Transcript Reminders

Announcements
Bring motors to lab this week.
 Bring eight 1N4001 diodes to lab per team.
Get them at the Scientific Supply Store (2nd
Floor Sciences Center Laboratory).
 Kits can be purchased at the Scientific
Supply Store

» 2 wheels + 1 lead with two connectors
1
Announcements

Most of your work on the Lego car will be
done outside of lab and lecture.
» Labs 3-7 and 9 are designed to provide the
technical knowledge and measurements required to
design and build your car
» Lab 13 is an (optional) open lab when you can
work on your car
– Week preceding the competition
– You should have your car designed and built by this lab
2
AC vs. DC

AC = alternating current (e.g. wall plug-in)

DC = direct current (e.g. battery)
Actually, AC and DC refer more to voltage
than current.
3
AC terminology
T = 50milliseconds = 0.05 seconds
T


f = 1/T = 1/0.05 = 20 Hz
Period (T) – the time to complete one cycle of the
waveform (Units: seconds)
Frequency (f=1/T) – the number of cycles per
second (Units: hertz, Hz=1 cycle/s)
4
AC terminology
Amplitude =
1V

Amplitude -- peak value of the signal
(Units are V if a voltage source)
» Measure from 0 to peak
5
AC terminology

RMS value -- “root-mean-square”, or
square root of average squared value. Equal
to amplitude/2 for sine wave.
» Example: Sine wave with amplitude of 5V
– RMS value = 5/1.4159 = 3.54V
6
AC to DC
Problem #1:
 Audio amplifier will not generate DC
waveform, since the ear cannot hear DC
(constant) or low frequencies (< 20 Hz)
7
Amplifiers

Amplifiers respond differently to different frequencies
8
Audio Amplifier Response
This plot shows the output of an audio
amplifier to an input sine wave of constant
amplitude but different input frequencies.
RMS Amplifier Response in Volts

Frequency in Hertz
9
AC to DC
Problem #2:
 Legos® motors are DC motors. They will
not turn with an AC power supply. (Or at
least they will not turn constantly in the
same direction!)
10
AC to DC

Another way to state the problem is that an
AC signal has a zero average value.

To drive the motor consistently in one
direction, we need an input signal with a
non-zero average value.
11
Converting AC to DC

Contains:
» Transformer
– Converts 120V to 9V
– Will not need this in
ENGR1110
» Rectifier
– Converts AC to DC
12
Converting AC to DC

Two types of rectifiers
» Half-Wave
» Full-Wave or Bridge
13
Half-Wave Rectifier
An AC signal has a zero average value.
 Can create a non-zero average value (a DC
component) by clipping off the negative
part.

AC Signal (average in red)
Clipped Signal (average in red)
14
Half-Wave Rectifier
15
Half-Wave Rectifier


For positive voltages (voltage drops in the
direction the diode points), the diode offers
no resistance.
» Resistor voltage = source voltage
16
Half-Wave Rectifier


For negative voltages (voltage rises in the
direction the diode points), a diode offers infinite
resistance.
» Acts like an open circuit
– Current (I) = 0
» Resistor voltage = 0V
Why?
V=IR and I=0
17
Half-Wave Rectifier
Source (input)
voltage
Resistor (output)
voltage
1
1
0.8
0.8
0.6
0.6
+
0.4
0.2
0.4
0.2
0
0
-0.2
-0.2
-
-0.4
-0.6
-0.8
-1
-0.4
-0.6
-0.8
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
-1
1
1
1
0.8
0.8
0.6
0.6
+
0.4
0.2
0
-0.2
-
-0.8
-1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.2
-0.2
-0.6
0.1
0.4
0
-0.4
0
-0.4
-0.6
-0.8
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-1
18
Half-Wave Rectifier
Input voltage
Output voltage
+
19
Half-Wave Rectifier
The half-wave rectifier “wastes” part of the
input signal, since the negative lobes of a
sine wave are just clipped off.
 It is possible to use both the positive and
negative lobes with a full-wave bridge
rectifier.

20
Bridge Rectifier

The circuit below inverts the negative lobes
of a sine wave and preserves the positive
lobes.
21
Bridge Rectifier
1
1
0.8
0.8
-
0.6
0.4
0.2
0.6
0.4
0.2
0
0
-0.2
-0.2
-0.4
-0.4
-0.6
-0.6
-0.8
-1
-0.8
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
+
1
-1
0
1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1
-
0.8
0.6
0.8
0.6
0.4
0.4
0.2
0.2
0
0
-0.2
-0.2
-0.4
-0.4
-0.6
-0.6
-0.8
-1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
+
-0.8
-1
0
22
Bridge Rectifier

The result of the bridge rectifier is shown in
the plot on the right.
AC Signal (average in red)
Clipped Signal (average in red)
23
Bridge Rectifier

Notice that the average value is higher than
for a half-wave rectifier but at the cost of
three extra diodes.
Half-wave rectifier
Bridge rectifier
24
Capacitor Filter

In the circuit below, the capacitor begins to
“fill up” when the source voltage is
positive. When the source is negative, the
capacitor begins to
discharge and acts like
a temporary battery to
keep up the voltage
across R.
25
Capacitor Filter

A typical plot of output voltage (across R)
for a capacitor filter is shown below:
26
Circuit with Lego Motor
+
3V
5sin(t)
Vout
-
27
Circuit with Lego Motor
Input voltage
Output voltage
28
Circuit with Lego Motor
+
3V
5sin(t)
Vout
-
29
Voltage Waveforms
5
Input
0
Voltage
-5
0
50
100
150
200
250
300
0
50
100
150
200
250
300
5
Output
0
Voltage
-5
30
Quiz
Put name, lab day and
time, and section
number on quiz!!
1. Let R1=10 ohms, and
R2=20 ohms. Find the
voltage V2 if V1=6V.
2. Electrical voltage is analogous to which quantity related to water
flowing through a pipe?
a) pressure b) flow rate c) volume
3. Electrical current is analogous to which quantity related to water
flowing through a pipe?
a) pressure b) flow rate c) volume
31