Diode Circuits and Applications Questions and Answers

Q: A \( 50 \ \mathrm{mA} \) load is now connected across that Zener. The diode current becomes

The resistor can supply only ten milliamps in total:\[ \begin{aligned} I_{R} &= 10 \ \mathrm{mA} \\ I_{Z} &= 10 - 50 < 0 \end{aligned} \]so the diode stops conducting and regulation is lost, with the output falling below ten volts. This is the design check that matters most: the series resistor has to pass the maximum load current plus a minimum keep-alive current for the diode.

Q: What is the maximum load current a shunt regulator can supply?

The load may take everything the resistor passes except the small current the diode needs to stay in breakdown. The power rating sets the other end of the range, the no-load case. Adding the diode's current rather than subtracting it inverts the relationship, and the input current is what the resistor passes rather than a separate quantity. A shunt regulator's whole design reduces to making the resistor's current cover the worst case at both extremes.

Q: Which condition dissipates most power in the Zener diode?

The diode takes whatever the load does not, so the highest input with no load is the worst case and is the condition the power rating must survive. Minimum input with full load is the other extreme, where the diode risks dropping out of breakdown instead. Checking both corners rather than the nominal case is what distinguishes a design that works on the bench from one that works in the field.

Q: Which breakdown mechanism dominates in a Zener diode below about five volts?

Below roughly five volts the Zener effect, a direct tunnelling process, dominates and has a negative temperature coefficient; above it avalanche multiplication takes over with a positive one. Thermal runaway is a failure rather than a mechanism, and punch-through belongs to transistors. The crossover is why diodes near 5.6 V have almost zero temperature coefficient, which is exactly why that value is chosen for references. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Q: Why is a 5.6 volt Zener often chosen as a reference?

At about that voltage the negative coefficient of the Zener mechanism and the positive one of avalanche cancel, giving a reference that barely drifts with temperature. Dynamic resistance is lowest around six or seven volts, power depends on the current chosen, and the series resistor follows from the input voltage. A parameter that passes through zero where two mechanisms cross over is worth exploiting, and reference diodes are the clearest example. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: What does a Zener's dynamic resistance describe?

Dynamic resistance is the slope of the breakdown characteristic, and it decides how much the output moves when the load changes - a few ohms in a good device. The series resistor is external, the sub-breakdown resistance is enormous and irrelevant, and the power to current ratio is just the voltage. Regulation quality in a shunt regulator is set by this slope and the series resistor together, which is why both appear in the line and load regulation figures.

Q: What is the chief advantage of a Schottky diode?

A Schottky's metal-semiconductor junction stores almost no minority charge, so it offers a forward drop around 0.3 V and negligible recovery time, which is why switching supplies use them. High reverse ratings are not their strength; they are usually modest. Voltage-dependent capacitance belongs to the varactor and negative resistance to the tunnel diode. Each special diode exists for one property, and naming that property is the whole of choosing between them.

Q: What property of a varactor diode is exploited?

A varactor is used as a voltage-controlled capacitor, because the depletion layer widens with reverse bias and the capacitance falls. That is what allows electronic tuning and the voltage-controlled oscillator at the heart of a phase-locked loop. The low drop belongs to a Schottky, light emission to a light emitting diode, and negative resistance to a tunnel diode. Every junction has this capacitance; the varactor is simply built to make it large and predictable.

Q: Why must a light emitting diode be driven through a series resistor?

The forward characteristic is so steep that a small voltage change produces a large current change, so the current must be set externally rather than by the applied voltage. The low reverse rating matters for protection, the temperature dependence affects brightness, and the colour-dependent drop matters when choosing the resistor's value - but none of them is why the resistor is needed. Any device with a steep characteristic must have its operating point set by something in series.

Q: How does a photodiode normally operate?

A photodiode is usually reverse biased, and the reverse current then rises in proportion to the incident light, which gives a linear and fast response. It can also be used unbiased in photovoltaic mode, which is what a solar cell does, but that is slower and less linear. Forward bias and breakdown are not used. The same device operated in two different modes trades speed and linearity against the need for a supply. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

31.

A \( 50 \ \mathrm{mA} \) load is now connected across that Zener. The diode current becomes

A.10 mA, unchanged by the load
B.50 mA, equal to the load current
C.60 mA, the sum of both currents
D.Zero, and regulation is lost

Answer: D

The resistor can supply only ten milliamps in total:

\[ \begin{aligned} I_{R} &= 10 \ \mathrm{mA} \\ I_{Z} &= 10 - 50 < 0 \end{aligned} \]

so the diode stops conducting and regulation is lost, with the output falling below ten volts. This is the design check that matters most: the series resistor has to pass the maximum load current plus a minimum keep-alive current for the diode.

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32.

What is the maximum load current a shunt regulator can supply?

A.The resistor's current less the diode's minimum
B.The diode's power rating divided by its voltage
C.The resistor's current plus the diode's maximum
D.The input current at the highest expected input voltage

Answer: A

The load may take everything the resistor passes except the small current the diode needs to stay in breakdown. The power rating sets the other end of the range, the no-load case. Adding the diode's current rather than subtracting it inverts the relationship, and the input current is what the resistor passes rather than a separate quantity. A shunt regulator's whole design reduces to making the resistor's current cover the worst case at both extremes.

Discuss in Forum

33.

Which condition dissipates most power in the Zener diode?

A.Minimum input voltage with no load
B.Maximum input voltage with no load
C.Minimum input voltage with full load
D.Maximum input voltage with full load

Answer: B

The diode takes whatever the load does not, so the highest input with no load is the worst case and is the condition the power rating must survive. Minimum input with full load is the other extreme, where the diode risks dropping out of breakdown instead. Checking both corners rather than the nominal case is what distinguishes a design that works on the bench from one that works in the field.

Discuss in Forum

34.

Which breakdown mechanism dominates in a Zener diode below about five volts?

A.Thermal runaway in the junction
B.Punch-through of the depletion region
C.The Zener effect proper
D.Avalanche multiplication

Answer: C

Below roughly five volts the Zener effect, a direct tunnelling process, dominates and has a negative temperature coefficient; above it avalanche multiplication takes over with a positive one. Thermal runaway is a failure rather than a mechanism, and punch-through belongs to transistors. The crossover is why diodes near 5.6 V have almost zero temperature coefficient, which is exactly why that value is chosen for references. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Discuss in Forum

35.

Why is a 5.6 volt Zener often chosen as a reference?

A.Its dynamic resistance is lower than any other value
B.It dissipates less power than a higher voltage device
C.It requires a smaller series resistor than other values
D.Its temperature coefficient is near zero

Answer: D

At about that voltage the negative coefficient of the Zener mechanism and the positive one of avalanche cancel, giving a reference that barely drifts with temperature. Dynamic resistance is lowest around six or seven volts, power depends on the current chosen, and the series resistor follows from the input voltage. A parameter that passes through zero where two mechanisms cross over is worth exploiting, and reference diodes are the clearest example. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

36.

What does a Zener's dynamic resistance describe?

A.The change in voltage per unit change in current
B.The resistance in series needed to limit its current
C.The reverse resistance below the breakdown voltage
D.The ratio of its rated power to its rated current

Answer: A

Dynamic resistance is the slope of the breakdown characteristic, and it decides how much the output moves when the load changes - a few ohms in a good device. The series resistor is external, the sub-breakdown resistance is enormous and irrelevant, and the power to current ratio is just the voltage. Regulation quality in a shunt regulator is set by this slope and the series resistor together, which is why both appear in the line and load regulation figures.

Discuss in Forum

37.

What is the chief advantage of a Schottky diode?

A.A negative resistance region in its forward characteristic
B.A low forward drop and fast recovery
C.A very high reverse breakdown voltage rating
D.A capacitance that varies strongly with reverse bias

Answer: B

A Schottky's metal-semiconductor junction stores almost no minority charge, so it offers a forward drop around 0.3 V and negligible recovery time, which is why switching supplies use them. High reverse ratings are not their strength; they are usually modest. Voltage-dependent capacitance belongs to the varactor and negative resistance to the tunnel diode. Each special diode exists for one property, and naming that property is the whole of choosing between them.

Discuss in Forum

38.

What property of a varactor diode is exploited?

A.It emits light when forward biased at low current
B.It exhibits a negative resistance region when biased
C.Its capacitance varies with reverse bias
D.Its forward drop is lower than a silicon diode's

Answer: C

A varactor is used as a voltage-controlled capacitor, because the depletion layer widens with reverse bias and the capacitance falls. That is what allows electronic tuning and the voltage-controlled oscillator at the heart of a phase-locked loop. The low drop belongs to a Schottky, light emission to a light emitting diode, and negative resistance to a tunnel diode. Every junction has this capacitance; the varactor is simply built to make it large and predictable.

Discuss in Forum

39.

Why must a light emitting diode be driven through a series resistor?

A.Its reverse breakdown voltage is unusually low
B.Its light output falls as its temperature rises
C.Its forward drop varies with the colour emitted
D.Its forward characteristic is very steep

Answer: D

The forward characteristic is so steep that a small voltage change produces a large current change, so the current must be set externally rather than by the applied voltage. The low reverse rating matters for protection, the temperature dependence affects brightness, and the colour-dependent drop matters when choosing the resistor's value - but none of them is why the resistor is needed. Any device with a steep characteristic must have its operating point set by something in series.

Discuss in Forum

40.

How does a photodiode normally operate?

A.Reverse biased, with current proportional to light
B.Forward biased, with its drop proportional to light
C.In breakdown, with the breakdown voltage set by light
D.Unbiased, generating a voltage proportional to light

Answer: A

A photodiode is usually reverse biased, and the reverse current then rises in proportion to the incident light, which gives a linear and fast response. It can also be used unbiased in photovoltaic mode, which is what a solar cell does, but that is slower and less linear. Forward bias and breakdown are not used. The same device operated in two different modes trades speed and linearity against the need for a supply. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

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