Here we are talking about controlling things like lamps, motors, heaters, and other similar electrical loads. The devices we use for this purpose are called thyristor and triac, OK? Both these devices, thyristor and triac, they can actually do this job like simple switch ON/OFF or they can also adjust the power given to these loads. But then, we must understand one big issue when we try to use thyristor for this purpose.
Thyristor Is Like Diode – Only One Direction
That big problem is, this thyristor, it is just like a diode only. It means it is a unidirectional component, right? So it allows current to move only in one direction, that is from Anode to Cathode – it does not allow current to flow in reverse direction.
Now this behavior is fine when we use thyristor in DC circuits because in DC circuit, current is always flowing in one direction. So once we trigger this thyristor ON then it stays ON and all DC power just flows straight to the load without any trouble.
Why Thyristor Does Not Work Well In AC
But now, when we try to use thyristor in AC circuit – like 230V mains AC or any other sinusoidal AC power – then we face problem. Because in AC, current keeps changing its direction every half cycle, you see.
But our thyristor allows current in only one direction. So it will conduct only during positive half cycle when Anode becomes positive with respect to Cathode. In negative half cycle, even if Gate is getting signal from controller, thyristor will remain OFF and block current.
So what actually happens? It behaves just like a half-wave rectifier and gives power only during one half of AC cycle. That means our load is getting power only half the time. So load is running at half power and this is not good when we want full power operation.
How Then We Can Achieve Full-Wave AC Control With Thyristor
Now then, if we want to have full power or full-wave AC control using thyristors, we need to use some extra tricks. One simple idea is, we can put one thyristor inside a full-wave bridge rectifier circuit, so it receives positive voltage pulses in both half cycles of AC and we can trigger it properly by gate signals every time we need.
Another method is we can connect two thyristors in reverse-parallel configuration, means back-to-back – so that one thyristor handles positive half cycle and the other thyristor handles negative half cycle. This way, both directions of AC get covered and load gets full power.
But then both these ideas, they increase total number of components. Also wiring becomes more complex and difficult to manage. So entire circuit becomes more bulky, more messy, and more complicated to build and troubleshoot later.
Analyzing Triac Circuit Topologies
What is Triac Actually, Let Us See
Now first of all, we know that apart from thyristor and SCR, there is this one more solid-state device. That one also belongs to the thyristor family only. We simply call it “Triode AC Switch” but for short we call it Triac.
This device is also used like power switch in electronic circuits, same as thyristor and SCR. But then the biggest plus point of Triac is that it is bidirectional. Not like thyristor which is one-way only. So that means you can control both halves of AC signal using one Triac.
Bidirectional Triggering – The Main Feature of Triac
So now what is this bidirectional thing? That means Triac can conduct current in both directions. It does not matter that voltage at its terminal is positive or negative.
It does not matter that gate pulse is positive or negative. If we apply gate pulse then Triac triggers and starts conducting both ways.
That is why we say Triac is two-quadrant gate-controlled switch, because both main terminals (MT1 and MT2) and gate (G) can get triggered by both polarities.
How Triac is Built Internally, Let Us See Deeply
OK now, if we look more deeply into Triac working, we will see something interesting. Internally this device behaves like two thyristors joined back-to-back. That is called inverse parallel connection. But since it is inconvenient to use two separate thyristors outside, all this arrangement is packed inside one single three-pin body. Also both internal thyristors share same Gate pin, so only one Gate terminal is needed. That makes things simpler.
Terminal Names – No Anode, No Cathode Here
Now since Triac can work both ways, we cannot use normal names Anode and Cathode like in thyristor or diode. Because then Anode and Cathode only work for one direction, right? So instead we use different names for main terminals. We call them MT1 (Main Terminal 1) and MT2 (Main Terminal 2). And Gate pin is still called G, like usual.
So in AC circuits, normally gate connection of Triac is done with respect to MT1. Just like how in thyristor, gate is connected with respect to Cathode. It is also a bit similar to how base and emitter are arranged in transistor. That is all about Triac, so now you know it is very useful device for switching AC loads in electronic circuits.
Construction, Layers and Symbol of Triac
Now first of all, we must understand that inside the body of this triac thing, the construction is not simple.
It has special P-N layer doping arrangement because it needs to handle current flowing in both directions, that is forward and reverse.
And if you see the circuit diagram symbol of triac, it looks like two thyristors placed in reverse directions but sharing the same gate terminal, so we can recognize it very easily by just looking at the symbol.
So since we already know that this triac is a bidirectional three-terminal power device then we must look a little deeper into how it is actually made from inside. Basically the triac is made using a four-layer structure.
When it is conducting in positive direction, the layers are arranged like PNPN. But when it conducts in negative direction, then the layer sequence becomes NPNP. This switching between both structures is what makes it special and different from normal devices.
OFF State Behavior – Like an Open Switch
Now when the triac is not triggered by any gate pulse, then it behaves just like an open switch. It does not allow current to flow from any terminal, no matter which direction you apply the voltage.
So in this OFF condition, triac just sits quietly like a dead open circuit switch, fully blocking any AC or DC current. You can think of it as doing absolutely nothing, just waiting.
ON State – Gate Pulse Starts the Show
But once we give a small gate triggering pulse, the triac suddenly jumps into conduction mode. And now here comes the fun part, you see, it does not care at all about the direction of current.
Whether current flows from MT2 to MT1 or from MT1 to MT2 in reverse, the triac allows both. It is like a wild beast that lets current pass freely once triggered, in either direction.
The Four Triggering Modes of Triac – Very Important to Know
Now because the triac is symmetrical in structure and can conduct current in both directions and also accepts both positive and negative gate signals, there are four different ways, or triggering modes, to turn it ON. Let us see these 4 modes carefully because they are very important to understand:
- Mode I+:
- MT2 current = Positive (+ve)
- Gate current = Positive (+ve)
This is the most sensitive mode and triggering is easiest here. We can easily turn ON triac in this mode.
- Mode I–:
- MT2 current = Positive (+ve)
- Gate current = Negative (–ve)
This mode also works well but since the gate current is negative, we need a little more gate current to trigger the triac.
- Mode III+:
- MT2 current = Negative (–ve)
- Gate current = Positive (+ve)
This mode works when the AC is in reverse cycle, and we give positive gate pulse. Triac conducts in reverse direction.
- Mode III–:
- MT2 current = Negative (–ve)
- Gate current = Negative (–ve)
This is the least sensitive mode among all four. It needs strong gate triggering pulse to make triac conduct.
So now you can see that this triac thing is not so simple but we can use it to control AC power in very smart way, because it handles both directions and accepts both positive and negative triggers.
That is why triac is very useful in AC dimmer circuits, motor speed control and many other power control applications.
Shown in I-V Characteristics Curve of Triacs
Now let us first try to see that all these four triggering modes of triac, we can actually observe them clearly in the I-V characteristics graph of the triac.
This graph shows us how the voltage and current behave in each quadrant when gate pulses are applied in different combinations.
That graph helps us to understand how the triac turns ON and how much gate current is needed in every case. It is very important to understand because this tells us the working principle of triac in full detail.
Triggering the Triac in Different Quadrants
So when we are using a triac, especially in Quadrant I, then the most common way to trigger it is by giving a positive gate current. This method is called mode I+ and it is the easiest and most sensitive one. That is why we use it in most cases, because it requires less effort and works more reliably.
But even in Quadrant I, we can also use a negative gate current to turn the triac ON and this one is called mode I–.
So both positive and negative gate currents work here, though I+ is more preferred, because it needs less gate current amplitude to trigger the device.
Now same is true in Quadrant III – there also we have two modes of triggering. One is with negative gate current, and that is called mode III– which again is more commonly used in circuits, because it is easier to trigger. The other one is mode III+ where we use positive gate current in the negative current region. This one is less sensitive, but still it works.
So now we can say that in total, we got all four modes of triggering:
- I+
- I–
- III+
- III–
Out of these four, modes I+ and III– are more sensitive and more reliable. That is why they are used more times in practical circuits than the other two modes.
I– and III+ Need Stronger Gate Current
But modes I– and III+ are not that sensitive, so these ones need stronger gate current to fire the triac ON properly.
We have to push more signal into the gate to make it respond, compared to I+ or III– which work even with smaller gate pulses. That is very important to remember because if we use weak gate signals in these modes, triac will not turn ON and circuit will fail.
Holding Current Needed – Just Like SCR
Now, just like SCRs, our triac also needs a small amount of current called holding current, symbol IH, to stay in conduction. Especially around the zero-crossing point of the AC waveform, this holding current becomes very important.
If current drops below IH then the triac will turn OFF automatically. This happens because the internal carrier concentration is not enough to sustain conduction anymore.
So we must always make sure that load current does not fall below IH during operation, or else triac will switch OFF unintentionally.
Even Though It Looks Like One Device
So even though triac looks like a single device from outside, we must remember that internally it is like two thyristors connected back-to-back.
That is why the two sides can show different electrical behavior, like different breakdown voltages, different trigger voltage levels, and also different holding current values.
This is important to understand because even though it is one package, it acts a bit like two separate SCRs packed together.
So when we design circuits using triac, then we must test and use it carefully, especially when we want clean switching across full AC cycle. If we do not consider this, then we may get erratic behavior, false triggering, or failure to turn ON.
Where We Use Triac – Applications in Real Life
So now when we look at where this triac is used in real life, we can say that it is one of the most widely used semiconductor parts for switching and controlling AC power.
The main reason behind this is that triac can be switched ON using either a positive or a negative gate pulse, and this works no matter what side the AC voltage is at that moment.
So if AC waveform is in positive half cycle or even in negative half cycle, then this triac will still trigger properly. It does not care about the supply polarity.
That makes it super useful for controlling various AC loads, like if we want to dim a lamp, adjust the speed of an AC motor, control a heater, or operate a fan. Triac can do all of that with very simple control circuits.
Because of this bidirectional ON behavior, we can build very simple AC control circuits using just one triac. The gate pulse can come from any low-power source, like microcontroller or simple circuit and still it can control big AC loads without any trouble.
That is why triacs are used in light dimmers, fan speed controllers, heater controllers, and many other AC power control applications.
Understanding Fundamental Triac Circuits
Now, above we are seeing one very basic and easy type of DC triggered triac power switching circuit. This one diagram above is showing how it is working.
When that switch SW1 is in open condition, then nothing is going into triac Gate pin, right?
So then no gate current is happening and at that time this triac is not firing, and that lamp is also staying OFF only.
But suppose we are closing that switch SW1 then one gate current is starting to flow from battery side (we are calling that VG) through that resistor R, and going straight into triac Gate.
So what is happening now? That triac is getting triggered and going into full conduction mode. It is behaving just like one closed switch. So because of this, now that lamp is starting to take full power from that AC mains, which is sinusoidal supply.
Important Note about Gate Current Polarity
Now one important thing is that this battery is always giving positive gate current whenever we are closing SW1.
That means this triac is getting gate signal properly during mode I+ and also during mode III+, no matter what is the polarity of that MT2 terminal.
So it is not caring about whether MT2 is going positive or negative, that DC gate signal is always keeping triac ON during those modes.
Problem of Extra Gate Supply Needed
But one small problem is coming here. For this circuit to work nicely, we must give extra gate supply – like one separate DC voltage, either positive or negative, to fire that triac properly.
That extra source we are needing always. So it is becoming not so convenient sometimes.
Better Way – Using AC Supply for Gate Triggering
But the good thing is, we are not always needing to use extra gate supply. We can even use that same AC supply voltage for giving gate triggering signal.
That means the AC voltage which is going to lamp, same we can use to trigger that triac also. So this is saving us from using battery or DC supply.
Basic Triac Switching Circuit – ON and OFF AC Power Control
So we can see here that we got one very simple triac circuit, yes, nothing complicated at all. That triac is used like static switch for controlling AC power.
We do not use it like fancy circuit, no, it is just giving plain ON and OFF action, just like how we did before with DC switching circuits. Nothing else, just simple ON or OFF, that is all.
Now when that switch SW1 is in open condition, then what happens? Then that triac is behaving like open circuit, means totally OFF.
So the AC current is not able to reach the lamp, you see? That is why the lamp is getting zero current, no light, nothing happening.
But since we close that switch SW1 then the story changes totally.
Now the gate of triac is getting triggered through the resistor R, which is actually working like current limiting resistor.
That resistor is not allowing excess current to hit the gate, because if too much current goes to gate, triac might get damaged. So resistor is there to save everything.
Triac Latching Process
Now once the gate gets correct trigger then triac turns ON. We also say that triac is getting “latched”. That latching happens shortly after each half-cycle of the AC starts. So the triac is sending full power through to lamp load, means lamp is glowing properly, very bright, as we expect.
What Happens During Each AC Cycle
We must remember that this supply is not DC, no, it is sinusoidal AC which is always going up and down, positive and negative, half-cycles. Because of this nature of AC, this triac has more behavior.
At end of every half cycle, the voltage of AC is going down to zero, yes? So in that moment, the current through load also becomes zero. So what happens then? Then triac is automatically getting “unlatched”.
That means it stops conducting by itself. No need for us to do anything manually, it happens automatically.
But when next half-cycle starts and if SW1 is still closed, then again the gate gets triggered and triac is getting latched again. But the other half of the triac conducts because it has two thyristors inside, back-to-back. So this way, triac again starts passing full current through the lamp.
Continuous ON/OFF Action
So this ON and OFF triggering keeps happening again and again on both sides of the AC waveform — positive and negative, every half cycle — as long as switch SW1 is kept closed.
We do not need to care about that, because this whole process is automatic due to nature of triac and AC.
We call this whole process full-wave control because both halves of the sinewave, full wave, are getting controlled by triac, not just half-wave. This is how we manage full power control using one simple circuit without complex parts.
Taking the Triac Switching Further – Gate Control
Now let us first understand something very important, that this TRIAC is nothing but just like two SCR’s connected in opposite direction internally, back-to-back.
So we can actually play around with how we give the gate triggering. We are not limited to simple ON/OFF action.
We can do much more than that, like phase control or dimming, if we modify the way the gate is getting fired. That means we can have much more control on switching.
Since TRIAC is conducting in both positive and negative half cycles, we can decide when to trigger it and how to trigger it.
We will see later in next part of the circuit below how to do that exactly.
How Lamp Works With Switch Positions A B and C
So first, we know that if we keep the switch SW1 at position A, and then we leave it open, then no gate current will flow into the TRIAC.
That means nothing happens. The lamp will stay completely OFF. Nothing glows. No current path. The TRIAC remains non-conducting.
But now if we move the switch to position B then gate current will start flowing during every half cycle of AC. That gate triggering happens just like it was happening in the basic simple way before.
So TRIAC starts conducting both in mode I plus and mode III minus. That way, full power gets delivered to the lamp and the lamp glows with full brightness.
We see full illumination because both positive and negative cycles are conducting fully.
Now if we shift that switch to position C, something different happens. Then the diode comes into action.
That diode is placed in such a way that it blocks the gate current when MT2 becomes negative.
That time the diode becomes reverse biased, so it stops that gate current from flowing. So TRIAC does not conduct during negative half cycles.
Then what happens is conduction occurs only during positive half cycles, only mode I plus. That means only half of the power goes to the lamp.
So lamp glows but now with half brightness. We see dim light compared to full brightness, because only half power is delivered.
Summary of Switch Positions and Lamp Behavior
So depending on where we keep the switch, we can have three possible states:
- At position A → Lamp stays completely OFF, no gate current flows, no conduction.
- At position B → Full Power ON, gate current flows every half cycle, full conduction both positive and negative, full brightness lamp.
- At position C → Half Power ON, gate current flows only in positive half cycles, because diode blocks in negative cycles, so half brightness lamp.
That is how we can control the lamp brightness using simple switch positions and TRIAC triggering arrangement.
Triac Phase Control for AC Power
Now we talk about another method which we call Triac Phase Control. This method is used very commonly when we want to control switching by cutting phase of AC signal.
That way we can vary how much voltage is going to the load. So that also means we can control power going to that load.
Mostly we use this method with motors or other AC devices. In this case, we can control both positive and negative halves of input waveform.
So this AC motor speed control works by adjusting firing angle. That gives us fully variable and very linear type of control. Because of that, we can change voltage from complete zero to full applied AC voltage.
That range is very smooth and controllable like shown in waveform.
How the Basic Phase Triggering Circuit Works
So in this basic phase triggering type circuit, we are using one triac device which we are putting in series with the AC motor. Then we are connecting this series combination directly across AC mains supply which is sinusoidal in nature.
Now to control phase angle, we are using one variable resistor which we are calling VR1. This VR1 is used for adjusting how much delay we want to give to gate triggering of the triac. Because of that gate delay, we can control when the triac turns ON in each AC half cycle.
That is how we are actually controlling voltage that reaches to motor. When we adjust VR1, then we change phase angle, and then motor gets more or less power depending on that.
How the Triac Gets the Trigger
The triac does not get gate voltage directly. It gets it through combination of VR1 and capacitor C1. These two components are working together to build up voltage slowly. Now in between this path, we are putting one diac also.
The diac is one special bidirectional semiconductor part. That means it can conduct in both directions, and it also helps us to create very sharp and sudden pulse which is very important for triggering triac properly.
How Charging and Triggering Happens
At starting point of every AC cycle, capacitor C1 starts charging through variable resistor VR1. This charging process keeps going until voltage across C1 becomes equal to breakdown level of diac.
When that happens then diac suddenly turns ON and allows capacitor C1 to release its stored charge directly into gate of triac. Because of this sharp discharge pulse, triac gets triggered and enters conduction mode.
What Happens After the Triac Turns ON
Once triac gets its gate pulse and turns ON, then it becomes fully conducting and enters saturation state. Now triac itself starts conducting full load current and takes over entire operation.
At this point, gate triggering components like VR1 and C1 do not matter anymore because triac is already ON. So during rest of that AC half cycle, triac stays ON and supplies power to motor.
Process Repeats in Every AC Cycle
Then when AC half cycle reaches zero crossing point, triac automatically turns OFF by itself because there is no more current flow. So in next AC half cycle, again same process repeats.
That means again C1 starts charging through VR1, and again diac breaks down and fires triac. So this keeps happening in every half cycle continuously without stop.
Triac Is Not Always Fully Symmetrical
But there is one thing we have to understand clearly. That is, triac does not always trigger exactly same way in both positive and negative cycles.
This is because in some conduction modes like mode I plus and mode III minus, gate current requirement is different.
So triac behaves in asymmetrical way. That means sometimes triggering angle may shift slightly between positive and negative sides of AC waveform.
Applications of This Circuit
This type of basic triac speed control circuit is not only useful for controlling AC motors. We can also use same idea for controlling brightness of incandescent lamps, or for regulating power in electrical heaters.
Actually this kind of circuit is very similar to common triac dimmer that we see in many household applications for controlling light brightness.
But we must not use standard commercial triac dimmer for controlling motors, because those dimmers are made for resistive loads only like filament bulbs, not for inductive loads like motors.
Conclusion Triac Operation
We know that triac is kind of thyristor device, which have three terminals and four semiconductor layers inside it. It is looking like SCR but the main thing is that triac can conduct in both directions but SCR can conduct only one way.
Now you must know that triac can be triggered in four different ways, but out of those four ways, only two ways are more reliable and commonly used by us in real circuits.
How we use triac
So now we can use triac to control AC power very easily and very effectively, especially when we work with resistive loads. These resistive loads are like heaters, light bulbs, or small universal type AC motors which are very commonly found in portable tools and home appliances.
That is why triac is very useful because we can switch or control power without big bulky mechanical switches.
Important Safety Advice
But now we must remember one very important thing, so do not forget this. Since triac circuits are directly connected to high voltage AC mains supply, we should never ever test or touch the circuit when it is powered on.
Now if we want to check something or modify the circuit then first we must completely disconnect it from mains supply, no shortcut.
Safety must come first always, because if we ignore this, then it can be very dangerous for us. Let us not be stupid and do proper safety first, so that we do not get electric shock or burn our hands.
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