PH337 Electronics
Experiment #5
DIODES CHARACTERISTICS LAB
OBJECTIVES
- To generate "typical" current vs. voltage data for different diodes, and,
- To learn something about the characteristics of Zener diodes and how they can
be used as DC voltage references.
DISCUSSION
You will first measure the forward and reverse current as a function of the applied forward and reverse
voltage across the diode. After understanding the function of a "standard" diode, you will characterize
a Zener diode. Then you will add a variable load resistor to see how the zener can regulate the voltage
of the circuit...and when and why it fails to do so.
Definition: A zener diode is a specially constructed diode intended to be used in the reverse
direction.
It is DC biased so that the PN junction has its polarity opposite from ordinary signal or
rectifier diodes. However, in this mode the zener diode approaches a breakdown and avalanche region
that gives a much steeper slope on the I vs V curve, and hence, a much lower resistance. Your
experiment today is intended to show you why that can be useful.
PROCEDURE
Apparatus:
Digital Multimeter (DMM), signal diode, zener diode, fixed resistor (150 Ohm), variable DC power supply,
and a variable decade resistance box. Two channel scope.
This experiment is done with DC voltages.
- Characterize the Signal Diode
by measuring the I vs V in the forward and reverse bias conditions. Connect the diode in
series with the decade box. Put the two DMM's in the circuit as in figure 1, to measure the
current through the diode and the voltage drop across the diode. Make a table from several volts
negative to maybe 1 volt positive (we don't want to draw too many amps.)
Plot the data and determine:
- The reverse bias resistance
- The forward bias resistance
- The forward bias voltage drop
- Characterize the Zener Diode
by measuring the I vs V in the reverse direction
- Connect the diode in series with the 150 Ohm fixed resistor and the DC power supply
as in figure 2.
Don't connect the load (decade box) yet. Use the DMM's as above to monitor the current and
voltage drop through the diode.
- Use the DMM to measure the power supply voltage, Vs, and the Zener diode voltage, Vz, as
you adjust the supply voltage Vs. I suggest that you take data points for Vs over the
range 5.0 V to 10.0 V every 0.5 V.
-
Since there is no load connected yet, the current through the 150 Ohm resistor = the current
through the Zener, Iz. You should, for one set of data measure all three voltages,
Vs, Vz, and Vr, the voltage drop across the 150 Ohm resistor, to convince yourself
that Vr = Vs - Vz. You can calculate Iz from the equation
Iz = Vr / R
(Measure the actual value of R with the DMM!)
- Using the Zener Diode as a Voltage Reference
- Now connect the variable resistor as the load, RL. Set it
to approximately 10kOhm. Then adjust the power supply voltage, Vx=10V.
- Without disturbing the 10 V power supply setting, decrease the load
resistance while measuring Vz. (which is also = VL). You will
probably have to go as low as 200 or 100 ohms to see a large change in Vz.
Take data sets on VL and RL so that you are able to
plot VL as a function of IL. You can compute IL
the current through the load from the equation,
IL = VL / RL
- Summarize your data in a graphical form that resembles figure 3.
Note that the "load line" without the Zener present in the circuit would
intersect the voltage axis at Vs. Try it, by removing the zener and convince
yourself that this is a straight line. What does the slope equal?
- Explain what the action of the Zener is on this circuit and how it acts as a
voltage reference.
- Scope Picture
If we use a triangular wave signal as a replacement for our voltage supply, we
should be able to view on the scope the entire I-V curve we plotted above.
Replace the current DMM in your circuit with channel one of the oscilloscope.
Channel 2 will be the triangular wave voltage source. See if the scope is able to
reproduce the plotted curve at 1 Hz. Change the frequency to 1kHz. Does the
pattern stay the same? Why or why not?