Download ELECTRONIC DEVICES AND CIRCUITS LAB #1

ELECTRONIC DEVICES AND CIRCUITS
LAB #3
THE SEMICONDUCTOR DIODE
WHAT IT’S ABOUT
You will examine basic diode properties in this lab and learn how to test
them using a DMM. Although several semiconductor materials might be
used, silicon is by far the most common and will be used here The
simplest semiconductor device and the first one you study is the diode (pn
junction)..
WHAT TO DO:
(1)
Most diodes use an identification number starting with the prefix “1N”.
Locate a 1N4004 or a 1N4005 from your lab kit. It is a small resistor sized
component…black…with a white stripe at the negative (cathode…N
material) end. The other lead is the positive (anode…
p type material) lead.
ANODE (P)
CATHODE (N)
(2)
The most basic property of the diode is that it will conduct only when the
anode is more positive than the cathode (by about 0.7V).
Set up the circuit of figure 1.
FIGURE 1:
(3)
1
The AVERAGE forward voltage drop across a conducting (forward
biased) diode is usually taken as 0.7V. The diode is a non linear device
which means its resistance is not constant. This is most evident as the
diode turns on with a high resistance initially, dropping as current rises. At
normal operating current the resistance is very low and reasonably
constant.
Measure and record, to 3 figures, the voltage across the diode.
VD =_______V
Measure the resistor voltage and from it calculate the diode current .
(remember ohms law?)
{ I D = (Vs-VD)/RL}
ID =_________ma
Determine the resistance of the diode as:
VD
RD =  = ________
ID
(4)
Repeat (3) for R = 1000 and R = 100Ω (1 watt)
R = 1K VD = _______V
ID = ________ma
RD = _______
R = 100 VD = _______ V
ID = ________ma
RD = _______
(5)
Take the average of the three diode voltage drops and round off the value
to one decimal place
VD = _______V
(6)
Set up the next circuit shown. You are using an LED (light emitting diode)
which is still just a diode, but with a different voltage drop and it glows
when forward biased.
The LED should now be illuminated.
Measure the voltage drop across the
LED.
VLED = _______V
(7)
2
Reverse the 1N4004. Did the LED turn off? If so this indicates that the
1N4004 is now OFF and is acting like an open switch. It is said to be
REVERSE BIASED.
(8)
Testing the diode with an ohmmeter will determine if the diode is open or
shorted internally. Normally a diode will fail open circuit.
Set the DMM to OHMS and on a range marked with a diode symbol.
An ohmmeter measures resistance by applying an internally generated voltage
of about 0.25V and it determines the unknown resistance from the resulting
current flow. 0.25V is not enough to turn on the diode, so for diode testing
a special ,diode marked range has an output which is high enough to turn on the
diode and measure its resistance. The test gives a reading which no particular
significance other than indicating whether the diode is conducting or not. When
the meter forward biases the diode there should be a reading, and when reverse
biased it should indicate an open circuit. Most DMMs show the voltage across the
diode if it is good, if it is bad it will show an open or 0
Leave the meter leads open and note how your meter indicates an open
circuit. Different meters have different ways of showing an open (or off
scale value).
Apply the red meter lead (+) to the anode and the black lead (- or gnd or
com) to the cathode . Record your reading as “OPEN” or “CONDUCTING”
READING _________________
READING _________________
(9)
Set up the circuit shown:
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Be sure that the peak to peak input is exactly 10V
Sketch Vout showing the exact peak voltage with respect to ground (use
the DC setting of the scope)
Vout
0V
This circuit which converts ac to dc is called a half wave RECTIFIER. You will
examine RECTIFICATION in your next experiment.
QUESTIONS:
(1)
The +lead of an ohmmeter is connected to the cathode and the negative
to the anode of a diode. The meter reads 0. Is the diode good or bad?
(2) Diode resistance decreases as the diode current increases
(true
false)?
(3) The voltage drop across a diode
( increases
decreases does not change) as diode current increases?
THE DIODE AS A RECTIFIER
__________________________________________________
WHAT IT’S ABOUT
One of the most important applications of the diode is in the conversion of
AC to DC, a process called RECTIFICATION . Electronic circuits require a
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DC source, usually a low voltage from about 3V to 50V, although a few
require higher voltages. The “half wave” rectifier discussed briefly in class
and seen in lab #1 is rarely used. We will examine the
“full wave rectifier in this lab.
WHAT TO DO
(1)
The bridge rectifier is the most commonly used circuit. Commercially the 4
diode array is available as a single package unit. You will use 4 discrete
diodes from your lab kit. Set up the circuit of figure 1.
1N4004
DIODES
The transformer
used here is 12.6V
center tapped. You
will use only one
half of the
secondary as
shown (6.3V)
FIGURE 1: FULL WAVE BRIDGE RECTIFIER
FOR ALL MEASUREMENTS CONNECT ONE SCOPE CHANNEL ONLY. IF BOTH
CHANNELS ARE CONNECTED YOU WILL BLOW ONE DIODE!
(2)
Connect scope channel 1 across the transformer secondary (Vs).
Measure it accurately and sketch the waveform showing the  peak
voltages and the frequency. Set the scope on DC and put 0V at center
screen.
+
Vs
0V

(3)
Measure the rms secondary voltage of the transformer using the scope.
{measured}
(4)
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Vs (rms) =________V
Calculate the rms secondary voltage from your measured peak to peak
value in (2). Compare to the value in (3)
{ rms = 0.707peak}
Calculated Vs (rms) = ______V
(5)
Move the scope to Vout and accurately measure the peak voltage and
sketch the waveform. (Set 0V at bottom screen). Show the peak value.
The scope input must be set
The DC input
Vout
0V
6)
Calculate the average voltage (the DC value) of Vout:
Voutpeak
Average Vout (VoutDC ) = 2 x  =__________V

Measure Vout using a DC VM. (or the “mean” value on the scope)
VoutDC = _________V
A dc meter always reads the average value of a waveform
(7)
Connect a 10f capacitor across RL in the circuit of figure 1 and be
certain that the capacitor is correctly connected.
(8)
Set the scope 0V reference level at bottom screen and display Vout.
Carefully sketch the waveform showing maximum and minimum voltage
levels, the peak to peak voltage, and the frequency. (the time varying
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portion of the waveform is called the RIPPLE voltage and represents the
amount of ac “left over” in the DC Vout)
Vout
0V
(9)
Fill in the following:
RIPPLE PP = ______________V
AVERAGE Vout = 0.5( Voutmax + Voutmin ) = ______________V
MEASURED DC Vout = ______________V
RIPPLE FREQUENCY fR
= ___________Hz
(10)
The 10f capacitor is too small for normal use but it clearly shows the
ripple waveform.
Add a 100f capacitor across the 10f capacitor. This time the ripple
voltage will probably be too small to measure when you have the scope
set to measure DC. Measure Vout using a DVM {or the mean value on
the scope}
Vout = __________VDC
Set the scope to AC input and increase the vertical sensitivity to give a
large display of the ripple portion of Vout. Sketch the waveform showing
the peak to peak voltage.
When the scope is set to AC input, there is no zero volt reference point on
the display, in other words the display is isolated from ground. The 1 or
the 2 arrow at the left side of the display identifies the channel and is not
0V. You can only measure peak to peak or rms values on AC input
setting.
RIPPLE VOLTAGE
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____________________________________________________
QUESTIONS:
(1) A transformer is rated at 120V/46V. What peak to peak voltage would
be displayed on a scope?
(2) A scope measures a voltage as 42.4Vpp, what would an AC meter
read ?
(3) If one diode went open in the bridge:
The ripple would (increase….decrease….not change)?
The DC output would (drop slightly….stay the same)?
(4) The % ripple is calculated as
pp ripple X 100%
VDC
Determine the % ripple for the 10µf and the 110µf capacitors used
in the experiment.
D FULLER 8/07
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