PHYS 343 Experimental Techniques
Laboratory #2
Diodes – Zener and Others
Due October 15
Summary. This laboratory will give you enough familiarity with the properties of Zener diodes to
enable you to design a simple voltage regulator employing these devices. The lab will also help you
appreciate the difference between resistors and non-ohmic devices such as diodes.
330 Ω
Preliminary Reading
Chapter 1 of Horowitz & Hill, through page 15.
+
DC
supply
-
A
V
Equipment
• At least two different Zener diodes. One
should have a Zener voltage greater than 5 V.
Also, some 1N4001 or 1N4004 diodes, and several colors of LEDs.
• Solderless breadboard.
• Digital voltmeter.
• Analog milliammeter.
• DC power supply.
• A 1K resistor and a resistance box.
Procedure
Characterize your Zener diodes
1. Examples of Zener voltages are: 1N746A:
3.3 v; 1N4740A: 10 v; 1N4729A: 3.6 v; and
1N759D: 11.6 v. Use an internet search
engine to look up characteristics of your
diodes For example, a Google search for
“1N746A data” turns up a dozen or more
data sheets in .PDF format. Note the
Zener voltage, power rating, and maximum continuous working current.
2. Hook up the circuit shown. Use a separate
330 Ω resistor; the resistance box will be
used later. Use the milliammeter in the
100 mA range, depending on the Zener.
Be careful not to exceed the rated Zener
current so as not to destroy the diode.
Usually the band on the diode is on the
cathode side:
3. By varying the input voltage, collect
enough data to draw a characteristic I–
V plot for each diode, with the current
on the vertical axis. Take current readings using both forward and reverse biasing. (You need not take many points
in the reverse–biased direction until the
diode reaches the Zener break-down region and starts conducting.) Plot these
both on the same graph. The range on
the horizontal (voltage) axis will be from
-10 V (or something below -Vz ) up to +1
V. Take current readings to at least ±20
mA. The forward bias voltage will not be
above 1 V. The reverse breakdown current should be negative on the plot. (Do
not just graph the range over which you
got nonzero current. The plot must contain the origin.) From your plot, estimate
the Zener voltage for each diode — the
value given by the spec sheet is only an approximation. The Zener voltage is not the
voltage at which the diode barely starts
to conduct. It will be approximately the
voltage at a 10 mA current.
Note: if you are using a 1/2–W resistor,
you need to keep the current below 23 mA
(right?). For a 1–W resistor, you could allow current up to 30 mA.
A Zener diode voltage regulator.
1. Remove the ammeter and connect the resistance box as a load for your circuit.
Now show that under this condition, the
Zener diode current is
1
R
IZ =
VS − Vz 1 +
R
RL
3. Use only one of your Zener diodes for the
following procedure. Set the power supply
at 4.0 v above Vz . Now vary RL , starting high and decreasing this resistance until the Zener no longer regulates. (You
should use the resistor “plug box” for RL .)
Make a table:
VS
RL
VL
2. Let VS denote the power supply voltage
and Vz be the Zener voltage.
R
+
VS DC
supply
-
V
RL
One may redraw the circuit as shown:
VS
R
RL
Show that for the Zener diode to conduct
a current, the condition is
VS > Vz (1 + R/RL )
Let me be clear about this: in your notebook, you are going to derive this equation, with a narrative describing your reasoning. Equations alone are not a derivation. Your narrative will contain several
complete sentences.
VL is the voltage drop across the load resistor. The table should contain about 10
values. Plot VL as a function of RL , with
RL on the horizontal axis. Use 3–cycle
semi–log paper, with the resistance RL
on the logarithmic axis, oriented horizontally. The easiest way is to use a spreadsheet. In Excel you can choose the scale
to be logarithmic or linear for either axis.
Now, predict the value of RL below which
the voltage should no longer be regulated,
and describe in your notebook how you
arrive at this predicted value. (Again, a
narrative with complete English (or German) sentences!) From your graph, determine the value of RL where regulation
ceases, and compare this with your prediction.
We can do better than this. For values of
RL below the “critical” value, show that
the expression for the load voltage drop is
VS
VL =
1 + R/RL
(Hint: the Zener diode is not conducting.)
Enter this expression in your spreadsheet,
and plot the predicted points on the same
graph your measurements are on. For the
“theoretical” points, use a line only; no
symbols. Comment on how closely your
predicted values match the measured values. They should match very well.
Now, looking at the equation above, it is
clear that if we plot VL on the vertical
axis and (1 + R/RL )−1 on the horizontal
axis, we will get a straight line. Plot these
values using a spreadsheet. Again use the
graph to figure out the minimum value RL
can be for good regulation.
4. How can you increase the range over
which RL can be varied with good regulation? Describe your reasoning. Briefly
try this with your circuit to verify that it
works, but you need not record the data
in your notebook.
Problem: suppose you wish to install a digital
clock in your car, and you wish to power it
directly from the car battery. The clock requires 5 mA of current at 9 volts. You will
use a 9-volt Zener diode rated at 100 mA.
What is the largest series resistor you could
use (to minimize battery drain) and still provide good regulation? A car battery usually
provides 13.6 volts and can drop to 11 v when
the starter motor is turning over. The zener
current should be at least 10 mA at all times.
(Note: a device such as a car clock does not
act like a resistor, so it is not valid to calculate some kind of equivalent resistance for
the clock. It’s also unnecessary.) This regulator is not really recommended for automotive use, because it is wasteful of power.
There are much better solutions, as we shall
see.
1
Other diodes
In addition to Zener diodes, you have some ordinary silicon diodes (for instance, 1N4001 or
1N40041 ), germanium diodes (often encapsulated in glass), and several LEDs (red, yellow,
and green). Briefly characterize the following
diodes, by measuring the current versus voltage dependence of each: 1N4001 (or equivalent),
yellow LED, red LED, and green LED. None
of them will conduct significantly in the reverse
direction, so you do not need to measure the
reverse-bias current. But you will find that they
differ a lot in the forward-bias direction. Take
enough measurements to plot roughly the I-V
curve for each diode. Put all the curves on the
same graph for comparison. Do not exceed a
current of 25 mA with any of the diodes.
Final Problem: Suppose the diodes in the
circuit below are all 1N4001 (silicon type)
diodes. Detetermine the voltage across each
of the resistors. This is not a long problem
— if you spend more than 15 seconds, you
need to review some concepts.
+
12.0
V 20 Ω
5.0 Ω
Diodes labeled 1N4001, 1N4002, 1N4003, . . . 1N4007 are all the same diode, but with different maximum reverse
bias voltage ratings. You may often substitute them for each other.