What Is A Thyristor
A thyristor with a p-n-p-n-structure has three junctions: PN, NP, and PN.
If the anode is a positive terminal with respect to the anode, then the outer junction, PN and PN, are forward-biased, while the center NP junction is reverse biased.
Therefore, the NP junction blocks the positive flow from the anode to the cathode. Thyristor is said to be in a major inhibitory state.
Similarly, the flow of a negative current is blocked by external PN junctions. The thyristor is in a state of a reverse block state.
They are amazingly versatile, but that doesn’t mean they can’t do it all.
Although we can use them to turn small electric currents on and off (and this is the basic principle behind computer memory) and make small currents slightly larger (this is why an amplifier works), they Not useful when handling it.
Another drawback of many large currents is that they turn off completely when the switching is turned on, meaning that they are not so useful on devices such as alarms that you want the circuit to turn on and stay on indefinitely.
First, let’s zero in on some words. Some people replace the silicon-controlled rectifier (SCR) with the term “thyristor”.
In fact, a silicon-controlled rectifier is a brand name introduced by General Electric to describe a particular type of thyristor.
There are also a variety of thyristors (also known as dikes and tricks, which are designed to work with the alternating current), so the terms are not entirely synonymous.
Anyway, this article is about keeping things simple, so we’ll only talk about thyristors in the most general terms and assume that SCRs are exactly the same thing. We consider them a small ester.
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| What is a thyristor |
Types Of Thyristor
Silicon Controlled Rectifier (SCR)
Silicon controlled rectifier is normally in the OFF state but when a tiny current enters its gate G it belongs to the ON state.
If the gate current is eliminated the SCR stays in ON condition and then turns it off the anode to cathode current has to be removed or the anode has to be set to a negative voltage concerning cathode.
Working with SCS is very similar to SCR but also it could be turned off by using a positive pulse on the anode gate. The SCS can also be turned ON by using a negative pulse on the anode gate.
The current flows only from anode to cathode. SCS is employed in counters, lamp drivers, logic circuits, etc.
Triac
Triac is very similar to SCR but it conducts in both directions, which means it can switch AC and DC currents. The triac stay in ON state only if there is present in gate G and changed OFF when this current is eliminated. The current is flowing in both directions between MT1 and MT2.
Four Layer Diode
Four-layer diode has 2 pins and works like a voltage-sensitive switch. When the voltage between the two pins exceeds the breakdown voltage it turns ON, otherwise, it’s OFF. Current flows from anode to cathode.
Diac
Diac is similar to a four-layer diode but it can conduct in both directions meaning it can contact both AC and DC currents.
Basic SCR Applications
In this circuit, an SCR is used to form a fundamental latching circuit. When S1 is pushed momentary a tiny present goes into the gate of SCR and turning it ON, thus lowering the load.
To turn it off we must push the S2 push-button so the current through SCR stops. Resistor RG is used to place the gate voltage of SCR.
In this circuit, an SCR is used to alter a sinusoidal signal so the load receives less electricity than of what would get if source voltage was applied right.
The sinusoidal signal is applied to the gate of SCR through R1. During the negative part of the sine wave that the SCR is in OFF state.
Increasing R1 has the effect of decreasing the voltage applied to the heart of SCR and so creating a lag in the conduction time.
This was the load is getting electricity for less time and therefore the average ability to load is reduced.
This is a variable speed DC motor control with a UJT, an SCR, and few passive components.
UJT along with resistors and capacitors form an oscillator that provides AC voltage to the gate of SCR.
When the gate voltage exceeds the triggering voltage of SCR, the SCR turns ON and the engine is operating.
By adjusting the potentiometer the output of the oscillator is changing and thus the times the SCR triggered is changing, which then changes the rate of the engine.
This way the motor is getting a series of pulses that average over the rate is adjusted.
Basic TRIAC Applications
This is an AC light dimmer formed by a diac, a triac, and a few passive components.
The capacitor is charging through both resistors and when the voltage on one end of the diac exceeds the breakdown voltage it goes ON and sends a current into the gate of Triac placing the triac to ON state and therefore powering the lamp.
Then the capacitor is billed and so forth. So the lamp is just powered for a portion of time during the complete sinewave. This occurs very quickly and the lamp looks dimmed.
Uses Of Thyristor
When a positive voltage is applied to its anode with regard to its own cathode, the intersection J1 and J3 gets forward biased. The junction J2 gets reverse prejudice.
Since the intersection J2 is reverse biased, the thyristor doesn’t run and stay from the off-state present. But, there’s still leakage current called off state present ‘.
If the implemented anode-cathode voltage raises quite high to a certain limit, it is going to break the reversed bias intersection j2.
This phenomenon is called’ avalanche breakdown’ and this voltage is known as forwards collapse voltage’.
Following the conclusion, the thyristor will switch-ON leading to the conduction of a large forward present because junction J1 and J2 were currently ahead biased.
Practically, this kind of change could be harmful and forward voltage ought to be kept below the breakdown voltage.
The appropriate method of switching on a thyristor is by applying a positive voltage pulse for its own gate related to the cathode. The intersection J2 will turn out to be forward bias.
Throughout the conduction condition, the thyristor behaves like a diode. It will run present continuously with no outside controller.
When it’s triggered with gate input signal, the system stays latched in on-state. It doesn’t want the constant gate distribution to stay in the conduction state.
But, there’s a grab; the anode present shouldn’t decrease from a limitation called’ latching current’. The’latching current’ is higher than the’holding present ‘.
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