Homework 1
1. A 50 Ω load consumes 1 W of power. What is the output voltage of the power supply?
Ans 7.071 v
2. A battery can provide 1.5A of current for 24 h. What is its ampere-hour rating?
Ans 0.0625 Ah
3. If a battery is rated at 650 mAh, how much current will it provide for 48 h?
Ans 31.2 A
4. A certain power supply provides a continuous 2 W to a load. It is operating at 60% efficiency in a 24 h period. how
many kilowatt-hours does the power supply use?
Ans W = Pt = (0.6x2W)x(24h) = 28.8 Wh = 0.0288 kWh
5. By varying the rheostat (variable resistor) in the circuit of the following figure, you can change the amount of
current. The setting of the rheostat is such that the current is 750 mA. What is the resistance value of this setting?
To adjust the current to 1 A, to what resistance value must you set the rheostat? The rheostat must never be
adjusted to 0 Ω. Why?
Ans The resistance value of the first setting is R = V/I = 100V / 750 mA = 133.33 Ω. Then, to obtain current of
1 A, the rheostat should be modified to 100V / 1A = 100 Ω. Moreover, we must never adjust the rheostat to
0 Ω because the circuit at the resister will be short and the excessive current will be drawn from the source.
As the result, the source will be damaged.
6. You have the following resistor values available to you in the lab in unlimited quantities: 10 Ω, 100 Ω, 470 Ω, 560
Ω, 1.0 k Ω, 2.2 k Ω, and 5.6 k Ω. All of the other standard values are out of stock. A project that you are working
on requires an 18 kΩ resistance. What combination of available values can you use to obtain the needed value?
Ans
Any combination of resistors can be used in order to get 18 kΩ. However, to obtain the minimum
number of resistors we can use three 5.6 kΩ series with one 1 kΩ and two 100 Ω.
7. Find all the unknown quantities in the following figure.
R1 = 220 Ω
R3 = R4 = 285 Ω
R5 = 330 Ω
R6 = 280 Ω
V2 = 2 V
V3 = V4 = 5.7 V
V6 = 5.6 V
V1 = 4.4 V
9. Four 1/2 W resistors are in series: 47 Ω, 68 Ω, 100 Ω, and 120 Ω. To what maximum value can the current be
raised before the power rating of one of the resistors is exceeded? Which resistor will bum out first if the current is
increased above the maximum?
9.1) Imax = 64.5 mA
9.2) R4 will be burn if the current through four resistors is greater than Imax
10. Develop a variable voltage divider to provide output voltages ranging from a minimum of 10V to a maximum of
100V using a 120V source. The maximum voltage must be at the maximum resistance setting of the potentiometer.
The minimum voltage must be at the minimum resistance (zero ohms) setting. The maximum current is to be 10
mA.
Ans There are several ways to design the circuit. The following answers are just examples.
R1
DC
Vs
R2
Vout
R3
R1 = 11 kΩ
R2 = 1 kΩ
R3 = 0 – 54 kΩ
Vout depends on the value of R3
Vout = 10 V – 100 V
Vout = 10 V -> R3 = 0
Vout = 100 V -> R3 = 54 kΩ
หรื อ
R1
DC
Vs
R2
R3
Vout
R1 = 2 kΩ
R3 = 10 kΩ
R2 = 108 – 0 kΩ
Vout เปลียG นตามค่าความต้ านทาน R3 คือ
Vout = 10 – 100 V
Vout = 10 V -> R2 = 108 kΩ
Vout = 100 V -> R2 = 0 kΩ