Syscomp CTR-201 User manual

Syscomp Computer Controlled Instruments
CTR-201 Curve Tracer
Syscomp Electronic Design Limited
http:\\www.syscompdesign.com
Revision: 2.1
November 2017

Revision History
Version Date Notes
1.00 November 2014 First revision
1.10 April 2015 Feature Update
1.11 March 2016 Solar cell option added
2.1 November 2017 New software and hardware
Contents
1 Introduction 1
2 The CTR-201 Curve Tracer 2
3 Measurements 4
3.1 Device Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 Instrument Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.4 Zener Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5 NPN Transistor (BJT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.6 PNP Transistor (BJT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.7 N-Channel MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.8 P-Channel MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.9 N-Channel JFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.10 General 2-Terminal Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.10.1 2-Terminal Measurement, Voltage Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.10.2 2-Terminal Measurement, Current Mode . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.11 Small Signal Transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.12 Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.13 Lambda Diode, Negative Resistance Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.14 Vacuum Tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4 How It Works 16
5 Calibration 21
6 Installing the Software 23
6.1 Packaged Executable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.2 Source Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.2.1 Obtaining the Source Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6.2.2 Installing the Tcl Interpreter on a Windows Machine . . . . . . . . . . . . . . . . . . . . 24
6.2.3 Executing the Source Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7 Commands 24
7.1 Testing the Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7.2 Command List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7.3 Export and Import Data, Compare Measurements . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.4 48 Volt Power Enable/Disable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.5 Pulsed VI Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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List of Figures
1 VI Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Typical Control Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Cursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4 Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5 1N3022 Zener Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6 NPN Transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7 PNP Transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8 N-Channel MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9 P-Channel MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10 N Channel JFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
11 2-Terminal Measurement, Voltage Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12 2-Terminal Measurement, Current Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
13 2N4401 NPN BJT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
14 2N4403 PNP BJT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
15 2N7000 N-Channel MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
16 Solar Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
17 Lambda Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
18 Lambda Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
19 Vaccum Tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
20 Curve Tracer Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
21 Attaching to the Ground Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
22 Calibration Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
23 BJT Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
24 48V Power Enable/Disable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
ii

1 Introduction
Electronic devices are useful because they cause a specific relationship between voltage and current. A curve
tracer displays the VI characteristic of these devices, leading to better understanding of their operation.
(a) Resistor, 1kΩ(b) Diode, MR851
Figure 1: VI Characteristics
As a very simple example, it’s possible to plot the VI characteristic of a resistor on a curve tracer. A resistor
plot appears as a diagonal straight line (figure 1a), the slope inversely proportional to the resistance. There are
simpler and less expensive ways of measuring resistance, but it’s useful to keep that example in mind when
viewing a VI plot.
For example, a diode allows current to flow in the forward direction, and blocks it in the reverse direction.
As well, there is a small voltage drop across the diode when it is conducting. This voltage drop increases with
current, so that the voltage is a non-linear function of the current. We refer to the graph of this function as the
VI Characteristic for the forward-biased diode. Different diodes have different shaped curves. For example, the
voltage drop across a germanium diode is about 0.3 volts. Across an LED, it is 2 or 3 volts. Figure 1b shows the
VI characteristic of a 3 amp silicon power diode.
The curve tracer can also plot a family of VI Curves for NPN and PNP transistors, MOSFETs, JFETs and
many other semiconductor devices.
Applications of a curve tracer include the following:
Education: Viewing Device Characteristics A lab exercise to measure device characteristics - such as the col-
lector curves of a transistor - helps students to retain important information about devices. It also teaches
students how to interpret the curves to determine device parameters and models.
Modelling a Device Detailed knowledge of the characteristics of a device allows one to model it for a simulation
program. For example, the characteristic of the diode in figure 1(b) can be modelled by an ideal diode in
series with a pure voltage source and resistor. The voltage source models the threshold voltage of the diode.
The resistor models the linear portion of the curve at higher currents.
Measuring the Range of a Parameter A device data sheet generally specifies the behaviour of the device under
certain conditions. The curve tracer can determine the behaviour under a range of other conditions. For
example, the datasheet of a transistor may not show its incremental collector resistance. This is readily
determined from a curve tracer plot.
1

Testing a device to determine if it is Functional It can be useful not only to know whether a device is func-
tional, but if it has failed, whether it is a short circuit, open circuit, or some other state.
Testing a device to determine it meets its Specifications One can quickly determine whether a device meets
certain performance requirements, such as current gain.
Matching Devices by Comparison In certain specialized applications it is useful to be able to match device
characteristics. For example, one wishes to use a pair of matched JFETs for the input to a low-noise
differential amplifier. The two JFETs should be matched, but a matched pair are not available commercially.
For low volume production or a one-off scientific instrument, a group of single JFETs can be sorted and
matched according to their curve-tracer measurements, and then used in pairs.
Testing a Two-Terminal Circuit Two terminal circuits, such as a constant current or constant voltage device,
can be constructed from component parts. The curve tracer is ideal for measuring the properties of these
circuits. As another example, a negative resistance device (Lambda Diode) can be synthesized using two
junction FETs. The curve tracer can be used to plot its VI characteristic.
Testing Unknown Device If you obtained a large quantity of a particular transistor, it would be possible to de-
termine its principal characteristics, such as polarity (NPN or PNP) and current gain.
Why do we need a curve tracer to determine device characteristics? Isn’t the information
in the datasheet?
The data sheet for a semiconductor device will specify some or all of the maximum, minimum and typical values
for some parameter. For example, the forward voltage drop of a diode will be specified at some value of forward
current. The datasheet may also show a typical curve of forward voltage vs current. However, in the process of
electronic design and troubleshooting it’s often important to be able the exact behaviour of a given device.
For example, the forward voltage of a silicon diode is often quoted a 0.6 volts. A curve tracer shows that the
forward voltage of an MR851 diode, when conducting 1 amp of current, is actually about 1.4 volts. This would
be important to know when designing a power supply1.
2 The CTR-201 Curve Tracer
Early examples of the curve tracer instrument generally included a cathode ray display, much like an oscilloscope.
AC line-operated power supplies swept the device voltages and currents, typically at a rate of 60 or 120Hz. The
instrument had many manual controls and extensive analog circuitry. These instruments were excellent for their
time, but they were large, heavy and expensive.
The CTR-201 system uses a completely different approach. The test hardware connects to a personal computer
via USB, and the computer runs a control program to operate this hardware. The hardware unit contains various
controlable voltage and current sources that actuate the device under test while measuring the voltages and currents
in the device. The measurement results are then handed back to the host PC for display, manipulation, or storage.
There are many advantages to this approach:
•The hardware can be relatively simple, which reduces its size and cost. Where an electronics lab could
perhaps have one curve tracer that was shared by all staff or students, it is now feasible for each work
station to do its own dedicated measurements.
•The measurement algorithms are defined in software, so the capabilities of the instrument can be modified
and extended.
1The curve tracer measures the characteristics of a specific device. For a production design where many units are to be produced, one
should use the worst case parameters of a semiconductor in the design. So you should in general not read the measurements from a curve
tracer of one particular device and use those results directly in a design. But you could use the curve tracer to ensure that a given device meets
or exceeds its datasheet specifications.
2

•Measurements are conducted in pulsed mode. That is, the measurement conditions are applied to the device
under test for a brief interval. The hardware captures the device behaviour during the measurement interval.
The measurement conditions are then removed, allowing the device and the driver electronics to cool. This
approach minimizes the size and cost of the hardware, and allows measurements of a semiconductor device
up to and beyond its rated values.
For example, light emitting diodes are often operated in pulsed mode at peak currents well in excess of their
maximum allowable continuous current. The CTR-201 can take pulsed measurements up to a maximum of
1 ampere forward current.
The interval between measurements automatically increases at higher currents to control the power dissipa-
tion in the device.
•Legacy instruments provided a repeditive display of the measurement curve. In the CTR-201, data is
captured in one measurement sweep, minimizing the dissipation in the device under test.
•Legacy instruments often used dissipation limiting resistors in series with the device under test. Then, as
the current was increased in the device, the voltage across the device decreased. This was a useful approach
to protect the device under test2. However, it limited the measurement at the corner values of voltage and
current. You could measure the device at maximum voltage or current, but not both at once.
The CTR-201 contains no dissipation limiting resistances, and the current sensing resistances are small. As
a result, the device can be tested at the full limit of the curve tracer capabilities - about 35 volts at 1 ampere.
•Legacy instruments typically provide voltage-current curves. A computer-based instrument like the CTR-
201 can provide many more modes of display and analysis, such as the variation in current gain of a BJT
over a range of operating currents, or the incremental collector resistance as a function of collector voltage.
The data can be captured and transferred to a spreadsheet or other program for further analysis.
•Because the CTR-201 hardware is hosted by a computer it is straightforward to capture screen shots, save
the data to a file for further analysis, or project the screen image in a teaching environment.
2As well, the voltage across the dissipation resistance could be used as a measurement of the current through the device.
3

3 Measurements
3.1 Device Connections
3.2 Instrument Overview
Figure 2: Typical Control Panel
A typical control panel configuration is shown in figure 2. Some sections of this display change according to
the type of device under measurement.
Here we list the various functions on the control panel of the Curve Tracer. These are the functions that apply
to all measurements.
Menu Bar: File
Save Preset Saves the current control settings.
Load Preset Loads the previously saved control settings.
Menu Bar: View
Trace Smoothing Enable and disable dot connection algorithm that smooths the trace. Disable to see the actual
measurement points.
4

Menu Bar: Tools
Calibration Provides access to the calibration facility of the curve tracer. You will need multiple digital volt-
meters and ammeters to complete this process. Extreme caution is required.
Menu Bar: Data
Save, load and clear reference trace. This allows saving and displaying a trace for comparison purposes.
Menu Bar: Hardware
Connect Provides access to the routine for connecting to a USB port on the host computer.
Pulsed VI Measurements When enabled, the measurements include a ’cooling’ period between each measure-
ment to minimize dissipation in the device under test.
Menu Bar: Help
Manual Accesses this document.
Change Log Accesses the software changes with this version of the software.
About States the version number of the software.
Firmware Upgrade The firmware is the code installed in the hardware unit. It is not the computer code that runs
on the host computer. Forces an upgrade to the firmware. Do not access this unless a firmware upgrade is
essential. An internet connection is required of the host computer.
Check for firmware upgrades on startup Recommended to leave this enabled. To upgrade the firmware, an
internet connection is required of the host computer.
Device Status
Temperature Shows the internal temperature of the instrument.
USB Voltage Displays the voltage supplied by the USB connection. Should be around 5 volts.
Input Voltage When taking a measurement, shows the supply voltage from the AC adaptor. Should be around
48 volts.
Device Safety
These controls allow the operator to set the current limit or the power limit for the device under measurement. If
the instrument tries to exceed this value, it stops measurement and post a warning message.
Current Range
Allows selection of the current measurement range. There are four settings: Auto, 1 amp, 30mA, 1mA. If you
select Auto, the instrument will try to select the optimal measurement range. There will be seen a small glitch in
the trace when switching between ranges. Also, Auto can fail under certain situations, in which case a manually
selected range is necessary.
Select Device
Allows selection of the type of device under measurement. These selections can be used for other purposes as
well. For example, the N-Channel JFET setting has also been used for measuring MESFETs and Russian Vacuum
tubes.
5

Voltage-Mode, Current Mode
The VI measurement setting allows one to set the measurement mode. In Voltage mode, the instrument adjusts
the terminal voltage and measures the current. In current mode, the instrument adjusts the current and measures
voltage. For example, in a diode-like device, when measured by voltage mode the current increases very rapidly
after the device threshold is reached. It’s much more controllable to operate the in current mode, ie, adjust the
measurement current and report the device voltage
Control Settings
These entry boxes set the parameters for a measurement. Move the cursor to the entry box and left click to place
the cursor in the entry box. Then edit the value. Carriage return is not required. When you click on START,
the instrument software will read the values in these boxes. Usually some experimentation is required to get the
desired curve.
Curve Data
This section of the control panel logs the measured values as they are completed. The ’Save CSV’ function (see
above) copies this data to a .CSV file.
Measurement Cursors
Figure 3: Cursors
A left click in the plot area deposits a measurement point and shows the coordinates of that point.
A second left click deposits a second point, which shows the coordinates of that point. The second point also
generates a line between the two points, a readout of the slope, and a readout of the inverse of the slope. This
provides a semi-automatic method of determining incremental conductance and resistance.
For example, in figure 3 the operator has deposited two measurement points on one of the collector character-
istic curves. The collector incremental resistance is 420 ohms. This is useful information for building a model of
the transistor for circuit analysis or simulation.
6

3.3 Diode
The control settings for a typical diode measurement, and the results of that measurement, are shown in figure 4.
(a) Control Settings (b) Result
Figure 4: Diode
A diode VI characteristic could be measured by placing a voltage between its terminals and measuring the
current (voltage-driven), or passing a current through it and measuring the terminal voltage (current-driven).
Either one will work, but current increases very rapidly with voltage once the voltage exceeds the threshold for a
diode. Conversely, forward voltage increases very slowly with current, so a current-driven measurement is more
controllable and precise.
The current source/sink is available at the centre (Blue) terminal. For the Diode measurement, the software
selects sourcing of current from that terminal. The left (Red) terminal acts as a constant voltage source for return
for the current.
3.4 Zener Diode
The zener diode should be measured using the Diode measurement configuration, which is current-controlled.
Figure 5 shows the characteristics of a 1N3022 zener diode. Figure 5(a) is the forward characteristic, which
is similar to the forward characteristic of any silicon diode.
Figure 5(b) shows the reverse characteristic. The inverse of the slope of this characteristic is the zener in-
cremental resistance. The 1N3022 is billed as a 12 volt zener. The measurement shows it is more like 10 volts,
highlighing the importance of a curve tracer measurement to verify a device characteristic.
7

(a) Forward (b) Reverse
Figure 5: 1N3022 Zener Diode
8

3.5 NPN Transistor (BJT)
The control settings for an NPN Power Transistor (2N3055), and the results of that measurement, are shown in
figure 6.
(a) Control Settings (b) Result
Figure 6: NPN Transistor
Notes
•The collector current of a BJT is more-or-less constant with increasing collector-emitter voltage. That
is, it behaves as a current source (or, more accurately, a current sink in the case of an NPN transistor.)
Consequently, it is best to test this characteristic by setting a base current, sweeping the collector-emitter
voltage and measuring the collector current. The base terminal is supplied with a current that increases
stepwise with each sweep.
•The measurement can generate considerable power in the transistor. For example, at a test collector-emitter
voltage of 30 volts and collector current of 1 ampere, the transistor dissipation is 30 watts. A small signal
transistor might have a maximum power dissipation of 0.5 watts. Consequently, it’s very easy to destroy a
transistor by exceeding the power dissipation. To prevent this:
–While learning how to operate the curve tracer, use a power transistor as the device. The 2N3055 is a
good choice, since it is inexpensive, readily available, has a large semiconductor chip and is mounted
in a metal TO-3 housing.
–Do a rough calculation of the maximum expected current in the transistor. The collector current is
controlled by the base current times the current gain (DC beta β, or hF E ). A typical value for β
is 50. Consequently, a maximum base current of 1mA will cause a maximum collector current of
50mA. With a maximum collector-emitter voltage of 5 volts, the maximum dissipation is 250mW. For
example, a 2N4401 in a TO-92 case can dissipate around 500mW at ambient air temperature of 40◦C,
so this measurement should be safe on a 2N4401.
9

–Use the Current Limit or Power Limit setting to protect the transistor from excessive current.
If the collector current tries to exceed this value, the measurement halts and the measurement currents
and voltages are removed.
–Because the measurement is pulsed with time between each pulse, it may be possible to exceed the
actual power dissipation of the device without destroying it. This is however not guaranteed, and
should be approached very carefully - or with a stack of spare devices.
3.6 PNP Transistor (BJT)
(a) Control Settings (b) Result
Figure 7: PNP Transistor
For a PNP transistor, setup and operation of the curve tracer is similar to the NPN BJT transistor, section 3.5,
page 9.
The control settings for an PNP Power Transistor (TIP32C), and the results of that measurement, are shown
in figure 7.
As mentioned in section 3.5, one must exercise caution not to destroy the PNP device under test by excessive
current or power dissipation.
3.7 N-Channel MOSFET
The control settings for an N-Channel MOSFET (IRF820), and the results of that measurement, are shown in
figure 8.
Notice that the MOSFET drain current is essentially zero when the gate-source voltage is less than the thresh-
old voltage. It then increases very rapidly as Vgs is increased.
Like the BJT, the drain current of a MOSFET is more-or-less constant with increasing drain-source voltage.
That is, it behaves as a current source (or, more accurately, a current sink in the case of an N-MOSFET).
Internally, the gate-source voltage originates with the same circuit that generates base current for a BJT. A
4k7Ωresistor is switched between the current generator and ground, converting it into a voltage source with
10

(a) Control Settings (b) Result
Figure 8: N-Channel MOSFET
an internal impedance of 4k7Ω. (The DC impedance of a MOSFET gate is essentially infinite, so the internal
impedance of this voltage source is inconsequential.)
3.8 P-Channel MOSFET
For an P-Channel MOSFET, setup and operation of the curve tracer is similar to the N-Channel MOSFET, de-
scribed previously.
3.9 N-Channel JFET
The control panel settings and a typical result are shown in figure 10.
For the JFET, the value of Idss (drain current with zero gate-source voltage) is often of interest. This can
be obtain from the VI plot. However, to eliminate any possible voltage between gate and source, it’s useful to
connect only the source and drain terminals to the curve tracer, as in figure 10. The gate terminal is connected to
the source terminal of the JFET, and not connected to the curve tracer. Now the characteristic curve will show the
drain voltage-current characteristic with zero Vgs.
11

(a) Control Settings (b) Result
Figure 9: P-Channel MOSFET
(a) Control Settings (b) Result
Figure 10: N Channel JFET
12

3.10 General 2-Terminal Measurement
For general voltage-stimulus measurement, use the 2-Terminal setting.
This measurement operates in voltage mode, where the voltage is adjusted and the current measured, and
current mode, where the current is adjusted and the device voltage measured.
3.10.1 2-Terminal Measurement, Voltage Mode
This measurement is capable of sweeping from a negative to positive voltage while plotting the device current, so
that all quadrants are visible.
The total range of voltage is 30 volts, that is:
Stop voltage −Start Voltage <= 30V
For example, the range could be set as −15V to +15V, or −5V to +25V.
(a) Control Settings (b) Result
Figure 11: 2-Terminal Measurement, Voltage Mode
A typical measurement is shown in figure 11. The device under test is an arrangement of diodes: two of
1N4001 diodes in series, in parallel with a further three 1N4001 diodes in series. The result is a device with
approximately 1.4 volt threshold in one direction and a 2.1 volt threshold in the other direction.
This test is voltage driven. Testing a constant voltage device such as a diode or zener can easily result in a
measured current that exceeds the maximum current setting. In that case, a warning window will pop up. Reduce
the test voltage and then try the measurement again. For example, in the case of the diode array, a starting voltage
of -3 volts resulted in excessive current. The starting voltage was reduced to -1.8 volts, which allowed the analysis
to run to completion.
For a device with a very constant voltage drop - such as a zener with 6 volt or greater breakdown - it may be
necessary to use the current mode configuration. That configuration forces and controls the device current while
measuring the device voltage so there is less likelyhood of an over-current being triggered.
The connections are different for 2-terminal voltage and current mode. The instrument warns of this when
switching from voltage and current modes.
13

3.10.2 2-Terminal Measurement, Current Mode
This measurement is similar to the 2-terminal voltage mode, with the difference that the current is controlled and
the voltage measured. The maximum range of current is +/-1 amp and the resultant voltage +/-40V.
Notice that the device connections are different for voltage and current mode. A warning dialog pops up
when switching between them.
Figure 12 shows the previously described diode array measured in current mode.
(a) Control Settings (b) Result
Figure 12: 2-Terminal Measurement, Current Mode
3.11 Small Signal Transistors
In the following images we show the control settings and curves for three small-signal devices: 2N4401 NPN BJT
(figure 13, page 15), , 2N4403 PNP BJT (figure 14, page 16) and 2N7000 N-Channel MOSFET (figure 15, page
17). All these devices are packaged in a TO-92 package and are very limited in dissipation.
These settings and results will be useful in providing a starting point for setting the controls for other small-
signal devices.
3.12 Solar Cells
The 2-terminal VI curve control panel for measuring solar cells is shown in figure 16(a) on page 18. The solar cell
is connected with its positive terminal to the collector (red) terminal of the curve tracer. The solar cell negative
terminal is connected to the emitter (black) terminal of the curve tracer.
A typical curve is shown in figure 16(b) on page 18.
At zero terminal voltage the cell is delivering Isc, the short-circuit current, about 2.9 milliamps. At zero output
current, the cell is delivering Eoc, it’s open circuit voltage, around 5.9 volts. The intersection of the curve with the
vertical (current) axis is Isc, the short-circuit current. This varies with light level. The intersection of the curve
with the horizontal (voltage) axis is Eoc, the open-circuit voltage.
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(a) Control Settings (b) Result
Figure 13: 2N4401 NPN BJT
The move the cursor into the curve area and the cursor display then shows the voltage and current at the cursor
position.
3.13 Lambda Diode, Negative Resistance Device
The lambda diode is a circuit that behaves as a negative resistance over part of its VI characteristic. The circuit
and characteristic are shown on pages 18 and 19.
From the literature3, we expect the shape of the curve to have a peak, followed by a valley, followed by a
rising portion. Should we test this using a current source or voltage source?
A current source creates a load line that is horizontal, so varying the test current causes this horizontal line to
change it’s vertical position. This load line can intersect with the VI characteristic simultaneously at as many as
three points, which leads to an ambiguous result.
A voltage source creates a load line that is vertical and changes its horizontal position. This load line can
intersect with the VI characteristic at one point only, so the result is unambiguous.
Consequently, the device should be driven by a constant voltage source, while measuring the current. In the
CTR-201, that implies connecting it between the collector (Red) and emitter (Black) terminals, which are both
voltage sources. The current drive (Blue) terminal is not used.
This is similar to the test method for an NPN BJT and N-Channel MOSFET, where the characteristic is tested
by applying a voltage and measuring the current.
3.14 Vacuum Tube
Figure 19 on page 19 shows the it plate characteristic of a low-voltage vacuum tube, Russian type 1zh18b. The
vacuum tube operates in a similar manner to an N-Channel JFET, and so the JFET setting was used on the curve
tracer. An external power supply of 1.2 volts was also required to power the filament in the tube.
3Google Lambda Diode and negative resistance.
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(a) Control Settings (b) Result
Figure 14: 2N4403 PNP BJT
4 How It Works
A block diagram of the curve tracer is shown in figure 20. It consists of two programmable voltage sources and
one programmable bidirectional current source. These sources are controlled by 12 bit D/A converters.
The measured voltages and currents are shown conceptually on the diagram as voltmeters and ammeter. These
are actually A/D converter inputs that are 12 or 16 bit resolution.
By convention, the three terminals are labelled Collector,Base and Emitter, which refers to the con-
nections for a BJT (transistor). For a MOSFET, the corresponding terminals are Drain,Gate and Source.
The curve tracer can perform measurements on a wide variety of semiconductors besides these two devices.
Notice that a ground terminal is not available to the outside world. Measurements are performed between the
three terminals.
The base current generator actually consists of two current sources. Isource supplies current out the Base
terminal. Isink accepts current into the Base terminal. At any given time, only one of these sources is active. The
compliance of the current source is equal to the full voltage range of measurements, about 42 volts.
In addition to the circuitry shown in figure 20, the unit includes:
•Case temperature sensor, to monitor the temperature of the metal case and shut down supplies if the tem-
perature becomes excessive.
–The temperature warning threshold is indicated by the temperature readout changing to an orange
colour at 45◦C.
–The software critical temperature is 50◦C. Above that temperature the software will not run.
–The firmware critical temperature is 52◦C. Above that temperature the firmware will disable the out-
puts.
•Voltage monitors for the 5 VDC USB supply and 48 VDC main supply.
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(a) Control Settings (b) Result
Figure 15: 2N7000 N-Channel MOSFET
•Indicator LEDs. From left to right these are:
Green USB Power ON.
Red 48V Power ON
Flashing Red Communications traffic.
Amber Status. Illuminates when a measurement is in progress. Flashes to indicate over-temperature con-
dition.
The hardware microprocessor accepts commands from the host to set the various D/A converters and acquire
various measurements. It then sends the acquired data to the host, for display and storage.
Commands are in the form of ASCII strings, so it is quite straightforward for some other software to control
the CTR-201 hardware.
Example: Diode Measurement
A diode is connected with the anode connected to the Base terminal and the cathode to Collector terminal.
The base current source forces current through the diode. The ammeter in the Collector circuit measures this
current4. The voltage across the diode is equal to the base voltage reading minus the collector voltage reading.
Example: PNP Transistor Measurement
The PNP transistor is connected to the like-named terminals on the curve tracer. The emitter and collector voltage
sources are raised to some positive voltage. The base current generator is configured to sink the first value of base
current. The collector voltage is then swept downward so that the emitter is more positive than the collector and the
4The diode current is known from the current source setting, which is quite accurate, so the collector current measurement is redundant.
However, the collector current measurement is very precise, with high resolution, so we use that.
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