In this blog, we will exemplify how will Draw the V-I characteristics of thyristors. In the previous blog, we saw what is Thyristors. Working Principal and Classify thyristors with their control characteristics.

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The V-I characteristics of thyristors

We know that a thyristor is turned on by a gating signal and its anode current is greater than the holding current, the device persists to conduct due to positive feedback, even if the gating signal is removed. A thyristor is a latching device. When the gate current Ig=0 and anode current falls below a certain value called holding current during the ON state, it again advances its forward blocking state. Thyristors can either be forward biased or reverse biased. We will see how it performs in both conditions.





There are two conditions 
1. Forward biased, 
2. Reversed biased 

Forward biased:

When the anode is produced positive, the PN junctions at the ends are forward biased and the middle junction (NP) becomes reverse biased. It will remain blocked also known as the forwarding Blocking Stage mode till the time it is activated by Gate current pulse or the applied voltage reaches the forward break-over voltage.

Gate Current Pulse – When it is triggered by the gate current pulse, it initiates operating and will act as a close switch. The Thyristors stay in the ON-state,  Here the gate fails its control to turn off the device.

Breakover Voltage – When a forward voltage is applied, a leakage current starts to flow through the blocking (J2) in the middle junction of Thyristors. When the voltage exceeds the forward break over voltage or critical limit, then J2 breaks down and it reaches the ON state.

Reverse Biased 

If the anode is negative with concern to the cathode, with the application of reverse voltage, both PN junctions at the end i.e. J1 and J3 become reverse biased, and the middle junction J2 becomes forward biased. Just a small leakage current flows through it. This is the reverse voltage blocking mode or the OFF state of the Thyristor. When the reverse voltage is increased further, then at a certain voltage, avalanche breakdown of J1 and J2 occurs and it starts conducting in the reverse direction. The maximum reverse voltage at which a thyristor starts conducting is known as Reverse Breakdown Voltage.


Importance Term

Forward leakage current

A small leakage current flows from the anode to the cathode terminals of the SCR. This small leakage current is known as forwarding leakage current. This small leakage current is known as forwarding leakage current. The region OA of V-I characteristics is known as forwarding blocking region in which the SCR does not conduct electric current.

Reverse leakage current

When a diode is reverse biased, the width of the depletion region increases. It is seen that in a reverse-biased diode, some current flows through the depletion region. This current is called leakage current.

 Latching Current

It is the minimum anode current required to maintain the thyristor in the ON state immediately after a thyristor has been turned on and the gate signal has been removed.

Holding Current

 It is the minimum anode current to maintain the thyristor in the on-state.

Breakover voltage of the thyristor

It is the minimum forward voltage, gate being open, at which SCR starts conducting heavily i.e. turned on.