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SCRs: What are They and How Do They Work?
Jan 26th, 2000



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, mwescoat@vegas.cirquedusoleil.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, stan.price@juno.com

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.

M. White

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, khebert@dksolutions.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 considered noise.

Tom Bryan, salami@ameritech.net

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., jwest@psihq.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.

Earl Dean

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, brian.purvis@ieee.org

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