be repeated
here. Fig. 3 shows one of the most popular circuits, the series motor speed control.
For many years, tool and appliance manufacturers have sought a way not only to
adjust the speed of the series motor but, more important, to furnish governor action
that would maintain the necessary torque, even at the low-speed settings. This is
accomplished in the circuit shown in Fig. 3. Essentially the SCR performs three
functions here: first as an on-off switch, second as the means for voltage reduction,
and third as the sensing mechanism making feedback control possible. The counter-e.m.f.
of the rotor gives us a tachometer indication of speed which is compared with the
desired setting of the speed-adjust potentiometer. Any differential then varies
the phase angle to maintain the necessary speed. Thus, both the sensitivity and
the power-handling capability of the SCR are essential.
Other phase-control circuits use the unijunction transistor as a sensitive preamplifier.
The circuit shown in Fig. 4 has a control gain of over 10 million and can be constructed
with less than $16 worth of parts even in relatively small quantities. Sensors such
as cadmium-sulphide photocells, thermistors, and silicon strain gages can all be
used, making possible functions that were economically unfeasible just a few years
A variation of phase control is shown in Fig. 5. This is a time-delay relay for
photographic enlargers, garage lights, etc. Since phase control is a form of time
delay, it is only necessary to extend the RC time constant to time out intervals
of approximately one minute or so.
Full-Wave A.C. Phase Control
As useful as the above circuits are, a simple, economical method of controlling
alternating current was needed. Previously, a-c. control required two SCR's plus
a number of auxiliary components to control both halves of the sine-wave cycle.
Recently, the bilateral switch emerged. A most useful version is the gate-triggered
unit known as the Triac.
Fig. 6 shows a comparison of the SCR method with the Triac method to accomplish
phase control. Not only is the Triac much simpler but it also includes protection
from line-voltage transients that can otherwise harm semiconductor circuitry. The
circuit is suitable for the speed control of small a.c. motors of the shaded-pole
and permanent-magnet split-capacitance types. This control is not suitable for a.c.
motors with starting mechanisms, such as split-phase, split-capacitance, or synchronous
motors. However, when used with a motor properly designed for the control, variable
speed of furnace blowers, circulating pumps, and air-conditioning blowers will bring
a new level of comfort to the household of the future. Such a system replaces the
manual potentiometer with a special thermistor located in the furnace bonnet. With
a variable speed control the blower idles continuously, eliminating the stratified
air layers, with speed increased only when the thermostat calls for more heat and
bonnet temperature rises.
SCR lighting controls were among the earliest applications of phase controls
to be used in the home and in industry. Hundreds of thousands of 600-watt SCR dimmers
are now used for control and dimming of ceiling fixtures. The larger table- and
floor-lamp market can now be served with the more compact Triac devices in circuits
similar to that shown in Fig. 6B. Indeed, an entire low-power dimmer circuit can
be readily built into a standard lamp socket.
The Inverter-Chopper Control
The basic circuit shown in Fig. 7 essentially changes the state of electric power,
functioning in reverse, as it were, from the familiar transformer-rectifier power
supply. The latter takes 60-cps, 117-volt a.c., for example, and converts it into
12 volts d.c. The inverter takes the 12-volt d.c, from an automobile battery, for
example, and converts it to 117-volt, 60-cps a.c. by means of electronic switching,
enabling the voltage to be stepped up by means of the transformer.
While this job has been accomplished for years by both the automotive breaker
points of an ignition system and the older vibrator power supplies, the availability
of higher power, more efficient solid-state switches in recent years has greatly
increased the market. The wide variety of uses, such as operation of fluorescent
lamps from low-voltage d.c. standby power supplies for telephone communications
systems and computers, and operation of satellite electronic equipment from very
low voltage fuel cells, are all based on the use of the inverter. By employing SCR's
rather than simple power transistors in the inverter, control circuits may be incorporated
into the gate circuits to adjust output.
But an even more interesting use is the conversion of 60-cps power to higher
frequencies. This is done by rectifying the 60-cps line and using the resultant
d.c. to power an inverter whose output is at some higher frequency. This technique
is used to increase the synchronous speed of induction motors or to reduce the size
of inductive or capacitive elements in power supplies or filters. The use of higher
frequencies is also important when fluorescent lights are used. Such lighting systems
may operate more efficiently at, say, 3000 cps.
Of considerable interest today is the "chopper" concept, whereby inverter techniques
are used to provide efficient regulation of direct current for motors or other loads
from a d.c. source, such as a battery. Controlling the speed of a d.c. motor with
resistors and rheostats generally results in poor regulation and worse efficiency.
Switching-mode operation results in higher efficiency along with simplicity of circuitry
(Fig. 8). The motor or heater load averages out the variations, as will a capacitive-filtered
power supply or a lamp, at sufficiently high frequencies. Among practical applications
are speed controls for golf carts and lift trucks.
While the lower power applications are undoubtedly of greatest interest to the
electronics engineer and technician, very high power applications are also making
history. Motor drives for up to 13,000 horsepower using SCR phase-control concepts
are employed in several modern steel mills. The chopper concepts are used in electric
trains, while special circuitry permits SCR's to operate at frequencies from 10
kc. to 50 kc., thanks to improved devices. Large blocks of power, such as required
for electrochemical processes, are being controlled by SCR's. Wide-scale use depends
only on lower SCR costs in the near future.
Other new applications for SCR's include the carrier-current remote-control system,
capacitive-discharge automotive ignition system, and regulated battery-charging
systems (Fig. 9).
The carrier-current remote control is based on the synchronization of transmitter
and receiver by means of a sine wave. It is more practical today because the great
sensitivity of the SCR simplifies the circuit and allows inexpensive, reliable tone-filtering
systems. An SCR ignition circuit for a car can use the principles of capacitive
discharge, employing an inexpensive silicon controlled rectifier to discharge a
storage capacitor through the car's ignition coil. In battery-charging circuits,
the use of an SCR results in simplified circuitry. Note that in the circuit shown
in Fig. 9, the single SCR serves as rectifier, regulating control unit, and as a
stage of amplification permitting a more precise battery-charging cycle.
The SCR applications engineer is always tempted to ask his customer, "What else
can the SCR do for you?", for the best applications for the SCR seem to be those
in which it ends up accomplishing more than one function and of making practical
a device or concept beyond the reach of conventional techniques.
If the electronics enthusiast finds himself somewhat at a loss because of the
growth of new SCR applications, imagine the dilemma of the mechanical or electrical
engineer not in the semiconductor field who is suddenly faced with an entirely new
technology to learn. We hope that the task has been made simpler for both by the
introduction of the experimenter-hobbyist concept of SCR education. G-E and other
manufacturers have a "hobby line" of SCR's along with training manuals showing simple
construction projects using these devices. These are certainly of interest to the
experimenter, but they were primarily intended as an educational vehicle for the
non-electronics engineer or to show the electronics engineer some new semiconductor
control concepts. The educational information is organized into complete small bites
ideal for those scarce extra hours of the busy engineer.
The SCR has revolutionized solid-state electronics. The devices can control motors
ranging in size from flea power to thousands of horsepower, can transmit intelligence
at hundreds of kilowatts, and can switch power into massive research equipment.
Future growth will be limited only by the ingenuity of the device and circuit engineer.
Posted August 4, 2022