ECEL 301 ECE Laboratory I
Dr. A. Fontecchio
Assignment 7
Zener Diodes in LabVIEW, MATLAB, and PSpice
Goal
You have done a preliminary design for a voltage regulator. With a
1N4734 zener diode the nominal output is 5.6 V. The nominal output is 6.2 V with
a 1N4735 diode. Your manager would like to know if your circuit would operate
safely under the conditions he/she has in mind. What are the minimum and
maximum input voltages that can be applied and maintain regulation and the safe
operation of the components used?
Pre-Lab Reading
ECEL 301 Week 7 Lecture Notes
Textbook pp. 216 – 220
Parts Required
1.0 kΩ resistor, 1/4 Watt minimum
100 Ω resistor, 1/4 Watt minimum
1N4734 or 1N4735 zener diode
Software Required
Cadence PSpice/Probe
MATLAB
IV Sweep 1.1.exe LabVIEW VI
Voltage Sweep LabVIEW VI (optional)
Introduction
A voltage regulator compensates for variations in input voltage and load
resistance to maintain a nearly constant output voltage (Figure 1). The regulator
to be studied here uses the special characteristics of the zener diode to perform
the voltage control. The zener has forward bias characteristics similar to signal
diodes like the 1N4006/7, but is special for it’s reverse breakdown. Different
models of zener diodes have this breakdown at different voltages, Vz. The zener
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voltage VZ is given for a test current, IZT. These details are shown on the device’s
datasheet.
Figure 1. Regulation of the Load Voltage
The block diagram of Figure 1 can be implemented as the circuit in Figure 2. The
regulator is made up of RS and D1. RL is the load, while VS is the supply. In a
general regulator application, both VS and RL can change.
Figure 2. Simple Voltage Regulator
The IV curve for the 1N4734 zener diode, a device with a zener voltage
(VZ) of 5.6 V and power rating of 1.0 W is shown in Figure 3 and in more detail in
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Figure 4. Note that breakdown curve is nearly vertical. This means that if we can
keep the regulator diode operating in this portion of the IV curve, large changes
in the diode current will cause very small changes in the diode (output) voltage. If
you keep the zener biased below the ”knee”, but do not exceed the maximum
power dissipation limit of the diode, you can have a successful regulator.
Figure 3. Simulated IV Curve for the 1N4734 Zener Diode. This diode has a
zener voltage of 5.6 V.
Figure 4. Reverse Bias Behavior of the 1N4734 Diode (simulated)
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Your manager needs to know if the regulator you have designed will
operate safely with the input voltage variation they have in mind. The maximum
power dissipation rating of the diode limits the current you can pass through it for
a given VZ. The source voltage that generates this current would be VSmax. If the
load is fixed, the source voltage that brings the operating point to the knee would
be VSmin
Theory
To work with the zener diode we will adopt a simplified model that will
ease our calculations (Figure 5).
Figure 5. Simple zener model. The zener diode, with cathode up (left), is
replaced by the source VZ0 and resistance RZ on the right
VZ0, with it’s positive terminal at top, slides the IV curve to the left. Resistor
rZ gives the IV curve a positive slope. If rZ is small the IV curve will be nearly
vertical. This is a reasonably good replacement for the behavior we see in Figure
4. Since the real diode IV is not straight, the effective resistance will depend on
where you are on the IV curve. This varying value is called the “dynamic
resistance”.
Finding VSMAX
To find VSMAX we will assume the worst case - your load has failed or has
been disconnected, so all current passes through the diode. Apply KVL to the
regulator circuit of Figure 2 using the model of Figure 5 and assuming the load is
not present.
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−VS + I × RS + VZ 0 + I × rZ = 0
(1)
The maximum power that can safely be dissipated by the diode is
PMAX = I × VZ
(2)
and is given on the diode’s datasheet.
From (1)
VS = I (RS + rZ )+ VZ 0
Put (2) into (3)
VSMAX =
(3)
PMAX
(RS + rZ )+ VZ 0 (4)
VZ 0
assuming VZ ≈ VZ0
Use the test condition (right side of Figure 5) to find VZ0
VZ 0 = VZT − I ZT × rZT
(5)
Put this solution into equation (4) to find VSMAX.
Finding VSMIN
To find the lower limit of the supply we will assume that the operating point
of the diode has risen to the knee (Figure 4). The current from the source is split
between the zener and the load. To maintain the output voltage (and therefore
the current) constant as the source voltage falls, diode current has to be
redirected to the load. If the diode current does not exceed the knee current the
regulator will fail.
The current from the source is
I (RS ) =
VSMIN − VZ 0
RS
(6)
The load current and voltage can be expressed as equations 7-9.
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I (RL ) = I (RS ) − I ZK
(7)
V (RL ) = I (RL ) × RL
(8)
V (RL ) = VZK ≈ VZ 0
(9)
From equation (6),
VSMIN = I (RS ) × RS + VZ 0
(10)
Substituting equations 7-9 into 10,
⎛V
⎞
VSMIN = ⎜ ZK + I ZK ⎟ × RS + VZ 0
⎝ RL
⎠
(11)
Tasks
Figure 6. Diode Test Hardware. R = 1 kΩ
1. Build the test circuit of Figure 6. The line on the glass or plastic diode
package indicates the cathode end. Measure the zener diode using the IV
Sweep 1.1 LabVIEW VI. Gather diode current and voltage data in forward
and reverse bias while sweeping the dc power supply from -20 V to +10 V.
Capture a minimum of 50 sets of points to a data file for your diode.
Create an IV curve plot for your report.
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2.
3.
4.
5.
a. Part way through your voltage sweep, when VS ≈ 0 V, you will get a
dialog box prompting you to swap leads at the supply. Just swap
the V+ and GND leads and click OK to dismiss the dialog. The
sweep will then continue.
Write a MATLAB m-file that will read the diode data as captured and
determine the dynamic resistance of the diode measured. The dynamic
resistance is only important in reverse bias. You can use as much of the
code in Example 5.5 in the textbook as is useful.
Look up a manufacturer’s data sheet for your diode. Compare your value
of the dynamic resistance to the published value.
Calculate the VSMIN and VSMAX voltage limits for your regulator assuming
RS = 100 Ω and RL = 1000 Ω.
Simulate the regulator circuit in PSpice to confirm your calculations. Some
options of how to do the verification were given in the lecture notes, but
there are others.
Deliverables
Put the following deliverables into our standard lab report format, in the
proper places. All graphics must have the team member names.
1. IV curve of your zener diode as measured using LabVIEW. You can plot
the data any way you like. Put the graph in the body of the report.
2. MATLAB m-file that will read LabVIEW data, as captured, and calculate
the dynamic resistance. In addition to putting this in your report, submit
this m-file through WebCT. Insert the dynamic resistance graph into the
report.
3. Comparison of your dynamic resistance with a commercial datasheet. Cite
your source.
4. Results of the calculations of the VSMIN and VSMAX voltage limits. Put
sample calculations in the appendix.
5. Verification of the VSMIN and VSMAX voltage limits from PSpice.
References
PSpice and MATLAB for Electronics, J.O. Attia, CRC Press, 2002.
MATLAB internal documentation
Microelectronic Circuits, 4th ed. A.S. Sedra and K.C. Smith, Oxford
University Press, New York, 1998, pp. 172 – 175.
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