[ECEN 1400]
Introduction to Digital and Analog Electronics
R. McLeod
HW #4: Power Supply
1
Why Not Use Batteries? (10 pts)
Work this problem in symbols, then clearly state the values of any parameters you need before plugging in to get
final numbers.
1.1
How much current would be drawn, on average, by a 4-digit display? Each LED segment
of the display draws current when emitting light. Given the amount of current you know
the LEDs need (check your previous lab results if you cant remember)
1.2
Now consider supplying your clock using AA batteries. How many will you need and in
what arrangement? What will be the voltage and current capacity of this combination?
Think carefully about which quantities add and which do not. Hint: Two AA batteries
should have the capability to supply twice the energy of one AA battery. You can use
this to check your answer.
1.3
Estimate how long a 4 digit clock would run before depleting your batteries.
2
2.1
RL Circuit Charging (20 pts)
Assuming that there is no current through the inductor initially, find the current
through and voltage on the inductor as a function of time > 0 assuming the switch
is closed at t=0. Plot these two functions. At what time does the inductor current
reach 90% of its final value?
R1 = 1kΩ
V1 = 10V
3
3.1
L1 = 1mH
Basic Transformer Operation (10 pts)
If a transformer delivers 500 mA at 24 V peak from its secondary side when driven
with a sinusoidal voltage source with peak voltage of 170 V on its primary side, how
many turns are on the secondary if the primary has 3000 turns? What is the primary
current?
Version 2.0, September 24, 2014
Page 1
[ECEN 1400]
4
Introduction to Digital and Analog Electronics
R. McLeod
Tapped Transformer Operation (10 pts)
A transformer is wound with three taps on both the primary and secondary as shown. The number of turns
between each tap, as shown is black to white = 100, white to red = 200, blue to green = 400 and green to orange
= 200. A 100 V peak sinusoid is connected between the black and white wires and a 2000 Ohm resistor is connected
between green and orange.
4.1
What is the peak current by the source?
4.2
What is the peak voltage between the blue and green wires?
4.3
What is the peak voltage between the white and red wires?
5
A Load Seen Through a Transformer
In the circuit below, the peak current in the secondary circuit, I1 = 100 mA, N1 = 50, and N2 = 40.
5.1
First find the RMS current and voltage on the secondary side and the average power
dissipated in the resistor.
5.2
Find the RMS current through and voltage on the primary coil. The find the average
power delivered by the source and compare to the average power dissipated in the
resistor. Find each quantity in symbols, plugging in numbers only in the final step.
Version 2.0, September 24, 2014
Page 2
[ECEN 1400]
6
Introduction to Digital and Analog Electronics
R. McLeod
Effeciency of a Bridge Rectifier
Consider a full-wave bridge rectifier driven by a 120 V RMS function generator oscillating at 60 Hz (that is,
identical to US line voltage). Assume the diodes are silicon with a forward bias voltage drop of 0.7 volts.
6.1
What is the peak voltage of the rectified waveform delivered to the load?
6.2
What is the peak current of the rectified waveform delivered to the load?
6.3
How much average power is delivered to the load?
6.4
Defining efficiency of the rectifier as power delivered to the load over power expended
by the source, what is the efficiency of the rectifier?
7
Putting them Together (15 pts)
A 120 V RMS, 60 Hz source is connected to pins 1 and 2 of the transformer T1. The primary winding of this
transformer has 1000 turns and the secondary winding has 20 turns. The diode rectifier is made from silicon diodes
with a forward bias voltage drop of 0.7 volts.
7.1
Calculate the peak voltage and plot the voltage for t = 0 to 50 ms for the input voltage
across pins 1 and 2.
7.2
Calculate the peak voltage and plot the voltage for t = 0 to 50 ms for the transformer
secondary voltage across pins 3 and 4
7.3
Calculate the peak voltage and plot the voltage for t = 0 to 50 ms for and the rectifier
output voltage relative to ground.
Version 2.0, September 24, 2014
Page 3