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SCR Operation and Applications
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SCR Operation and Applications
SCR Operation and Applications
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1
Question
Which structure and function characterize a silicon-controlled rectifier (SCR)?
Page 1
Answer
A three-terminal, four-layer p-n-p-n semiconductor switch that rectifies AC and controls power delivered to a load.
2
Question
Why is silicon used instead of germanium in an SCR?
Page 1
Answer
Silicon has a much smaller leakage current, supporting reliable OFF-state blocking.
3
Question
Which SCR terminal connects to the inner p-type layer and controls firing?
Page 1
Answer
The gate (G), held at a small positive potential to control firing.
4
Question
In forward-blocking bias, which SCR junction blocks conduction?
Page 2
Answer
The middle junction J2 is reverse-biased; only a small leakage current flows, so the SCR remains OFF.
5
Question
How does reverse-blocking bias affect the SCR junctions?
Page 2
Answer
J1 and J3 are reverse-biased, while J2 is forward-biased; the SCR remains OFF.
6
Question
When does an SCR with its gate open begin conducting on its own?
Page 2
Answer
When anode voltage reaches the breakover voltage and J2 breaks down.
7
Question
How does a positive gate voltage trigger SCR conduction below breakover voltage?
Page 2
Answer
It forward-biases J3 and initiates regenerative current increases that collapse J2.
8
Question
What must happen to anode current to turn a conducting SCR OFF?
Page 2
Answer
It must be reduced to near zero, or below the holding current.
9
Question
How do the two transistors in the SCR analogy create regenerative switching?
Page 2
Answer
Each transistor's collector feeds the other's base, creating positive feedback that drives both into saturation.
10
Question
Which equation relates SCR anode current to transistor gains and gate current?
Page 3
Answer
\(I_A = \frac{\alpha_2 I_G}{1-(\alpha_1+\alpha_2)}\).
11
Question
How does increasing gate current affect the forward closing voltage of an SCR?
Page 3
Answer
It lowers the anode voltage required to switch the SCR ON.
12
Question
What happens when reverse voltage exceeds an SCR's reverse breakdown voltage?
Page 3
Answer
Avalanche breakdown causes sharply rising current and can destroy the device unless current is limited.
13
Question
When does an SCR turn OFF as its anode current decreases?
Page 3
Answer
When current falls below the holding current \(I_H\).
14
Question
Which quantity does the SCR circuit-fusing rating \(I^2t\) represent?
Page 3
Answer
The maximum surge energy it can absorb, expressed as forward surge current squared times surge duration.
15
Question
How does an SCR differ from a transistor in control current after turn-on?
Page 4
Answer
A transistor needs continuous current to remain ON; an SCR can stay ON after one small pulse.
16
Question
Which gate-drive arrangement triggers an SCR at a selected point in each positive half-cycle?
Page 4
Answer
AC gate triggering, when positive-half-cycle gate current reaches \(I_{GT}\).
17
Question
How can a rapid anode-voltage rise trigger an SCR without a gate signal?
Page 4
Answer
The junction's capacitance can trigger turn-on through \(dv/dt\) triggering; a snubber circuit suppresses this unwanted effect.
18
Question
What happens to a conducting SCR at each AC supply zero-crossing?
Page 5
Answer
It turns OFF as the supply reverses and current falls to zero.
19
Question
Why does a half-wave SCR rectifier provide controlled output?
Page 5
Answer
Gate current sets the firing angle, determining how much of each positive half-cycle reaches the load.
20
Question
Complete the half-wave SCR conduction-angle relation: \(\phi = _____\).
Page 5
Answer
180° − α
21
Question
Which average-output-voltage equation applies to the controlled half-wave SCR rectifier?
Page 5
Answer
\(V_{av}=\frac{V_m}{2\pi}(1+\cos\alpha)\).
22
Question
How does increasing firing angle affect half-wave average current?
Page 5
Answer
A larger firing angle produces a smaller average current.
23
Question
How does a centre-tap full-wave SCR rectifier maintain load-current direction?
Page 6
Answer
Two SCRs conduct on alternate half-cycles, making load current flow in the same direction on both.
24
Question
Which average-output-voltage equation applies to the SCR full-wave rectifier?
Page 6
Answer
\(V_{av}=\frac{V_m}{\pi}(1+\cos\alpha)\).
25
Question
How does forced commutation turn off a conducting SCR in an inverter?
Page 6
Answer
A charged capacitor discharges in reverse through the SCR, briefly driving current backward and turning it OFF.
26
Question
What is wrong with thinking an SCR inverter converts AC into DC?
Page 6
Answer
An inverter uses SCR switching to convert DC into AC.
27
Question
How does an SCR crowbar protect a voltage-sensitive load?
Page 6
Answer
Excess supply voltage fires the SCR, which shorts the supply and blows a fuse.
28
Question
How does light trigger a light-activated SCR (LASCR)?
Page 7
Answer
Light striking its depletion layers substitutes for gate current and switches it ON.
29
Question
Which thyristor-family device conducts for either polarity at its main terminals?
Page 7
Answer
A triac; it is bidirectional and can be triggered by a positive or negative gate pulse.
30
Question
In the worked SCR surge example, does a 50 A surge lasting 12 ms exceed a 90 A²s rating?
Page 8
Answer
No. Its calculated rating is 30 A²s, below 90 A²s.