I. Multiple Choice
1. The diac is
a. Transistor
b. Unidirectional device
c. Three-layer device
d. Bidirecitonal device
10. It is recommended to use UJT oscillator for gatetriggering of the thyristors mainly because
a. It is fairly simple
b. It provides sharp firing pulses
c. It is less expensive
d. None of the above
2.
The triac is equivalent to
a. A four-layer diode
b. Two diacs in parallel
c. A thyristor with a gate lead
d. Two back-to-back SCRs in parallel
11. In a UJT, intrinsic standoff ratio is typically
a. 0.2
b. 0.4
c. 0.7
d. 0.99
3.
The unijunction transistor acts as a
a. Four-layer diode
b. Diac
c. Triac
d. Latch
4.
The _____ can be externally programmed to turn on at a
desired anode-to-gate voltage level.
a. UJT
b. PUT
c. Diac
d. Triac
12. A PUT relaxation oscillator has values VBB = 15V,
R=22kΩ, R2 = 6kΩ, IP = 100µA, VV = 1V, IV = 7 mA,
C = 1µF, RK = 100kΩ, and R3 = 12kΩ. The value of Vp
will be
𝑅3
12𝑘Ω
a. 0.7
̅̅̅̅
η=
=
= 0. 66
𝑅3 + 𝑅2 12𝑘Ω + 6𝑘Ω
b. 10 V
c. 10.7 V
̅̅̅(15𝑉) + 0.7 = 10.7
𝑉𝑝 = 𝜂𝑉𝐵𝐵 + 0.7 → 0. ̅66
d. 15 V
5.
You need a very efficient thyristor to control the speed of
an AC fan motor. A good device to use would be
a. a 4-layer diode
b. a PUT
c. a Triac
d. a BJT
6.
You have a need to use a device to trigger an SCR. A
good one to use might be
a. an SCS
b. a UJT
c. a 4-layer diode
d. a diac
7.
The triac can be turned on or off by a pulse at the gate
a. True
b. Maybe
c. False
d. Cannot be determined
8.
Which of the following is not a characteristic of the UJT?
a. intrinsic standoff ratio
b. negative resistance
c. peak-point voltage
d. bilateral conduction
9.
The PUT is
a. much like the UJT
b. not a thyristor
c. triggered on and off by the gate-to-anode voltage
d. not a four-layer diode
13. In question no. 12, the value Rmax will be
a. 2 kΩ
𝑉𝐵𝐵 − 𝑉𝑝 15 𝑉 − 10.7 𝑉
b. 2.2 kΩ
R 𝑚𝑎𝑥 =
→
= 43𝑘Ω
𝐼𝑝
100 ∗ 10−6 𝐴
c. 14 kΩ
d. 43 kΩ
14. A triac and SCR are compared
a. Both are unidirectional
b. Triac requires more current for turn-off than SCR at
a particular voltage
c. Triac has less time for turn-off than SCR
d. Both are available with comparable voltage and
current ratings
15. The triac can be used only in
a. Inverter
b. Rectifier
c. Multiquadrant chopper
d. Cycloconverter
16. In a UJT, maximum value of charging resistance is
associated with
a. Peak point
b. Valley point
c. Any point between peak and valley point
d. After the valley point
II. Problem-Solving
1. Determine a value of R1 in the Figure that will
ensure proper turn-off of the UJT. The
characteristics of the UJT exhibits the following
values: 𝜂 = 0.5, VV = 1 V, IV = 10 mA,
IP = 20 µA, and VP = 14 V. (3 points)
3.
At what anode voltage (VA) will the PUT in the
figure begin to conduct? (3 points)
𝑉𝐴 = 𝜂𝑉𝐵 + 0.7
𝑉𝐴 =
𝑉𝐵𝐵 − 𝑉𝑉
𝑉𝐵𝐵 − 𝑉𝑃
< 𝑅1 <
𝐼𝑉
𝐼𝑃
30𝑉 − 1𝑉
30𝑉 − 14𝑉
< 𝑅1 <
−3
10 ∗ 10 𝐴
20 ∗ 10−6 𝐴
2.9𝑘Ω < 𝑅1 < 800𝑘Ω
2.
In a certain UJT, RB1 = 2.5kΩ and RB2 = 4kΩ.
Determine the peak-point voltage for the UJT if
VBB = 15V. (3 points)
𝑉𝑃𝑃 = 𝜂𝑉𝐵𝐵 + 0.7
𝑉𝑃𝑃 =
2.5𝑘
(15) + 0.7
2.5𝑘 + 4𝑘
𝑉𝑃𝑃 = 6.469 𝑉
10𝑘
(20) + 0.7
12𝑘 + 10𝑘
𝑉𝐴 = 9.79 𝑉