Tag: introduction to diodes

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Questions Related to introduction to diodes

Multiple choice physics electric circuits introduction to diodes circuit components junction diode and its voltage current characteristics

A light emitting diode (LED) has a voltage drop of 2 V across it and passes a current of 10 mA. When it operates with a 6 V battery through a limiting resistor R, the value of R is 

  1. 200 $\Omega$
  2. 400 $\Omega$
  3. 40 $k\Omega$
  4. 4 $k\Omega$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Light emitting diode is a forward biased P-N junction which emits light.

The voltage across it = 2 v

Battery voltage = 6 v

Hence total voltage in the circuit = V = 6 - 2 = 4 v

since a LED has 0 resistance in the forward biased region, total resistance in the circuit

= 0 + r = r

current I = 10 mA = 10/1000 A = 0.01 A

we know that,

R = V/I

=> r = 4/0.01 = 400Ω

Hence the value of the limiting resistor is 400Ω

Multiple choice physics electric circuits introduction to diodes circuit components junction diode and its voltage current characteristics

When a diode is heavily doped the 

  1. Zener voltage will be low

  2. Avalanche voltage will be high

  3. Depletion region will be thin

  4. Leakage current will be low
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The value of reverse voltage at which this occurs is controlled by the amount ot doping of the diode. A heavily doped diode has a low Zener breakdown voltage, while a lightly doped diode has a high Zener breakdown voltage. At voltages above approximately 8V, the predominant mechanism is the avalanche breakdown.

Multiple choice physics electric circuits introduction to diodes circuit components junction diode and its voltage current characteristics

The relation between dynamic plate resistance $\left( r _ { p } \right)$ of a vacuum diode and plate current in the space charge limited region, is 

  1. $r _ { p } \propto I _ { p }$
  2. $r _ { p } \propto I _ { p } ^ { 3 / 2 }$
  3. $r _ { p } \propto \frac { 1 } { I _ { p } }$
  4. $r _ { p } \propto \frac { 1 } { \left( I _ { p } \right) ^ { 1 / 3 } }$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

In a vacuum diode operating in the space charge limited region, Langmuir-Child law states that plate current Ip is proportional to V^(3/2). Dynamic plate resistance rp = dV/dIp. Differentiating the relation gives rp proportional to 1/(Ip^(1/3)).

Multiple choice physics electric circuits introduction to diodes circuit components junction diode and its voltage current characteristics

Zener diode is used as

  1. Half wave rectifier

  2. Full wave rectifier

  3. ac voltage stabilizer

  4. dc voltage stabilizer
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A Zener diode is specially designed to operate reliably in the breakdown region under reverse bias. Because the voltage across it remains nearly constant over a wide range of reverse currents, it is widely used as a dc voltage regulator or stabilizer.

Multiple choice physics electric circuits introduction to diodes circuit components junction diode and its voltage current characteristics

What is the work function of tungsten at $1500\ K$ temperature, when a diode valve with a tungsten filament works at $1500\ K$? Assume the work function of tungsten at $0\ K$ is $4.52\ eV$:

  1. $4.71\ eV$
  2. $0.39\ eV$
  3. $8.86\ eV$
  4. $1.25\ eV$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The work function at temperature T is given by phi(T) = phi(0) + 2kT. Given phi(0) = 4.52 eV, k = 8.617 * 10^-5 eV/K, and T = 1500 K, the increase is 2 * 8.617 * 10^-5 * 1500 = 0.2585 eV. 4.52 + 0.26 = 4.78 eV. Option A is the closest.