One way to measure the i-v characteristic of a device is to attach a DC voltage source to it, measure the voltage and current, thus obtaining one i-v combination (one point on a graph), and then repeat for many combinations. It is much more efficient to get the scope and the function generator to display the i-v characteristic directly on the screen of the scope. To do so the technique is as follows:
- Press the Main/Delayed key and then choose the XY option.
This puts the scope into XY mode.
- Apply the voltage "v" to the CH1 or "X" input terminals of the scope,
so that the horizontal axis of the scope can be interpreted as "v";
- Apply a voltage proportional to "i" to the CH2 or "Y" input terminals
of the scope, so that the vertical beam deflection is proportional to "i".
This sets up an "i" vertical axis on the scope;
- Use an external time-varying source to cause "v" and "i" to change
through a whole range of values, thus tracing out the i-v curve, and
record the trace on the scope.
It is also important that both CH1 and CH2 be set to DC. Explain why.
The Circuit Setup
Recall that the black terminal of the scope is the "ground" connection on the scope.
In this technique the voltage applied to the vertical input is -Ri, so that the display will be the shape of the i-v characteristic, but upside down. Fortunately by using the invert option for CH2, the vertical ("-Ri") component can be inverted so as to be correctly oriented. Then vertical deflection = i in mA, and horizontal deflection = v in volts.
To cause the device to experience a variety of i-v combinations, so as to trace out the characteristic curve, it is convenient to use the signal generator set to a triangle function. Different sections of the i-v curve can be viewed by changing the DC offset and the amplitude of the triangle.
Set up the circuit with NL = a 2.7K resistor, which is of course a linear device and should result in a linear i-v characteristic curve, through the origin and with a slope equal to 1/R. Position the appropriate axis of both CH1 and CH2 to the center of the scope, which becomes the origin of the i-v graph. Set the frequency of the signal generator to 60Hz. Display and record the i-v curve (as much of it as you can get with maximum signal amplitude and maximal variations of the DC offset). Be sure to record this graph in units of Volts (horizontally) and milliamps (vertically). All graphs in this course should be labeled in electrical units such as these. Compare the measured i-v graph with the theoretical expectation. If the 1 K resistor in the circuit is significantly different from 1000 ohms you may have to insert a correction factor for that, or you might try "building" a resistor which measures exactly 1000 ohms.
Turn down the generator frequency to 1 Hz so you can see just what is happening here - a tracing out of a lot of individual i-v combinations, so fast at 60Hz that they blend into an apparently solid curve. Incidentally you can do a little experiment here about human perception. Experiment to determine the lowest frequency (in the neighborhood of 20-30 Hz) where the scope trace appears to you to stop flickering and look "solid." Movies and TV must refresh their images at this frequency or faster in order to convey the impression of smooth movement.
Once you understand part II, go on to record the characteristics of other devices, as given in the figures below. The first one is of course a linear circuit and should give a linear i-v curve. It is constructed from the DC voltage source in series with a plain resistor. The other devices are not covered in this course but they have interesting i-v curves. You don't need to know anything about the diodes in order to graph their i-v characteristics! Some devices to try:
| Thevenin Equivalent Circuit |  |
| Silicon Diode #1N4004 |  |
| Zener Diode #1N4735
You may need to position the graph
|
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| Leaky Diode Simulation |  |
| Leaky Zener Simulation |  |
Note: diode polarity (direction) is indicated by a ring, as follows:
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