Download Experiment/Project 1 Diodes/LEDs/Polarity Checker

UNIVERSITY OF MASSACHUSETTS DARTMOUTH
DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
ECE 201 CIRCUIT THEORY 1
EXPERIMENT / PROJECT 1
DIODES AND DIODE CIRCUITS
INTRODUCTION TO THE PN JUNCTION DIODE
The PN Junction Diode is the basic electronic component. Combinations of the simple PN
Junction Diode are used to make more complex devices such as bipolar transistors (BJTs), fieldeffect transistors (FETs), silicon-controlled-rectifiers (SCRs), and integrated circuits such as
operational amplifiers (OP AMPS).
The physics behind the operation of a PN Junction Diode will be discussed in your electronics
course. A simple circuit model based upon the conditions of operation will be presented here.
CIRCUIT SYMBOL AND SIMPLE MODEL
The circuit symbol and terminal identifications for a type 1N4007GP PN Junction Diode are
shown below.
Anode (A)
D1
Cathode (K)
1N4007GP
The Anode terminal is the P (or +) side of the diode, and the Cathode is the N (or -) side. The
“arrow” is used to indicate the positive direction of conventional current flow through the diode.
The voltage drop across the diode has polarity + on the Anode, - on the Cathode.
The circuit model for the PN Junction Diode is the Single-Pole Single-Throw (ON – OFF) Switch.
The operation of the PN Junction diode is as follows. If the external circuit provides a voltage
across the diode (a condition called BIASING) that forces conventional current through the diode
to flow in the direction of the arrow in the diode symbol, (FORWARD BIAS), the diode behaves as
a CLOSED switch (or short circuit) as shown below.
Iext
D1
1N4007GP
Iext
1
On the other hand, if the external circuit biases the diode in a direction to force the current
through the diode to flow AGAINST the arrow in the symbol, (REVERSE BIAS), the diode
behaves as an OPEN switch (open circuit) shown here.
D1
Iext
1N4007GP
Iext
A SIMPLE DIODE APPLICATION
The circuit shown below is known as a Half-Wave Rectifier. A sinusoidal voltage is applied to the
series-connected diode and resistor. When the input voltage (the upper waveform) is on its
positive half cycle, the current will be in the direction of the arrow in the diode symbol, the diode
acts as a short-circuit, and the source voltage appears across the resistor (the lower waveform).
When the input voltage is on its negative half-cycle, the current would try to flow in the direction
opposite to the arrow in the diode symbol. The diode will act as an open switch, the current will
be equal to zero, and there will be no voltage across the resistor, as evidenced by the waveforms
shown above. In order to satisfy KVL, the input voltage appears with reverse polarity across the
diode. In practice, there is a maximum value of this voltage known as the PIV (Peak Inverse
Voltage) that the diode can withstand without breaking down.
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LIGHT-EMITTING DIODES (LEDs)
Another type of diode that has a wide variety of applications is the “light-emitting-diode”, or LED.
This diode is made from specific material which causes visible light to be emitted when the diode
is forward-biased and conducting current. The LED behaves as an open-circuit when it’s under
reverse bias. The circuit symbol and terminal identifications for the LED are shown below.
LED1
Anode
Cathode
A simple series circuit as shown here can demonstrate the operation of the LED. In the circuit
diagram on the left, the switch is open, no current flows, and the LED is not illuminated. When
the switch is closed, as in the circuit diagram on the right, current flows in the forward direction
and the LED is illuminated.
J1
J2
Key = O
Key = C
LED1
LED2
V1
12 V
V2
12 V
RLimiting
470Ω
RLimiting1
470Ω
The inclusion of the 470 Ω resistor is to protect the LED from excessive forward current (usually
not more than 20 mA). In practice, the LED has a forward voltage drop of 1.5 – 2.0 V depending
upon the material used in fabrication. The limiting resistor usually ranges from 220 – 470 Ω
depending upon the applied voltage. It should be noted that many LEDs have a “built-in” series
resistor. Check the diode spec sheet for complete information and ratings.
A CIRCUIT ACCESSORY COMPONENT – THE PIEZO-ELECTRIC BUZZER
A common circuit accessory component is the piezo-electric buzzer. The circuit symbol for a
commercial SONALERT 200Hz buzzer is shown below.
U1
SONALERT
200 Hz
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The symbol for a more generic buzzer is shown here.
U1
BUZZER
200 Hz
These devices emit a tone of 200 Hz when connected to the proper supply voltage. Although
there are no polarity designations on the circuit symbol, most of these devices are polarity
sensitive. For the MultiSim symbols, the upper terminal is the + terminal. In general, there are
polarity markings on a “real-world” device.
In most cases, these devices are rated at 9 VDC, but actually operate over a wider voltage range,
typically from 3-18 VDC. When using the devices in MultiSim, check the ratings and make
adjustments to the model(s) as needed. Usually you need to adjust the voltage rating to the
particular supply voltage in your circuit. If the buzzer still does not sound, the current rating must
be lowered. Some trial-and-error (and patience) might be needed.
Finally, the frequency of the device may be adjusted. Typical frequencies for devices used in
alarm situations are 1.9 KHz and 2.7 KHz – these frequencies usually get your attention and
become irritable if not silenced.
A simple buzzer circuit is shown here. If you are trying this in MultiSim, no adjustments to the
model need to be made.
J1
Key = B
U1
V1
12 V
BUZZER
200 Hz
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EXPERIMENT / PROJECT #1
DC VOLTAGE POLARITY CHECKER
PROBLEM STATEMENT/ SPECIFICATIONS
Design a circuit which will check the polarity of an applied voltage of 12 VDC.
If the voltage is applied with proper polarity, the circuit should respond by lighting a Green LED. If
the voltage is applied with the wrong polarity, have the circuit respond by lighting a Red LED and
sounding a buzzer.
The buzzer will operate on any DC voltage from 3 to 18 volts at a current of 10 mA. The LEDs
have a voltage drop of 1.5 V and should have a protection resistor installed in series to limit the
current to 20 mA in order to prevent burnout.
As part of the design process, you must use MultiSim software to simulate your designed circuit.
LABORATORY PROCEDURE/ RESULTS
Construct the circuit which you designed. Pay attention to the polarity of the LEDs and
Buzzer. Test your circuit to see if it performs according to the problem specifications. If it does,
demonstrate it to either the instructor or the TA.
If your first design does not work according to specs, you should make whatever
modifications that you think are necessary and try again. When you have a working circuit,
demonstrate it to either the instructor or the TA.
Each student should submit:
1 complete circuit diagram of the original design
1 complete circuit diagram of the final design, only if modifications
were needed
1 MultiSim simulation of your final circuit
1 Technical Description that explains how the circuit functions.
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