In SCR, the following statements are given : (a) It acts as a short circuit device. (b) It acts as an open circuit device. (c) ON state voltage is approximately 2-5 Volts. (d) ON state voltage is approximately 50-100 Volts. Which of the above statements are correct ?
(a) and (c)
Statement (a) — correct: once triggered, an SCR is a closed switch. The device latches into a low-impedance state and the two outer junctions carry current with only a small forward drop. From the circuit's point of view it is essentially a short between anode and cathode, which is why a series load resistance is always needed to limit the current.
Statements (c) and (d) fix the numerical scale, and they differ by more than an order of magnitude. The conducting drop of a thyristor is small — typically 1–2 V, and comparable with an ordinary rectifier diode, since the current path is through forward-biased junctions. Statement (c)'s 2–5 V is of that order; statement (d)'s 50–100 V is absurd.
Why (d) is not merely inaccurate but impossible. Consider a thyristor carrying 100 A. At the figure in (d) the dissipation would be
\(P=V_TI_T=75\times100=7.5\ \text{kW}\)
enough to destroy any package instantly. At the true drop of about 1.5 V it is 150 W, which a proper heat sink handles. Power semiconductors are useful precisely because the on-state drop is a volt or two: efficiency in a converter is roughly
\(\eta\approx\dfrac{V_{load}}{V_{load}+V_T}\)
which is high only while VT stays small.
Assemble. (a) and (c), which is option 2.
Why statement (b) is not the answer even though it sounds true. An SCR is an open circuit when it is off — in both forward and reverse blocking it passes only microamps of leakage. But statements (a) and (b) are offered as alternatives and only one appears in each option; the question pairs them with the on-state voltage figures, so the intended reading is the conducting device. Since (c) is unambiguously the sensible voltage and it appears only alongside (a), the pairing is fixed.
The switch model that summarises it.
| State | Behaves as | Voltage | Current |
|---|---|---|---|
| Forward conducting | Closed switch | ≈ 1–2 V | Load-determined |
| Forward blocking | Open switch | Full supply | μA leakage |
| Reverse blocking | Open switch | Full reverse | μA leakage |
The consequence of latching. Because the gate loses control once conduction begins, the device can only be turned off by reducing the anode current below the holding value — naturally at each AC zero crossing, or by forced commutation in a DC circuit.
Hence, the correct statements are (a) and (c).
Similar Questions
Which of the following statements are true for SCR ?
(a) UJT is a phase controlling device for SCR rectifier.
(b) The average load voltage of SCR rectifier is greater than average load voltage of ordinary diode rectifier.
(c) Power delivered to the load in SCR rectifier is maximum when SCR firing angle is zero degree.
(d) SCR can control the amount of power fed to the load by switching current OFF and ON upto many thousand times a second.
In two transistor model of an SCR the expression for anode current is given by:
Match the following lists :
List - I List - II a. SCR can be turned off by i. it turns off when anode current falls below this value. b. Turn on time of SCR can be reduced by ii. the rectangular pulse of high amplitude and narrow width c. Holding current for an SCR means iii. its anode current interruption d. A forward current can be applied to an SCR after iv. its gate recovery time Correct Codes are :
The voltage safety factor (Vf) of a thyristor is given by :
An SCR is turned off by :
For a two transistor model, the value of α1 + α2 is equal to :
Important Questions from Thyristors
_______ is the device which acts like an N-P-N and a P-N-P transistor connected base-to-base and emitter-to-collector.
SCR stands for
A conducting SCR can be opened by reducing __________to zero.
Which of the following statements correctly describes the structure of a Silicon Controlled Rectifier (SCR)?
The thyristors which have a turn-off time less than _________ are called inverter grade thyristors.
