ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Module: Energy/Environment: DC-AC Power Inversion
Experimental Procedure
Name:
Score: ____________ / 100
Partner:
Overall Objectives of this Module
Circuits Concepts:
1. Reinforce understanding of power and energy
2. Understand DC-AC inversion, how electronic circuits perform inversion, and the
energy efficiency of inversion
3. Develop an understanding of energy efficiency
4. Understand the PV solar panel as a power source
5. Understand why the grid uses AC
6. Continue to learn to use basic lab instrumentation, including a function
generator, oscilloscope, and multimeter
Systems Concepts:
1. Learn how a system is implemented as an electronic circuit to perform a specific
function
2. See how a system can decomposed into subsystems to aid both understanding
and debugging
Grading
Part 1
/ 15
Part 2
/ 15
Part 3
/ 20
Part 4
/ 20
Part 5
/ 15
Part 6
/ 15
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Important Concepts
1. You will construct and explore an inverter that converts DC power to AC power.
The resulting AC power can be applied to something called a load, which is EEspeak for something that does something useful with the power. So we can
focus on the inverter system, in this experiment our load is a simple resistor.
2. Alternating current (AC) circuits are circuits in which voltages and currents are
not constant, but change with time.
3. The pattern in which voltages and currents change with time is called a
waveform or a signal.
4. Ideally, AC waveforms are sinusoidal. As we will see, we can develop inverters
that generate an approximation of a sinusoidal waveform.
5. Power is the rate of change (or derivative) of energy. Conversely, energy is the
time integral of power.
Background
In this experiment, you will implement a system known as an inverter to perform a
function: the conversion of DC power to AC power. Probably the most interesting thing
about inverters is that they can take a constant-voltage input and produce a voltage on
the load that not only varies, but rapidly swings between positive and negative voltages.
To implement this system, you will integrate three subsystems, as shown in Figure 1.
The first is a clock source that generates a clock waveform (or signal) that determines
the frequency of the AC waveform; we will use a function generator as a clock source.
The clock signal drives the second subsystem, which is a control subsystem that
determines when the current direction is switched. The control subsystem will be
implemented using a digital logic chip with two flip-flops. The third subsystem is the Hbridge. We will implement this with six MOSFET transistors. An H-bridge consists of
four legs that control the flow of current, with a pair of legs connected to each side of the
load. In an ideal circuit, only one MOSFET would be needed in each leg. But because
of details (beyond the scope of this course) of how the selected MOSFETs work, each
leg of the final circuit needs one more MOSFET to control one of the MOSFETs that
actually carries the current to the load.
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Figure 1. The system-level diagram of the inverter system shows its three subsystems as
blocks and the waveforms/signals connecting them as arrows (power signals are bolded).
The inverter transforms DC power from the energy source into AC power that can be
delivered to the load. The control subsystem generates two signals that control the Hbridge.
A complete system requires one more subsystem to provide power to the inverter.
In the last part of this experiment, you will supply input power to the system with a
renewable energy source—a small photovoltaic solar panel.
The inverter you will build is small, and can only handle a few hundred milliwatts of
power. However, the same basic principles are also used in inverters that connect
photovoltaic solar farms to the power grid. These can handle megawatts of power.
Note: You may use some circuit components, such as flip-flops and MOSFETs, for
the first time. Don’t worry about the details of how these electronic components
work. If you are studying Electrical Engineering, you will learn about them in much
more detail in subsequent courses. If you are a non-EE major, our focus on how
circuits implement important systems will help you see how they are an important
part of modern engineered systems.
Resources
1. Videos for this learning module (watch these before working through this
laboratory experience)
2. A basic function generator
3. A dual-channel oscilloscope
4. A basic multimeter that can measure voltage and current
5. A power supply that can deliver 5 V DC (1 A is enough current capacity)
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
You will also need the electronic components in Table 1 (substitute part numbers
that will also work are shown in parentheses):
Subsystem
Part
Number
Description
Quantity
7474
TTL Dual D-Type Flip-Flop
1
BS170
N-Channel Enhancement Mode
Field Effect Transistor
Standard Recovery Rectifier
6
Various values
Resistors
8
Various values
Capacitors
5
1W Solar Panel
80x100
1 W Solar (PV) Panel (Vendor:
Seeedstudio.com)
1
Control subsystem
H-bridge
1N4007
(1N4001, 1N4002,
1N4003, 1N4004,
1N4005, 1N4006)
2
Renewable Energy
Source (Optional)
Table 1. Parts list for this experiment.
Part 1 – Control of the H-bridge transistors
To generate the AC current, the circuit should automatically switch the flow of
current through the load. Roughly, it should flow in one direction for one-half of an
AC cycle, and the other direction for the other half, and repeat this forever.
In this part, we construct and integrate two of the subsystems in Figure 1: the
clock source that produces the clock waveform, and the control subsystem.
We use the function generator to produce the clock signal. What should its
frequency be? Since the AC waveform is 60 Hz, it repeats every 1/60 Hz = 16.67
ms (note that the inverse of time (s) is frequency (Hz), and vice versa). But the
trick is that we need to switch the flow at four times that frequency, or 240 Hz.
The clock waveform is not rich enough in content to drive the H-bridge: to change
the direction of current through the load, we need to turn one dog-leg of the Hbridge off when the other is on, and vice versa. See Figure 2; the top waveform is
the clock waveform, and the two other waveforms activate the H-bridge dog-legs.
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Figure 2. Waveforms for the input (Clock) and outputs (Q1 and Q2) of the control
subsystem.
Here is what happens when we use these signals to control the H-bridge. When Q1
is high and Q2 is low, the load current will flow in one direction, and when Q1 is low
and Q2 is high, the load current will flow in the opposite direction. However, we
have a slight complication: what happens when both are high (or low)? In both
cases, the voltage across the load is zero, so no current will flow. Thus the two
states (Q1=1 & Q2=0) and (Q1=0 & Q2=1) are zero-current transition periods
between the states (Q1=1 & Q2=1) and (Q1=0 & Q2=1) when the load current
flows.
Figure 3. Implementation of the inverter control subsystem using two D-type flip-flops
integrated into a single chip. The numbers outside the chip boundary (labeling the wires)
are the physical pin numbers; the labels inside the chip boundary describe the logical
function; for example, Q1 is the output of flip-flop 1, and in our circuit, it is connected to the
input D2 of flip-flop 2. (We will see later that Q1 is also the control signal for one dog-leg of
the H-Bridge, while Q2 is the control signal for the other dog-leg.) Notice that /Q2 (the
inverted output of the second flip-flop) is connected to the input D1 of the first flip-flop.
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Now that we have a model for the signals that must be generated, we must develop
a circuit to generate them. It is an implementation of our control subsystem for the
AC-DC inverter. It uses two flip-flops; see the schematic in Figure 3. It will be
familiar to you if you’ve studied digital logic. If not, you can just build it and see
how it works.
Procedure
1. Set up the function generator to produce a square wave at a frequency of 120
Hz. The low voltage should be zero volts (0 V), and the high voltage should be 5 V.
Verify the waveform on the oscilloscope.
2. On your breadboard construct the control circuit according to the schematic in
Figure 3. See Figure 4 for a diagram of the 7474 flip-flop.
3. Connect the function generator output (CLK_IN in the schematic of Figure 3) to
the clock inputs of the flip-flop. Make sure the negative output of the function
generator is connected to the circuit ground.
Figure 4. Packaging of the 7474 dual D-type flip-flop. Note that pin 1 is directly below the
dimple at the left-hand side of the physical package. Note: while this is a very typical
package for the 7474, you may have a different package; check the datasheet for your
device.
4. Using the oscilloscope, verify that the signals look like those in Figure 2. (First
compare each Q output with the clock waveform, and then compare the two Q1
outputs.)
Have your lab instructor or aide review your results and sign below:
Signature ________________________________ Date ___________________
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Part 2 – H-Bridge
Here we construct the circuit that rapidly changes the direction of the current
through the load. The switches in each leg are MOSFETs, denoted by Q1 through
Q6 in Figure 5. The load is connected between OUT1 and OUT2. When current
flows from OUT1 through a load to OUT2 (Figure 5), Q1 and Q6 are on. When the
current flows in the opposite direction, Q5 and Q3 are on. Think of MOSFETs Q2
and Q4 as inverters, so, e.g., when Q2 is on, Q1 is off.
Note: Take time to understand how the schematic in Figure 5 implements an Hbridge, even though it does not look like an H.
Warning: Do not confuse different Q’s in the schematics for the inverter system.
The label Q is used for (i) flip-flop outputs in the control subsystem and (ii)
MOSFETs in the H-bridge subsystem. In more detail: According to convention, Q1
and Q2 are outputs of the flip-flops in the control subsystem. Alarmingly, Q is the
conventional prefix for transistors in a circuit, so Q1 and Q2 are also two of the
MOSFETs in the H-bridge. Use context to determine whether Qx refers to a flip-flop
output or a MOSFET. Engineering is sometimes messy, but we make it work.
Figure 5. Schematic for the H-bridge subsystem.
The MOSFETs are in TO-92 packages, as shown in Figure 6 (left). Use Figure 6
(right) to map the D (drain), G (gate) and S (source) pins on the physical device to
the schematic representation in Figure 5 (don’t worry about the diode connecting
the source and drain in Figure 6 (right).
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Figure 6. (Left) Physical package of the MOSFET. (Right) Schematic of the MOSFET.
Procedure
1. Build the circuit in Figure 5 on your breadboard. To assure a clean layout, keep
in mind that in the next part of this experiment the Q1 and Q2 signals will be
connected to the Q1 and Q2 outputs (respectively) of the flip-flip using resistors.
2. Note that you will connect the 5 V supply to all four DC_INPUT points on the
schematic. (In the next part, you will use the 5 V DC power supply to power both
the control subsystem and the H-Bridge. In other words, the 5 V DC power supply
is both “5 V” in Figure 3 and “DC_INPUT” in the schematic of Figure 5.)
Part 3 – Integration of the Subsystems
Here you will connect the Control and H-Bridge subsystems to complete the design.
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Figure 7. Schematic of complete inverter, including control and H-bridge subsystems.
Procedure
1. Connect the control subsystem to the H-bridge. First, connect the flip-flop
output Q1 to the gate of MOSFET Q2 using a 1K resistor. Similarly, connect flip-flop
output Q2 to MOSFET Q4 using another 1K resistor.
2. Connect the H-bridge to the load. Connect a 1K resistor between AC_OUTPUT_1
and AC_OUTPUT_2.
3. View the load voltage waveform on the oscilloscope. Sketch the waveform in the
box below.
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
4. How does the output waveform differ from a sinusoid? Also, suggest some
reasons.
Have your lab instructor or aide review your results and sign below:
Signature ________________________________ Date ___________________
Part 4 – Efficiency of Inversion
Whenever energy is transformed, some is lost; in an inverter, energy is dissipated
as heat by resistors as well as the unavoidable resistance of the other electronic
devices.
How efficient is our inverter? Efficiency can be measured as the ratio of
the output power to the input power. For example, if ¼ of the power is lost in the
inversion process, then only 3/4 is generated as AC output power; so in this case
the efficiency of the inverter would be 75%.
Procedure
1. Measure the input power (supplied by DC_INPUT): because the input is DC, use
DC measurements of the current and voltage to obtain the power.
2. Measure the output power dissipated by the load. Here, use the RMS
measurement mode of the multimeter to account for the fact that the voltage and
current are time-varying.
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
3. Compute the energy efficiency. Because power is the rate of energy, we can
compute energy efficiency as the ratio of the output power to the input power.
Have your lab instructor or aide review your results and sign below:
Signature ________________________________ Date ___________________
Part 5 – Smoothing the AC Waveform
The output of the inverter does not look sinusoidal, but rather like stair steps.
Sophisticated inverters use clever tricks to generate a smoother signal, but all
inverters rely on some kind of filtering to do this. A good way to think about
filtering is as a way to quickly store and release energy; when an input voltage
drops rapidly, the filter boosts it, and when the voltage rises, the filter can absorb
it. Here we explore filtering using a capacitor, which stores energy in an electric
field.
Note that from a design perspective, the capacitor is part of the inverter system,
even though on our breadboard it looks like we are making the load reactive by
putting capacitance in parallel with the resistance.
Procedure
You will put a capacitor in parallel with the load. Here is where you can
experiment. Try 10 uF, 100 uF, and 500 uF capacitors, and interpret your results,
noting not only what happens, but why.
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Have your lab instructor or aide review your results and sign below:
Signature ________________________________ Date ___________________
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Part 6 – Reliability
One method to improve the reliability of a system is to build it with better (that is,
more reliable) components. But this approach does not alleviate the problem of
single-point failures, where the failure of one component causes the entire system
to fail. One way to remove the possibility of single-point failures is to build
redundant systems. In our case, we might use two inverters, each connected to
the DC energy source and the load, with each running at half of its capacity.
Analyze the advantages and disadvantages of redundant design.
Have your lab instructor or aide review your results and sign below:
Signature ________________________________ Date ___________________
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ENFUSE Experiment: Energy/Environment: DC-AC Power Inversion
Part 7 (Optional) – Inverters in Renewable Energy Systems
If you have access to a 1 W solar PV panel, try connecting it to the DC_INPUT of
your inverter and placing the panel in the sun. Monitor the DC_INPUT voltage with
a multimeter. You will notice that the output is maximized when the panel is
perpendicular to the sun’s rays. Using a protractor, measure the power output of
the panel when it is at different angles to the sun.
Question (multiple choice question): the panel power output is proportional to the
(a) sine
(b) cosine
(c) arctangent
(d) none of the above
of the angle of the panel to the sun’s rays.
Try to fit your data to the above three models. Describe and explain your answer
below.
Have your lab instructor or aide review your results and sign below:
Signature ________________________________ Date ___________________
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