How do SCR works and basic circuits

Have ever you see SCR? Some may use it instead of switches and relays. Because we have seen it works high speed and quiet.

Sound good, doesn’t it?

If we are going to learn how does SCR Works.  Let me explain them to you.

What is SCR?

SCR is a semiconductor and thyristor device that is important for a power electronic circuit. SCR stands for Silicon Controlled Rectifier.

Sure, these words are not necessary for you. You don’t have to do exams. We just use it well enough, right?

how do scr circuit works

SCR Application

How can we use SCR? a lot of applications for example:

  • Use to load current, 0.8A to 10000A or more.
  • Lighting control
  • Speed ​​control of motor
  • Rectifier circuits to convert alternating current to direct current.
  • Or Convert from direct current to alternating current.
  • Use instead of a switch or a relay to turn the load on and off.
  • Switch circuits that require very high speeds.
  • Need a spark-free switch that works.
  • Used instead of a very high current diode. The advantage is that it is controlled by small currents. Like transistor.
  • And more.

Which is your want?

Structure and equivalent circuit

Imagine you take a hammer to smash an SCR. You see, it is a four-layer structure. And has 3 PN junctions. Some called 4-layer PNPN Diode. Because it passes current in only one direction.

Comparison of Diode and SCR structures

SCR symbol

Compare Diode and SCR symbol. The SCR is similar to a diode. Just have more a G leg only.

Compare Diode and SCR symbol

Also, it has 3 leads similar to a bipolar transistor. But names are a difference, include:

  • “A” stands for Anode
  • “K” stands for Cathode
  • “G” stands for Gate

See Structures of SCR again. Now, we compare it with transistors.

We are more familiar with Transistors than SCR.


Compare Transistors and SCR Structures

We can modify both types of transistor NPN and PNP to the SCR easily.

Connect transistor as SCR

Here is connecting two transistors as SCR.

If compare circuit both.

  • Gate of SCR is collector(C) of PNP transistor and Base(B) of NPN transistor.
  • Anode of SCR is Emitter(E) of PNP transistor.
  • The cathode of SCR is Emitter(E) of NPN transistor

We will learn it works from the example circuit diagram later(below).

What is more?

Working of SCR

It is difficult to explain how SCR works are easy to understand. But I will try to reduce Its in-depth details. Leaving only important principles for basic use only.

In a normal SCR circuit. We always connect a load in series with “A” lead and positive supply. But a cathode(K) connects to the negative supply.
Here is working of SCR under two heads:

Open gate mode

See in the circuit.

Working of SCR in No gate voltage

When no voltage to the gate.

There are 2 cases to learn:

  • Forward BiasedUnder PN junction of SCR is a situation as in an NPN transistor with a base open.So, no current flows through the load RL and the SCR.But, if we continually increase the voltage supply. Until a reverse-biased point breaks down.
    Make the SCR now conducts heavily. It is “Turn ON” state. But in normal we do not add high voltage supply like this. We called Forward Breakover voltage, in forward-biased.
  • Reverse Biased In contrast, we turn the polarity supply voltage. It is like a reverse bias of a diode. If we increase more voltage up and up.Until to one point will make SCR conduct heavily current, and damaged in end.We called the Reverse Breakover voltage.

So, we must use a lower voltage than the Breakover voltage. The characteristics of each SCR are different. We should study the information in detail.

Not only that.

You would not use SCR with no gate current, Right?

Enter gate voltage

While we trig the gate with entering a little current. It is positive if compare with the Cathode. Makes SCR works right ways. We often use this.

Working of SCR with gate current

But do not forget the voltage across A and K must be lower than break over voltage.

When SCR conducts current through Anode to Cathode. Then, it keeps working on this current. Even if we remove the gate current it already.

If we reduce the current flows through Anode to Cathode leads to a holding current. It causes SCR to stop.

See in the graph:

The holding current is the lowest current that makes SCR still works.

Both IG1 and IG2 indicate the gate current. If we enter this more current it will work fast.

Size and shape of SCR

Currently, there are many types of SCR depending on the application. And there are many SCR specifications or qualifications as well.

We can divide into 2 properties. These are:

  • Can withstand a current of 0.8-2000 amperes.
  • Withstand pressure drop across it. May be about 200V to more than 1000V etc.

With these qualities, there are different shapes of SCRs.

All shape of SCRs

Sometimes you may wonder. Can your existing SCR be used interchangeably? Or choose which one to buy Most economical.

Let me give you an example of 6 SCRs that should be used.

  • 2N5060: 30V 0.8A, TO-92. Looks like 2N2222 transistor.
  • T106D1: 400V 4A SENSITIVE GATE SCR, TO-220
  • C106B1: 200V 4A A, TO-202
  • 2N6507 SCR: 400V 25A, TO-220
  • C38M: SCR 600V 35A, TO-65
  • 2N6509: 800V 250A, TO-220AB

What is more?

How to pick SCR

When using the SCR, consider Its limits. Which the manufacturer will always specify.

Important things to know:

  • 1. VBO (Forward breakover voltage) is the voltage causes SCR to start conducting without no gate current. Or, The maximum voltage that SCR can tolerate.
  • 2. IFmax (Maximum forward current)is maximum currents can flow in SCR without damage to the SCR.
  • 3. IGTmin (Minimum gate triggering current) is the minimum gate current that can trigger the SCR.
  • 4. IH(Holding current) is the lowest current that flows through A-K. That can keep on SCR works.
  • 5. VB (Peak reverse voltage) is the return voltage that will fit the SCR Can break down

For example, T106D SCR is 400V 10A.
Means an SCR of VBO = 400 V and
IF (max) = 4A. But Average current: 2.5A, IH = 20mA , IGT = 200uA.

So we should use SCR, not over its capability. It may cause too heat and damage in the end.

How SCR is triggered

We can trigger in many ways. I would like to divide by 2 types of power supply:

1. Using DC Power Supply

We often use it with a DC power supply. It is so easy.


It shows the principles that make SCR conduct current by direct current.

In the figure has 2 circuits.

Trigger current is separate from the power supply
1. Trigger current is separate from the power supply.

Get trigger current from a single power supply with divider resistors
2. Get trigger current from a single power supply with divider resistors.

But both circuit has the same principle.

We apply a positive supply to an anode of SCR. And negative to the cathode.

Then, enter a little current to the gate of SCR. We called a trigger current. And, this voltage must be a positive only, when compared with a cathode.

If the level of the gate current still lower than the trigger current. SCR will not work. But when SCR conducts current now. It will keep on works at all. Even we will reduce or remove the trigger current.

2. Using AC Power supply

Most we use SCR as the power electronics switches. To turn on-off any load circuits. But SCR conducts current in one way only. Or DC voltage as above.

However, we can use SCR in AC power supply.

Imagine you can control a motor, a lamp, and more on AC main using SCR.

Sound good, doesn’t it?

I will divide into 2 cases.

1. Triggering with DC current.

Look image the circuit. The trigger circuit is DC voltage. It can control SCR on-off.

DC trigger current to SCR with AC source

Are you understand? Look:

SCR out waveform and switches mode

Look at the signal image at various points of the SCR circuit. The signal passing through the load will become a Pulsating DC voltage.

It is a half rectifier circuit and a switches circuit. We can control by the trigger circuit.

Suppose that we enter the DC trigger voltage (Turn ON). It is in phase with the AC supply. Makes SCR conduct current to load in a waveform.

I feel uncomfortable, Cannot describe words well. You look graph better.

Sample simple circuits using SCR

If you still do not understand. You may look at the circuit below. To increase your imagination

The transistor lat as SCR trigger

This circuit works similar to an SCR. When the trigger input is high, Both transistors Q2(PNP transistors) and Q1(PNP transistor) will work. Therefore, current flow to the base pin of Q2, it has voltage 0.7 volts at resistor R1.

The transistor lat as SCR trigger

When cutting the activation voltage, the transistors can continue to perform, because of the base current. The reset circuit will do this by cutting the power supply circuit. Which this circuit we press the switch Reset button only.

Audio magic using SCR 2N1599

This circuit can be a trickster. Through a permanent magnetic switch controls the wreath only. I can make a noise out.

Audio magic by SCR 2N1599

Working principle of the circuit. BZ1 is usually not out loud. If you like BZ1 rang out.Tap the wreath is a permanent magnet switch S1 and S2.
To close the circuit. The R1 and R2 acts divided voltage from the power source. The R3 is acting current limiting.To trigger the pin to the gate of SCR1, It conducts current.Is a current flowing through the BZ1, out loud. and If you want to come back in normal conditions.Is the permanent magnet from the wreath switches S1 and S2. Which make scr1 stop conduct current. Therefore, current flows through BZ1, do not clutter it.

Not only that, Read more:

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I always try to make Electronics Learning Easy.

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Apichet Garaipoom

I love the electronic circuits. I will collect a lot of circuit electronics to teach my son and are useful for everyone.
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