SCR - SILICON CONTROLLED RECTIFIER
What is an SCR?
Mike Holt's Comment:
Please respond in simple language
with examples that nonengineers can understand.
Response No. 1:
An SCR, or Silicon Controlled Rectifier, is a semiconductor,
or integrated circuit (IC), that allows the control of current using a small current. Basically, it
is a simple direct current (DC) light switch. More specifically, if you place an available current
on the cathode, a load on the drain, you can switch the current on by applying a small current to
the gate. To turn off the flow of current from the cathode to the drain, simply removing the current
from the gate won't do it. You need to stop the flow of current to the cathode externally. SCR's will
block reverse current polarity and only allow correct polarity, which is not very useful on AC circuits
unless you are fine with 50% of the voltage. This is where two SCR's back-to-back come in useful.
One will control current in one direction and the other in the opposite polarity. There is no worry
about turning off the current flow to get the SCR to turn off since the AC current does that for you
automatically when it crosses zero volts 120 times a second. Dimming a lamp using this method only
requires timing when you turn on the SCR's each 120th of a second. Also, a triac is simply two SCR's
back-to-back in one package, eliminating the need for two separate packages.
Michael Wescoatt, email@example.com
Response No. 2:
I will try to keep these answers simple, so a BSEE degree
is not required. However, some basic knowledge of electronic components is needed in order to familiarize
you with the components; otherwise, the explanations will require a course in basic electronics/ semiconductor
devices to go along with it.
What is an SCR? (Extremely brief explanation) – An SCR
is an acronym for Silicon Controlled Rectifier. It works similar to a typical diode, but is controlled
similar to a bipolar transistor as far as connections go. Connection points are Anode [A], Cathode
[K], and Gate [G]. Physical makeup: The SCR is made up of two "P-N" junctions with a "Gate"
attachment between them (a normal diode [power type semiconductor diode] has one P-N junction). The
gate is connected between the two P-N junctions with a current waiting in the forward bias direction
[+ to -] and the voltage is above 1-volt. A momentary pulse to the gate will cause the SCR to conduct
and current will flow across the device until the value changes. If this happens, the gate needs to
be pulsed again to cause conduction to resume; otherwise no current will flow across the device. If
used on AC, the device needs to be triggered [fired] in relationship to the points on the sine wave
that conduction is requested. Example: to chop a wave to be 1/2 peak, the SCR gate will need to be
pulsed with either a high-logic pulse or a positive-voltage pulse until the AC wave reaches 50% of
peak value in the positive flowing direction.
At this point, pulsing is discontinued until the 50% value
on the second half of the wave is present, where the gate will be once again pulsed until the wave
reaches the zero line. No logic or conduction is performed on the negative-direction wave, only on
the positive-direction wave. To control both waves on an AC sine wave/circuit, a device called the
TRIAC is used. It basically is like two SCR’s connected together, one forward and one backward with
both gates connected together. Although it resembles and functions similar to two SCR’s connected
together, it is more involved than this. FYI: There is another device called the SCS [Silicon Controlled
Switch] which can be "turned on or off" in certain parts of a sine wave. Please note: These
explanations are EXTREMELY brief! A lot of information has been withheld to keep the descriptions
simple. To understand in more detail, please get a book on Basic Semiconductor Devices or Semiconductor
Control/Switching Devices, either from the library or a good tech book store.
Scott Thompson, adst@SoCA.com
Response No. 3:
1. SCR: Silicon Controlled Rectifier.
A semiconductor device that functions as an electrically controlled switch. (a) An SCR is one in the
family of semiconductors that includes transistors and diodes. (b) The basic purpose of a SCR is to
function as a switch that can turn on or off small or large amounts of power. It performs this function
without mechanical parts.
2. Where in the circuit are you providing
transient voltage surge suppression? (a) You would most likely use a "Thyristor" in a generator
field circuit, or "Chokes" in voltage regulators and power supplies.
3. SCR's are normally found in electrical/electronic
equipment and are on the secondary side of your equipment supply transformer, effectively isolated
from your service electrical system. Result, no "noise" to spike your lamps.
Stan Price, firstname.lastname@example.org
Response No. 4:
An SCR in my book is a silicone conductor relay. This type
of electronic device can been used with motor control circuits. My experience with an SCR is that
they are very reliable. As for surge suppression, there are a number of devices you can purchase directly
from manufacturers or vendors to help suffice your installation.
Rudy F. Rangel
Response No. 5:
SCR stands for silicon controlled rectifier. It is essentially
a switch with no moving parts. It consists of a semiconductive path and what is called a bridge. When
you apply voltage across the bridge, the path becomes conductive and carries the current until something
interrupts the current ahead of the SCR. Then the path becomes semiconductive again. They are used
in almost all electronic appliances, as they are very reliable and inexpensive. However, I have never
seen a voltage spike from one that could possibly cause damage. While it is true that a spike does
occur when they are activated, it is only a small percentage of the voltage being used. In most electronic
appliances, that would mean between 6 and 9 volts. You would need a very large SCR to create any damage,
and you would not be likely to find one at home.
Response No. 6:
What is a SCR? An SCR is a Silicon Control Rectifier.
Giving the simplest explanation possible, it is similar to a regular rectifier except it is equipped
with a gate circuit or firing circuit, as some technicians are familiar with. By utilizing a firing
circuit you can control how fast the gate opens and closes, which gives you the means of obtaining
variable DC voltage from an AC source. This configuration is used in both electronic and power applications.
I am most familiar with power applications. As an example, let’s say we have a 2500hp DC traction
motor. Unless you have a football field full of batteries, you’d better find a better alternative.
In comes the SCR unit. You can utilize your 480V, 3-phase service or generator and send it to your
SCR bay (cabinet). Once you start your motor, you may input the desired speed or have it configured
automatically by pressure or flow using either 20Ma transducers or 0 to 5 volt sensors. You can also
use temperature-monitoring devices such as RTD's to control the speed, but these uses are not that
popular or useful. As your speed input increases, your firing circuit will begin shortening its time
between impulses to the gate which, in turn, increases your DC voltage which, of course, increases
your motor’s speed. There are many ways of controlling and monitoring the DC output of a SCR unit
but I don't want to turn this short E-mail into a very large book. If anyone has specific questions
or problems, I will be happy to try and help out.
Kenneth Hebert, email@example.com
Response No. 7:
An SCR is a solid state device that controls DC voltage.
There is a control circuit involved that tells the SCR when to "fire," allowing the DC to
pass through it. TRIACS are used to control AC voltage. A TRIAC is basically two SCR's configured
back-to-back. Again, the control circuit for the TRIAC tells it when to fire, allowing only a certain
portion of the sine wave to pass through it. The amount of voltage is determined by where on the waveform
the TRIAC gets turned on and off. One of the downfalls of both SCR's and TRIACS are the harmonics
generated by the fast rise times of the waveform when it is turned off and on. These harmonics are
Tom Bryan, firstname.lastname@example.org
Response No. 8:
It would depend on how the SCR is installed and the application. With more information,
I can assist. In simple terms this is an electronic switch. What it is switching, how it is installed,
and the voltage will determine the surge protection. More information is required.
John West, Sr., email@example.com
Response No. 9:
SCR stands for Silicon Controlled Rectifier. First, think of a simple diode. This
can be thought of as an electronic 'check valve' that only allows current to pass in one direction
(assuming, of course, that it is used within its engineering specifications). The diode conducts forward
current and it blocks reverse current. An SCR adds to this concept. It blocks reverse current just
like a diode. The difference is that it has a gate in order to control forward current. Without a
control signal to the gate, forward current will not conduct. When a control signal is applied to
the gate, forward current conducts just like a standard diode. SCR's are used in those small, inexpensive,
light dimmers and fan controls. Think of the voltage sinusoid being applied to the input of the SCR.
If the control signal is kept off the gate until the sinusoid reaches some trigger point, the output
of the SCR will remain zero until the SCR triggers. At this point, the output will match the input.
When the output falls below the trigger point, the SCR will shut off and the output will fall to zero.
(Some of this is oversimplified. I'm glossing over some things simply to not cloud the issue. If I've
offended any hard-core electronic colleagues, my apologies are offered in advance).
Eric Stromberg, ERStromberg@dow.com
Response No. 10:
SCR stands for Silicon Controlled Rectifier. It allows AC motors to be speed controlled
as if they were DC, but it also causes chopping of the waveform, thereby contributing to harmonics
and noise on the feeders. Think of SCR’s as electronic relays that are so fast they can turn on and
off again within a single cycle. SCR’s are controlled by a small control current through the terminal
called the 'gate' or 'trigger'. The output current controlled is huge by comparison (just like a relay),
but there is no arcing and no contacts to pit. Like all diodes, SCR’s can allow current to flow in
one direction only; therefore, they are used in pairs for the conduction of AC current.
Response No. 11:
SCR is a Silicon Controlled Rectifier. It is a semiconductor
switch that is used to control large amounts of power with small signal input. It is turned off by
the current through it, naturally becoming zero. This happens twice every cycle in AC systems so it
is very suitable for AC to DC power supplies that provide tight voltage regulation on the DC side.
The voltage regulation is achieved by controlling when the SCR is turned on.
Brian E. Purvis, firstname.lastname@example.org