Measurement of Diode I-V Characteristics
and
Calculation of Device Parameters for a PSpice LED Model
1. Wire a 100k resistor in series with a LED
a. 100k will be tied to the + side of the arbitrary waveform generator
b. n-side/cathode (short leg) of the LED will be tied to ground
2. Apply a triangular waveform across the components where the amplitude is at its maximum and the offset is half
of Vpp. On the Velleman oscilloscope, the output will vary from 0V to +10V with these conditions. If f = 500
Hz, set the time division to 0.2 ms.
a. The maximum possible current that can flow through the diode is Isc = 10V/100k = 100 A. If you
look closely, you can see the chip inside the package is barely glowing.
i. Replace the diode with a short circuit and calculate the current flowing through the short circuit.
Turns out that the Norton current is the current limit for this circuit.
ii. The Thévenin voltage is the maximum voltage that could possibly be dropped across the diode
when the series resistor value was 0.
3. Measure the applied voltage and the voltage across the LED using the oscilloscope. [I found my oscilloscope
settings were best when the measurement of Channel 1 was the voltage across the diode. All of the instructions
are written for this case.]
a. Use the trigger to stabilize the signals if they are jumping, it doesn’t matter if the trigger is on the rising or
falling edge of the waveform measured on channel selected.
b. Use DC coupling to measure the voltage outputted by the arbitrary waveform generator.
c. While still DC coupled, measure the DC Mean of the voltage across the LED
d. Then, select AC coupling for the voltage measurement across the LED
e. Set the Volt/Div at the smallest value that takes up most of the graph in the vertical direction for both
channels.
f. Under Options, set the Average to 128.
i. The quirks of the scope are easily seen in this experiment. You have to turn the Average to Off
and/or the trigger off to get the waveform displayed to change as you adjust your circuit.
g. Export DSO data.
4. Open the file in Excel.
a. Perform the following calculations
i. Remove the digitizing offset by subtracting the appropriate cell in Row 8 from CH1 and CH2
ii. Convert the data in columns CH1 and CH2 to voltage using the conversion listed in Rows 6 and
7, respectively.
iii. Add the DC Mean to the CH1 voltage.
iv. Calculate the current flowing through the diode by subtracting the diode voltage (CH1) from the
voltage in CH2 and then divide the result by the value of the resistor.
b. Plot the voltage across the diode vs. the current as an X-Y plot.
c. Find Is and n, parameters used in the Ideal Diode Equation.
i. Take the log of the current using only the data were the current is a positive number.
ii. Plot the voltage across the diode vs. the log of the current
iii. Right click inside the plot and select Add Trendline.
1. Select Linear as the Trend/Regression Type.
2. Select Display Equation on Graph.
3. Click OK.
iv. (nkT)-1 is the slope of the curve where n is the ideality factor
v. Is is equal to 10y-interspect where Is is the reverse saturation current.
5. Back to the oscilloscope, set the arbitrary waveform generator to 10 Vpp with a -5V offset. Now the applied
voltage ranges from 0V to -10V.
a. The voltage across the diode should be essentially identical to the voltage from the arbitrary waveform
generator.
i. The diode is acting almost like an open circuit because the only current that can flow through the
diode, at least until it breaks down (which will permanently damage the diode) is Is, which will
be extremely small – unmeasureable given the Velleman oscilloscope.
6. Reset the arbitrary voltage generator to 10Vpp with a +5V offset and change the series resistor to 330 . Note
that the LED is glowing brightly.
a. Your eye can’t respond to changes faster than about 30 Hz so you are average the fact that the LED is
turning on and off. If you set the frequency down to 0.5 Hz, you can see the LED turn off. It will then
slowly turn on as the voltage applied across the diode increases above the turn-on voltage.
b. Repeat Step 3.
7. Import the data into Excel and convert the data to diode voltage VD (CH1) and diode current ID (CH2).
a. Plot VD vs. ID
b. Select a subset of the data where the diode current is increasing linearly and plot this range of data.
c. Find Rs and Von, parameters used in the piecewise model of a diode.
i. Right click inside the plot and select Add Trendline.
1. Select Linear as the Trend/Regression Type.
2. Select Display Equation on Graph.
3. Click OK.
ii. Rs is the inverse of the slope
iii. Von is the x-intercept.
d. If you right click on graph and Select Data, you can edit the data used for the plot, switching the cells
used for X and Y. Then Rs is the slope of the line and Von is the y-intercept.