Thyristor Characteristics and Triggering Questions and Answers

Answer: B

A triac is a pair of converter grade SCRs connected in anti-parallel.

A triac behaves like two SCRs connected in inverse parallel with a common gate, built as one device. It can therefore conduct in both directions and be triggered in either half cycle, which makes it suitable for AC control such as light dimmers and fan regulators.

A pair of UJTs would not give controlled conduction of load current. Two SCRs in plain parallel both conduct in the same direction, and two in series also conduct in one direction only.

Answer: C

Fibre optic triggering is preferred because it provides a very high level of insulation/isolation.

In high voltage applications such as HVDC valves, SCRs sit at potentials of many kilovolts above ground, while the gate control electronics are near ground. An optical fibre carries the trigger pulse as light, and since glass does not conduct, it isolates the control circuit completely from the high voltage. It is also immune to electromagnetic interference.

An optical fibre carries a signal, not power, so it does not provide low impedance, gain or high current for the power circuit.

Answer: C

The key gives 60 m sec; the paper writes 'm sec' where microseconds are meant, since thyristor turn-off times are measured in microseconds.

Turn-off time is the time the anode current must stay at zero, with reverse bias, before forward voltage can be reapplied without the thyristor turning on again. It depends on how quickly the excess carriers stored in the inner junction recombine. Carrier lifetime increases with temperature, so the stored charge takes longer to disappear, and turn-off time rises roughly in proportion to junction temperature in the working range. The standard textbook statement used for this question is that doubling the temperature from 50°C to 100°C doubles the turn-off time, from 30 to 60.

The turn-off time therefore does not stay the same or fall to 15, and 100 is not supported by that proportionality.

Answer: D

The conduction angle is changed by changing the gate current.

An SCR blocks forward voltage until a gate current pulse triggers it, after which it conducts until its current falls below the holding current. Varying the gate current (or the timing of the gate pulse) changes the firing angle at which the SCR turns on in each cycle, and so the conduction angle and the average power delivered to the load. This is phase control.

The anode voltage is the supply waveform being controlled. The anode current is set by the load once the SCR conducts. The forward current rating is a fixed device specification, not a control variable.

Answer: A

Once an SCR is conducting with a current above the holding current, it stays on and the voltage across it is the small forward drop, about 1.4 V (option A), a little above one volt at the usual currents.

Zero volts (B) would be an ideal switch, but the junctions of a real SCR keep a drop of a volt or more. The supply voltage (C) appears across the SCR only in the forward-blocking state, before it is triggered, and half the load voltage (D) is not a state of the device.

Answer: A

The values are \(R_{B1}\) = 6 k\(\Omega\) and \(R_{B2}\) = 4 k\(\Omega\), option (A). The intrinsic stand-off ratio of a UJT is \(\eta = R_{B1}/(R_{B1}+R_{B2}) = R_{B1}/R_{BB}\), where \(R_{BB}\) is the inter-base resistance.

\[ R_{B1} = \eta R_{BB} = 0.6\times10 = 6\ \text{k}\Omega \]

\[ R_{B2} = R_{BB} - R_{B1} = 10 - 6 = 4\ \text{k}\Omega \]

Option (B) reverses the two values. Option (C), 5 k and 5 k, would be the case of \(\eta = 0.5\). Option (D) gives each resistance as the whole inter-base resistance, which is not possible since they add up to \(R_{BB}\).

Answer: B

The triggering voltage of an SCR is close to 0.7 V, the second option. The SCR is turned on by a gate current, and the gate-to-cathode junction behaves like a silicon diode, so a gate voltage of about 0.7 V appears when the device triggers.

A voltage of 0.3 V is typical of a germanium junction and is too low, 3 V is far more than the gate junction needs, and the breakdown voltage is the forward breakover voltage, at which the SCR turns on without a gate signal and not by gate triggering.

Answer: C

A thyristor can be used as a switch, the third option. It is a four-layer device that stays off until it is triggered and then conducts heavily, and it can be turned off only when its current falls below the holding value, which makes it a latching power switch.

It has no linear region in which a small signal controls a larger one, so it is not an amplifier. It is not a resistor, and it is a controlled switch and not a source of power.

Answer: C

An SCR can be formed by using one npn and one pnp transistor.

In the two transistor model the four layers p-n-p-n are split into a pnp and an npn transistor, joined so that the collector of each feeds the base of the other. This positive feedback makes the device latch on once triggered.

Two transistors of the same type cannot supply the feedback, because the collector current of one would not drive the base of the other correctly. Two diodes have only two junctions between them in series and cannot be triggered by a gate.

Answer: C

The correct matching is a-iii, b-iv, c-i, d-ii, which is code (C).

An SCR conducts in one direction (iii), a triac conducts in both (iv), a chopper controls the supply to a dc drive (i) and a cycloconverter is used for induction motor speed control (ii).

Codes (A), (B) and (D) give the SCR or the triac the wrong direction of conduction or a drive of the wrong type.

An SCR is a unidirectional device because it conducts from anode to cathode only once triggered. A triac behaves like two SCRs in inverse parallel, so it controls current in both halves of the AC cycle. A chopper changes a fixed DC into a variable DC, which suits dc drives.