Orange Coast College
Physics 280
Experiment #4
Diodes
Goal: To determine the current versus applied voltage for a diode, in forward and reverse bias.
In this experiment we want to measure the relationship between the voltage and the current in a
device that does not obey Ohm’s law, namely, in a diode. In most materials (but not all), the
voltage ΔV across the material is linearly related to the electric current I through the material,
∆! = ! !
The constant R is called the resistance and its value depends on the
geometry, temperature, and intrinsic properties of the material. The equation above is known as
Ohm’s law. Ohm’s law is not a fundamental principle of electricity and magnetism. In fact, the
utility of many devices, such as integrated circuits made from semiconducting materials, hinges
on deviations from Ohm’s law. The simplest semiconductor device is the diode, symbolized by
or simply
Unlike other circuit elements discussed in the lab, the diode is a “nonlinear” device. There is a
relatively abrupt change in operating characteristics as the bias voltage changes polarity. A diode
acts like a one-way gate.
Forward bias: For current passing in the “on” direction, the resistance is small, ideally zero, and
the diode is said to be in forward bias. Essentially the diode acts like a wire with zero resistance
when in forward bias (! ≈ 0). The “on” direction is indicated by the arrow in the circuit symbol.
A band at one end of most diodes indicates the end corresponding to the bar in the electric
circuit.
Reverse bias: For current passing in the reverse direction, the “off” direction, the resistance is
much larger, ideally infinite, and the diode acts like an open switch (! ≈ ∞).
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Since diodes do not obey Ohm’s law, diodes are rated by two parameters: the maximum current
without overheating in the “on” state and the maximum reverse bias voltage without breakdown
in the “off” state. It is very important to protect your diodes by limiting the currents and the
voltages to the ranges allowed by the diode’s rating.
The direction of the current in the diode is determined by the sign of the voltage that is applied
across the diode. This voltage is known as the bias. A plot of the current versus bias voltage for
a diode appears in the figure below. For reverse bias a very small (typically microamperes)
current flows. For forward bias, above a certain minimum potential ΔVmin (typically around
0.8 volts) the diode becomes a very good conductor. Effectively the resistance in the forward
direction is very small; in the reverse direction it is very large.
Measure the current versus voltage characteristic of the diode for both positive and negative
voltages in the range - 60V to +1V. (Do not pass a current in excess of 1 amp through the
diode.) Use a digital multi-meter as a an ammeter to measure the current and another digital
multi-meter as a voltmeter to measure the voltage.
A. Forward bias
Maximum forward current: 1 Amp
Set the voltmeter to the 2 volt scale and increase the voltage output of the power supply carefully
from 0 volts to less than 2 volts making sure the current in the diode does not exceed 1 amp.
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I (mA)
ΔV (volts)
Forward bias
Observations:
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B. Reverse bias
Maximum reverse bias: - 60 volts
Vary the voltage on the power supply from 0 volts to about - 60 volts and be sure to use the most
sensitive scale to measure the reverse current in the DMM. You may find that there is no
measureable reverse current!
I(mA)
ΔV (volts)
Reverse bias
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Observations:
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Make a plot of current versus bias voltage for the diode. That is, plot the current in the vertical
axis and the bias voltage in the horizontal axis and connect the data points. Make sure you input
the -60 V data as the first data point, then the -50 V data point, etc… up to about +0.8 V.
Questions:
1. Does the diode obey Ohm’s law?
2. What is the effective resistance of the diode in the forward direction (at the largest value of
current you measured)?
3. What is the effective resistance of the diode in the reverse direction?
C. Half-Wave Rectifier
Goal: Observe a rectifier circuit made with a single diode.
The simplest diode circuit is a half-wave rectifier. When the diode is forward biased, a current is
established and a voltage drop appears across the load resistor. When the diode is reverse biased,
no current is established and the whole voltage appears across the diode. Hence, the voltage
across the load resistor appears on half the cycle (half-wave rectification). More complicated
circuits based on this principle are used to convert AC voltages to DC voltages.
Assemble the circuit shown in the figure below. Set the function generator for a 1 kHz sine wave
output, above 5 volts peak to peak and use your (~ 1 kΩ) resistor. Display the function generator
voltage ΔVAC in channel 1 of your oscilloscope and the voltage across the resistor ΔVBC in
channel 2. Sketch the waveforms grid below.
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