1
Q
What is the most basic semiconductor device?
A
The P-N junction, also known as the Junction Diode or simply Diode.
2
Q
What is the main function of a diode?
A
To allow or block current flow in a circuit.
3
Q
What are the main types of diodes and their applications?
A
Rectifier diode, Silicon Controlled Rectifier (SCR), Light Emitting Diode (LED), Photo diode, and Varistor.
4
Q
What does a Rectifier Diode do?
A
It converts AC (alternating current) to DC (direct current).
5
Q
What does SCR stand for in electronics?
A
Silicon Controlled Rectifier.
6
Q
What is the function of a Silicon Controlled Rectifier (SCR)?
A
It controls and rectifies high voltage and current with gate-triggered switching.
7
Q
What is an LED?
A
A Light Emitting Diode that emits light when current flows through it.
8
Q
What is a Photo Diode?
A
A diode that generates current when exposed to light.
9
Q
What is a Varistor?
A
A voltage-dependent resistor used to protect circuits from voltage spikes.
10
Q
What is the circuit symbol for a diode used to represent?
A
It shows the direction in which conventional current can flow.
11
Q
In a diode symbol, which direction does the arrow point?
A
In the direction of allowable conventional current flow.
12
Q
What happens if voltage is applied in the opposite direction of the diode symbol’s arrow?
A
The diode blocks the current flow.
13
Q
What are the two most common materials used to make semiconductor diodes?
A
Silicon and Germanium.
14
Q
Do silicon and germanium diodes operate differently in principle?
A
No, their principle of operation and circuit symbols are the same.
15
Q
What kind of document provides technical specifications for a diode?
A
A datasheet.
16
Q
What key information is found in a diode datasheet?
A
Maximum Forward Bias Current, Leakage Current in Reverse Bias, Optimum Operating Temperature, Frequency Response, and Maximum Power Dissipation.
17
Q
Why must engineers consult a diode’s datasheet when designing circuits?
A
To ensure the diode meets the electrical and thermal requirements of the application.
18
Q
What is Forward Voltage in a diode?
A
The voltage at which a diode begins to conduct in forward bias (anode more positive than cathode).
19
Q
What is an example of a forward bias connection that would make a diode conduct?
A
Anode at +3V, cathode at 0V.
20
Q
What is another example of forward bias using negative voltages?
A
Anode at -52V, cathode at -53V.
21
Q
What is the typical forward voltage for a Germanium diode?
A
0.1V to 0.2V.
22
Q
What is the typical forward voltage for a Silicon diode?
A
0.6V to 0.7V.
23
Q
What happens if a diode is operated with too much reverse voltage?
A
It can become permanently damaged, causing an open or short circuit.
24
Q
Where is the maximum allowable reverse voltage found?
A
On the diode’s datasheet.
25
What is the maximum reverse voltage for the silicon diode shown in Figure 2?
150V.
26
What is the maximum reverse voltage for the germanium diode shown in Figure 3?
75V.
27
Can some diodes be designed to operate in reverse bias?
Yes, but only within their rated reverse voltage limits.
28
What do rectifier diodes do?
Convert AC to DC, a process known as rectification.
29
What is the resistance of a rectifier diode in forward bias?
Low resistance to current flow.
30
What is the resistance of a rectifier diode in reverse bias?
High resistance to current flow.
31
What is Peak Inverse Voltage (PIV)?
The maximum reverse voltage a diode can withstand without breaking down.
32
What must the breakdown voltage of a rectifier diode be at minimum?
At least equal to the peak voltage of the AC being rectified.
33
What happens in a simple rectifier circuit during forward bias?
The diode conducts and produces unidirectional (DC) pulses.
34
What type of rectification occurs in a simple rectifier with one diode?
Half-wave rectification.
35
How can the polarity of the rectified output be changed?
By reversing the diode connections.
36
What material are rectifier diodes usually made from?
Silicon.
37
What is the typical maximum forward current rating for silicon rectifier diodes?
Approximately 200mA to 2kA.
38
What is the typical maximum voltage rating for silicon rectifier diodes?
Around 3kV to 4kV.
39
What are some physical advantages of silicon rectifier diodes?
They are small, lightweight, impervious to shock, and operate up to ~200°C.
40
What is another name for the SCR?
Thyristor.
41
how does an SCR operate?
by switching between two stable states: ON and OFF.
42
What type of semiconductor junctions make up an SCR?
Both n-type and p-type materials arranged as PNPN or NPNP.
43
What are the three terminals of an SCR?
Anode, Cathode, and Gate.
44
In a PNPN SCR, which region is the anode?
The outer p-type region.
45
In a PNPN SCR, which region is the cathode?
The outer n-type region.
46
In a PNPN SCR, which region is the gate?
The inner p-type region.
47
What is the SCR’s behavior in its OFF state?
The middle junction is reverse biased, blocking current flow except a small leakage current.
48
How is an SCR turned ON?
By increasing the supply voltage to the breakover voltage or applying a current pulse to the gate.
49
What happens once the SCR reaches breakover voltage?
Current increases rapidly, limited only by the circuit resistance.
50
What is the Holding Current in an SCR?
The minimum current required to keep the SCR in the ON state.
51
What happens if the current drops below the Holding Current?
The SCR reverts to the OFF (blocking) state.
52
Can the SCR remain ON without continuous gate current?
Yes, once fired, the gate current can be removed.
53
What does increasing gate current do to the breakover voltage?
It reduces the breakover voltage needed to switch the SCR on.
54
What is a key advantage of the SCR?
A small gate current can control a large load current.
55
What is an example of high-current switching using an SCR?
Controlling a DC lamp circuit using gate and supply switches.
56
How much current can an SCR switch?
Up to 1kA.
57
In AC control, when does an SCR conduct?
Only during the positive half-cycles of the waveform.
58
How can you control the portion of the AC waveform an SCR conducts?
By adjusting the gate firing point using a resistor and capacitor.
59
What does advancing the gate firing toward the AC peak do?
Reduces the conduction time, controlling power to the load (e.g., dimming a light).
60
What kind of circuit uses SCRs for variable brightness control?
Light dimming circuits using synchronized gate triggering.
61
What is an LED?
A Light Emitting Diode – a P-N junction that emits light when forward biased.
62
When does an LED emit light?
When current flows through it in forward bias.
63
What is the most common LED colour?
Red.
64
What other colours are available for LEDs?
Green and yellow, although at lower intensity.
65
What is the typical voltage drop across an LED?
Approximately 2V.
66
What happens if the voltage across an LED exceeds its normal drop?
The current increases rapidly, risking burnout.
67
Why is a series resistor used with an LED?
To limit the current to around 10mA and protect the junction from burnout.
68
What current is typically used to operate an LED safely?
Around 10mA.
69
Why are LEDs replacing incandescent lamps on aircraft fault panels?
Because they consume less power and are more reliable (no filament to rupture).
70
What makes LEDs more reliable than incandescent lamps?
They have no filament, so there's nothing to rupture or burn out.
71
What physical principle does a photodiode use?
Photoconductivity.
## Footnote
Photoconductivity is the increase in the electrical conductivity of a material when exposed to light.
72
How is a photodiode typically biased in a circuit?
In reverse bias.
## Footnote
In reverse bias, the voltage is applied in the opposite direction to the flow of current, which enhances the photodiode's sensitivity to light.
73
What causes current to flow in a photodiode?
Exposure to light on the P-N junction.
## Footnote
The P-N junction generates electron-hole pairs when light photons hit it, leading to current flow.
74
What is the current called when no light is present in a photodiode?
Dark current.
## Footnote
Dark current is the small amount of current that flows through a photodiode even when it is not exposed to light.
75
What is the typical dark current of a photodiode?
Approximately 1.5nA.
## Footnote
This value can vary based on the specific type of photodiode and environmental conditions.
76
What is the output current of a photodiode in bright sunlight?
Approximately 3.5µA.
## Footnote
The output current can change significantly based on the intensity of the light exposure.
77
What happens when a reverse-biased photodiode is exposed to light?
It produces its own current and also allows current from the external supply to flow.
## Footnote
The photodiode generates current from light exposure while also being able to amplify this current with an external power source.
78
What is one application of a photodiode's light-sensitive current output?
Controlling another component that requires more current than the photodiode can produce alone.
## Footnote
This functionality is essential in circuits where low-power signals need to control high-power devices.
79
How does a photodiode differ mechanically from a standard diode?
Its P-N junction is exposed to ambient light.
## Footnote
This exposure allows photodiodes to detect light, unlike standard diodes which are typically enclosed.
80
Why are photodiodes useful in measurement applications?
Because leakage current is directly proportional to light intensity over a wide range.
## Footnote
This feature makes photodiodes suitable for precise light measurements in various applications.
81
What material is commonly used for photodiodes?
Silicon.
## Footnote
Silicon is favored due to its semiconductor properties and efficiency in converting light to electrical signals.
82
In which light region does silicon photodiodes have peak response?
Infra-red region.
## Footnote
Silicon photodiodes are most sensitive to infrared light, making them useful in various optical applications.
83
What do the arrows on a photodiode symbol represent?
Light entering the device.
## Footnote
The arrows indicate the direction of light flow, which is essential for understanding the device's operation.
84
How do the arrows in a photodiode symbol differ from those in an LED symbol?
Photodiode arrows point inward (light entering); LED arrows point outward (light emitted).
## Footnote
This distinction is critical for identifying the function of each component in a circuit.
85
What does 'varistor' stand for?
VARiable resISTOR.
86
What is a varistor commonly made from?
Metal oxide, typically zinc oxide.
87
What is a common type of varistor called?
Metal Oxide Varistor (MOV).
88
What is one application of a varistor?
Removing noise spikes from AC voltage.
89
What are noise spikes?
Short-duration, high-amplitude voltage disturbances.
90
Why are low-pass filters ineffective against noise spikes?
Because noise spikes are too fast for the filters to attenuate effectively.
91
When does a varistor conduct?
When the voltage across it exceeds a certain threshold in either direction.
92
What does a varistor do when it conducts?
It clips off noise spikes by conducting them to ground.
93
How does a varistor behave similarly to a Zener diode?
It conducts in both directions when a threshold voltage is exceeded.
94
In what way is a varistor better than a Zener diode?
It responds much faster to voltage spikes.
95
What kind of voltage disturbances can a varistor suppress?
Both positive and negative overvoltage spikes.
96
What is the purpose of a rectifier circuit?
To convert AC voltage to DC voltage.
97
What is half-wave rectification?
When only one half of the AC waveform (positive or negative) is used.
98
What is full-wave rectification?
When both halves of the AC waveform are used to produce DC pulses.
99
Why is smoothing required after rectification?
To reduce voltage ripple and produce a more constant DC output.
100
What is the purpose of voltage stabilisation after smoothing?
To maintain consistent output voltage regardless of load changes.
101
What component is used in the most basic half-wave rectifier?
A single diode.
102
When does the diode conduct in a half-wave rectifier?
During one half-cycle of the AC input (forward bias).
103
What does the output of a half-wave rectifier look like?
Pulses of DC separated by zero voltage for half the cycle.
104
What is a drawback of half-wave rectification?
It only uses 50% of the available AC power.
105
What are the two types of full-wave rectifiers?
* Full-wave centre-tap rectifier
* Full-wave bridge rectifier
106
How many diodes are used in a full-wave centre-tap rectifier?
Two diodes.
107
What is the function of the centre-tapped transformer in a full-wave rectifier?
It provides a return path for current during each half-cycle.
108
In a full-wave centre-tap rectifier, which diode conducts during the positive half-cycle?
D1.
109
In a full-wave centre-tap rectifier, which diode conducts during the negative half-cycle?
D2.
110
What percentage of the transformer's secondary winding is used in a centre-tap full-wave rectifier?
50%.
111
How many diodes are used in a full-wave bridge rectifier?
Four diodes.
112
Which diodes conduct during the positive half-cycle in a bridge rectifier?
D2 and D4.
113
Which diodes conduct during the negative half-cycle in a bridge rectifier?
D1 and D3.
114
What is the advantage of a full-wave bridge rectifier over a centre-tap design?
It uses the entire secondary winding and delivers 100% of available power.
115
What is the output frequency of a full-wave rectifier compared to the input?
Twice the input frequency.
116
What is the mean DC level of a half-wave rectifier output?
32% of the peak AC voltage.
117
What is the mean DC level of a full-wave rectifier output?
64% of the peak AC voltage.
118
Why can't the raw rectified outputs be used directly as power supplies?
Because they are pulsed and not constant DC without smoothing.
119
What is the purpose of a rectifier circuit?
To convert AC voltage to DC voltage.
120
What is half-wave rectification?
When only one half of the AC waveform (positive or negative) is used.
121
What is full-wave rectification?
When both halves of the AC waveform are used to produce DC pulses.
122
Why is smoothing required after rectification?
To reduce voltage ripple and produce a more constant DC output.
123
What is the purpose of voltage stabilisation after smoothing?
To maintain consistent output voltage regardless of load changes.
124
What component is used in the most basic half-wave rectifier?
A single diode.
125
When does the diode conduct in a half-wave rectifier?
During one half-cycle of the AC input (forward bias).
126
What does the output of a half-wave rectifier look like?
Pulses of DC separated by zero voltage for half the cycle.
127
What is a drawback of half-wave rectification?
It only uses 50% of the available AC power.
128
What are the two types of full-wave rectifiers?
* Full-wave centre-tap rectifier
* Full-wave bridge rectifier
129
How many diodes are used in a full-wave centre-tap rectifier?
Two diodes.
130
What is the function of the centre-tapped transformer in a full-wave rectifier?
It provides a return path for current during each half-cycle.
131
In a full-wave centre-tap rectifier, which diode conducts during the positive half-cycle?
D1.
132
In a full-wave centre-tap rectifier, which diode conducts during the negative half-cycle?
D2.
133
What percentage of the transformer's secondary winding is used in a centre-tap full-wave rectifier?
50%.
134
How many diodes are used in a full-wave bridge rectifier?
Four diodes.
135
Which diodes conduct during the positive half-cycle in a bridge rectifier?
D2 and D4.
136
Which diodes conduct during the negative half-cycle in a bridge rectifier?
D1 and D3.
137
What is the advantage of a full-wave bridge rectifier over a centre-tap design?
It uses the entire secondary winding and delivers 100% of available power.
138
What is the output frequency of a full-wave rectifier compared to the input?
Twice the input frequency.
139
What is the mean DC level of a half-wave rectifier output?
32% of the peak AC voltage.
140
What is the mean DC level of a full-wave rectifier output?
64% of the peak AC voltage.
141
Why can't the raw rectified outputs be used directly as power supplies?
Because they are pulsed and not constant DC without smoothing.
