Diodes and Thyristors

Question 1

How does a diode function?

Answer 1

> When the diode is forward biased then it conducts.

> When it is reverse biased it has a very high resistance.

Question 2

What are the ideal characteristics of a diode? What is the graph?

Answer 2

Forward bias:

> V_D > 0 ⇒ R_D = 0Ω

Reverse bias:

> V_D < 0 ⇒ R_D = ∞Ω [Picture9]

Question 3

What are the practical characteristics of a diode?

Answer 3

Forward bias:

> V_D > 0 ⇒ R_D = smallΩ

Reverse bias:

> V_D < 0 ⇒ R_D = largeΩ

> There is a small leak current in the reverse direction. [Picture10]

Question 4

What is the symbol for a thyristor?

Answer 4

[Picture11]

Question 5

How does a thyristor function?

Answer 5

> Current will not flow when the thyristor is reverse biased.

> Current will only start flowing when forward biased and the gate voltage is pulsed.

> Current will only stop flowing when the thyristor next becomes reverse biased.

Question 6

When V_T > 0 and the thyristor is fired, what are the ideal and practical characteristics?

Answer 6

Ideal:

> Current flows with 0Ω resistance

> Flows immediately when V_T > 0

Practical:

> Current flows with lowΩ resistance

Question 7

When V_T > 0 and the thyristor is NOT fired, what are the ideal and practical characteristics?

Answer 7

Ideal:

• No current flows

Practical:

• No current flows until V_T > V_F . At this point the thyristor is forced to turn on.

Question 8

When V_T < 0 what are the ideal and practical characteristics for a thyristor?

Answer 8

Ideal:

• No current flows

Practical:

• Only a small leak current flow until V_T < V_R when the thyristor is forced to conduct.