ECE 252 Introduction to Electrical Engineering
Lesson 18. Batteries, Voltage Regulators, Transformers, and Sensors
Version S16
Homework partner name: _______________________Homework partner name: ________________________
1 Quickies. Fill in the blank or circle the correct answer.
a) A rechargeable battery is also called a _______________________________ battery.
b) List the nominal voltage of one cell of each of the following batteries:
zinc-alkaline ______________________ V
sealed lead-acid (SLA) ______________ V
nickel-cadmium (Ni-Cd) ____________ V
nickel-metal-hydride (NiMH) ________ V
lithium ion (Li-ion) ________________ V
c) List the type of battery (alkaline, SLA, Ni-Cd, NiMH, Li-ion) which would most likely be found in the
devices below:
Mobile phone _______________________
Motorcycle _________________________
Prius hybrid automobile ______________________
TV remote _________________________
d) A NiMH battery has about 150 mΩ of internal resistance. If the battery is connected to a load that draws 1.6
A, what is the voltage at the load? ______________________ V
e) How many SLA cells does it take to make an automobile battery? ________________________
f) It is usually not a good idea to connect batteries in parallel, because good batteries can discharge through
_________________________ batteries.
g) Linear voltage regulators waste energy. ________________________ voltage regulators are much more
energy-efficient.
h) What is the device shown below?
________________________.
i) Which one of these voltages cannot be used with a transformer? (AC, DC, High frequency, high power)
j) When charging a device with Li-ion batteries, what are the two most common practices that reduce battery
life?
________________________________________________
_____________________________________________________
k) A current sensor produces an output that is 5% less than the correct value of current when the current is
increasing. The sensor produces an output that is 5% greater than the correct value of current when the current is
decreasing. This error is probably caused by ________________________.
l) It is poor engineering practice to ground a sensitive system in more than one place. That is because noise
currents can be set up in a ground ________________________.
m) A(n) ______________________ is a sensor that changes its resistance based on temperature.
n) What is the approximate efficiency of an LED light bulb? _______________________%
o) What is the circuit below called?
______________________
o) A sensor-amplifier interface with one side grounded is said to be _________________________-ended.
p) The load cell is a type of ________________________ sensor.
Signatures: ________________________________________________________________________
2. A naive student has a project that requires a 5 V power supply. He wants to adapt a 12 V DC power supply to
provide the required 5 V, so he designs a voltage divider circuit consisting of the 12 V supply, a 15 kΩ and a 21
kΩ resistor. He measures the open circuit voltage of his voltage divider, and finds that it has the desired voltage (5
V). a) Sketch and label the voltage divider circuit the student used. b) When he connects the voltage divider to his
project, the project doesn't work. Where has the student gone wrong?
a)
b)
Signatures: ________________________________________________________________________
3. The student in Problem 2 consults his kindly old professor who recommends that he use an LM7805 linear
voltage regulator instead of the voltage divider. a) Draw and label the circuit for the power supply made with the
voltage regulator. b) If the student's project draws 600 mA, how much power is wasted in the LM7805?
a)
b)
Signatures: ________________________________________________________________________
4. The student in Problem 3 finds that his project works OK for a few minutes, but the LM7805 eventually burns
out. He again consults his professor. What does the professor recommend?
Signatures: ________________________________________________________________________
5. A student project is required to be portable and hand-held. It requires 12 V DC power at a current of 80 mA.
The batteries for the power supply must last for a minimum of 24 hours of continuous operation. NiMH
rechargeable batteries in AA size are to be used. a) How many batteries are needed? b) What mAh capacity
should the batteries have?
a)
b)
Signatures: ________________________________________________________________________
6. The transformer below has a 110 V primary and a 20 V secondary and is rated for 80 VA. Z2 is 8 Ω. a) What is
the current in the secondary? b) What is the current in the primary? c) What is the minimum value of Z2 for safe
operation of the transformer?
a)
b)
c)
Signatures: ________________________________________________________________________
7. An experimenter mounts a small strain gauge on a cockroach’s foreleg to measure the strength of that limb. The
voltage from the strain gauge is very small (microvolts). She amplifies the strain gauge's signal using an ordinary
inverting op amp. She finds that the output signal is contaminated by hum and other noise. What op amp circuit
can she use instead of the inverting circuit to eliminate much of the noise?
Signatures: ________________________________________________________________________
8. The experimenter in Problem 7 finds that the noise has been reduced using the new circuit, but that it is still
unacceptably high. She has hooked up the strain gauge to the op amp circuit using two ordinary copper wires.
Name two alternative wiring choices she has that might help eliminate the noise.
Signatures: ________________________________________________________________________
BONUS (No partial credit): A woman and her husband rent a cabin in a remote forest in Maine. They do the usual
things a couple does on vacation. Approximately one month after their vacation, the woman calls the sheriff of the
closest town to their rented cabin. She says, “There’s a dead man in the woods near our cabin.” The sheriff
responds, “I know. We found him last week, but why did you take so long to call?” The woman explains, and the
sheriff says, “OK. I understand. It’s alright then.”
What was the woman’s explanation?
Signatures: ________________________________________________________________________