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Semiconductor Curve Tracer This semiconductor curve tracer will allow you to dis­play the dynamic characteristics of semiconductors such as tran­sistors, FETs and diodes on an oscilloscope. It uses readily available parts and is easy to use. Design By CHARLES HANSEN* If you look through any semiconductor data book you will find that each device, be it a transistor, junction FET, MOSFET or zener diode, comes with a family of characteristics which tell a lot about its performance as circuit conditions are changed. This tester allows you to generate similar operating curves. It incorporates a collector supply and base step genera­ tor which together produce voltage and current signals that are applied to the device-under-test (DUT). The tester can be used to measure and display a number of bipolar-transistor parameters simultaneously. For example, it can be used to plot collector current (Ic) versus collector volt-age (Vce) characteristics of a tran54  Silicon Chip sistor, determine saturation vol­tage, calculate gain (hFE) and look at the spacing and slope of hFE curves. Nor is the Semiconductor Curve Tracer limited to bipolar transistors; it can also be used to test JFETs, MOSFETs, SCRs, diodes and zener diodes. The big picture Fig.1 is a block diagram of the Curve Tracer, with an NPN transistor shown as the DUT. The block diagram shows the two parts of the circuit, a collector supply and base step generator. The collector supply is essentially a low voltage transformer feeding a bridge rectifier. This supplies the collector current to the transistor as unfiltered DC. The base step generator feeds base current to the transistor, stepping it up in equal incre­ments, starting from zero and increasing to a maximum of nine steps. Note that the emitter of the transistor is grounded and so is one side of the base step generator. The collector supply on the other hand is fully floating which allows the DUT’s emitter to be grounded. Now let us consider current flow in the circuit. The col­lector current Ic flows from the positive side of the bridge rectifier into the DUT’s collector, out the emitter and via the emitter resistor Re back to the negative side of the bridge rectifier. The base step generator on the other hand produces a step current waveform which flows into the base of the DUT, back out through the emitter and then back to the negative side of the base step generator which happens to be grounded. This last point is most important because it means that the base current does not flow through the emitter resistor Re as it would in conventional transistor circuits. Hence, the current flowing through Re is only the collector current. The voltage waveform across Re is inverted by the fol­lowing op amp to correct its sense and then it becomes the Y signal to one channel of the oscilloscope. The collector voltage waveform becomes the X signal to the oscilloscope and the two are combined in a Lissajous display to produce the characteristic family of Ic vs Vce waveforms. Circuit description Fig.2 shows the circuit of Semiconductor Curve Tracer. It has four ICs, four transistors, 21 diodes, a fullwave bridge rectifier and two power transformers. At first sight, it bears no resemblance to the circuit of Fig.1 but stay with us and all will be revealed. The reason why the full circuit has two transformers is that it needs two completely separate power supplies; one to feed the base step generator and various op amps and the other to provide the collector supply circuit. The collector circuit uses transformer T2 which has a 12V centre-tapped winding which is switched by S4 before being fed to the bridge rectifier comprising diodes D18-D21. The resulting unfiltered DC of 10V or 20V (nominal peak voltage) is applied to two poles of a 3-position switch, S6a and S6c. Switch S6 allows the user to select one of three sweep modes: NPN, PNP or AC. The third pole of S6, S6b, is used to ground the base when the circuit is set for the AC mode. The collector current passed by the DUT is monitored by one of six resistors (Re) selected by switch S7 and the voltage across the selected resistor is inverted by op amp IC4b which has a gain of -1. Its output at pin 7 becomes the Y signal to the oscilloscope. The positions of switch S7 increase in a 1.2.5 sequence; ie, 1mA, 2mA, 5mA, 10mA, 20mA and 50mA. These values do not indicate the amount of collector current flowing but relate to the deflection sensitivity of the oscilloscope display; eg, 1mA/div, 2mA/div and so on. In use, S7 is set to produce the optimum Ic display. Base step generator The base step generator comprises a clock, counter, step-level converter and a step amplifier. The clock circuit consists of diodes D1 & D2 and transistors Q1 & Q2. Diodes D1 & D2 derive a 100Hz signal from the Fig.1: block diagram of the Curve Tracer, with an NPN transistor as the DUT. There are basically two parts to the circuit: a collector supply and a base step generator. This photo shows the collector current vs the collector-emitter voltage of an NPN transistor for different values of base current. The increments in the base current are produced by the step generator. secondary winding of transformer T1 and this is supplied to transistors Q1 & Q2 which turn on hard to provide a 100Hz square-wave clock signal to the 4017 decade counter IC1. Nine of its outputs are coupled via resistors and diodes which results in a step waveform with 1V increments and nine steps. Note that the first decoded output (pin 3) is not used and this represents Special Notice *This project and article has been adapted with permission from an article in the May 1999 issue of the American magazine Popular Electronics. The original design did not have a PC board and this has been produced by SILICON CHIP staff. the zero-level base step. The step waveform is fed to trimpot VR1 and op amp IC2a which functions as a unity gain buffer. Switch S3 is included to provide a 1V HOLD setting which is used to check the step genera­tor’s output during initial tests. IC2a drives two op amps, IC2b which is a unity gain invert­er and IC4a which is connected as a comparator. When the step signal at pin 3 of IC4a reaches the DC level set by potentiometer VR2, its output at pin l goes high, and this resets counter IC1 to zero via diode D13 and so the step waveform starts from zero again. In effect, VR2 is used to set the number of steps, up to the maximum of nine. Diode D13 is included to prevent any negative output vol­ tage swing from IC4a from damaging IC1. Switch S1 is used to select between positive polarity steps from IC2a or October 1999  55 negative polarity steps from IC2b. The step signal from S1 drives two circuits. The first is a voltage divider, which provides the gate voltage steps necessary to test FETs. So the first four positions of switch S2 provide step signal increments of 1V, 0.5V, 0.2V and 0.1V. The larger steps are required for power MOSFETs. Voltage to current converter In order to generate the base current steps required to test bipolar transistors, a voltage-to-current converter is required and that function is performed by the dual op amp IC3 and transistors Q3 & Q4. Note that the voltage-to-current con­verter produces an inverted output so S1 selects negative voltage steps to produce positive current steps and vice-versa. The eight output current steps are determined by the resistors selected by switch S2 and the steps are 5µA, 10µA, 20µA, 50µA, 0.1mA, 0.2mA, 0.5mA and 1mA. Finally, the output lines to the oscilloscope input chan­nels are fed via 560Ω resistors, to isolate the scope input ca­pacitance. Similarly, the connections to the collector and emit­ter of the DUT are isolated via ferrite beads. These measures are included to prevent the possibility of spurious oscillation in a device under test (DUT). The oscilloscope waveforms of Fig.4 demonstrate the opera­ tion of the Curve Tracer circuit. The bottom trace is the output of the base step generator while the top trace is the collector current waveform from the output of op amp IC4b. Apart from the already mentioned collector voltage supply involving transformer T2, the power supply of the Semiconductor Curve Tracer is quite conventional. Transformer T1 has a 24V centre-tapped secondary which feeds a bridge rectifier involving diodes D14-D17 to produce positive and negative supply rails. These are fed to positive and negative 3-terminal 12V regulators to produce ±12V. Construction The Semiconductor Curve Tracer is housed in a standard plastic instrument case measuring 260 x 190 x 81mm. There are two PC boards inside, one behind the front panel, accommodating all the circuitry on 56  Silicon Chip the righthand side of Fig.2, and one on the floor of the case, accommodating all the power supply circuitry. Before you begin assembly, check the PC boards for etching faults and for any undrilled holes. While these are relatively rare, it is much easier to check and fix them while the board is blank. PC board 1 Starting on PC board 1, fit the resistors and diodes first, followed by the transistors, regulators, PC stakes and capacitors. The component layout is shown in Fig.3. If you want to use a socket for IC1, fit it now, otherwise leave the 4017 until you have mounted the two power transformers. The PC board has been laid out for 2N2222 TO-18 metal can transistors but they may also be supplied as plastic TO-92 types. If you get the TO-92 type, bend the centre lead towards the flat before you insert them in the PC board. Both the power transformers are PC-mounting types but we do not solder the primary (240VAC) lugs to the board. Instead, bend the primary leads out and solder short lengths of mains-rated hookup wire to them. The secondary lugs are then inserted into the PC board holes and soldered. A cable tie is threaded through the holes in the PC board and used to anchor each transformer firmly in place. The primary wires are then connected to 2-way insulated terminal strips which also clamp insulating shields made of Elephantide to keep unwary fingers away from the mains. Fig.7 shows the dimenensions of the two shields. The tabs at either end fold back and go under the cable ties which secure the transformers. DO NOT OMIT THESE SHIELDS AS THE LIFE YOU SAVE MAY BE YOUR OWN. Check the polarity of the diodes, regulators and capacitors and then mount the board on the floor of the case using the four self-tapping screws. Front panel PC board The front panel board layout is shown in Fig.5. It is assembled in the same way as the main board, beginning with the links then the resistors Fig.2: the circuit has two completely separate power supplies: one to feed the base step generator (Q1, Q2 & IC1) and the various op amps and the other to provide the collector supply circuit. and diodes. The large number of odd value 1% resistors means that you should use your digital multi­meter to check each value as it is installed. We used PC stakes even though the external wires are soldered to the copper side of the board. This prevents the copper pads from lifting. The three rotary switches should be pushed hard against the PC board before soldering the pins. October 1999  57 Fig.3: the parts layout for PC board 1. The board has been laid out for 2N2222 TO-18 metal can transistors but they may also be supplied as plastic TO-92 types. Note that the pin configurations of the two types are different: if you get the TO-92 type, bend the centre lead towards the flat before you insert them in the PC board. Below: the fully-assembled front panel PC board. Note that the fuse (F2) is mounted on the copper side of the board, as shown in the photograph. 58  Silicon Chip The fuseholder and the trimpot are mounted on the copper side of the board to allow access to them. We did not fit the power LED at this stage but we did mount the step control potentiometer (VR2) on the PC board to allow initial testing. Once the tests are completed the LED can be fitted and VR2 can be mounted on the front panel. Don’t forget the wires from the pot lugs to the PC board. Before you solder the toggle switches in place, you must drill all the front panel holes. Use the front panel label as a template. The lefthand rotary switch (S6) must have its detent washer set for three positions (two clicks), then a nut fitted to hold it in position. The centre rotary switch (S7) must be set for six positions before the nut is fitted. The righthand switch uses all 12 positions and does not need a detent but you should fit a flat washer before the nut to keep the front panel spacing correct. Fit a nut and a star washer to each toggle switch and push it into the PC board. Make sure that the 3-position toggle switch (S5) is in the correct place. Mount the front panel on the rotary switches, using a second nut on each one, then make sure that the toggle switches protrude through the front panel far enough to get a nut on their threaded bushes. They should be pushed right up to the PC board but you may have to move them out a little to get everything just right. Once you are satisfied, solder the switch lugs, remove the front panel, fit the label if you haven’t already done so and put it to one side. It can be fitted after the unit has been tested and is working properly. We now come to the most critical stage, the mains wiring. As you can see from the wiring diagram of Fig.6, we have kept it simple. The mains switch is mounted on Fig.4: these oscilloscope waveforms demonstrate the opera­tion of the circuit. The bottom trace is the output of the base step generator, while the top trace is the collector current waveform from the output of op amp IC4b. the back panel close to the fuseholder and mains cord entry. We used a double pole switch, which ensures that both the Active and the Neutral are disconnected in the off position. Naturally, all the mains wir­ing, including that to the two transformers, must be run in 250VAC-rated hookup wire. Make sure you use a generous length of heatshrink to shroud the fuseholder and the switch. Each wire (or pair of wires in the case of the transformer leads) should be individually sleeved on the power switch before the larger outer sleeve is fitted. Twist the mains leads together as shown and secure them with cable ties, so that if a lead comes adrift, it can not contact any other parts. Also ensure that there are no strands of wire protruding from the terminal blocks on the PC board. We fitted two BNC connectors to the rear panel for connec­ tion to the oscilloscope, which means you will need two BNC to BNC coaxial leads. Quite often it is just as convenient to use the existing oscilloscope probes and to this end we have also fitted a couple of 3mm screws as tie points adjacent to, and wired in parallel with, the BNC sockets which lets you clip the probes straight onto them. Of course you will also need to clip the earth wire of one of the probes onto one of the BNC sockets. The wiring between the two boards must be run as shown in Fig.6. Testing Re-check your wiring between the two PC boards and make sure you have fitted the mains fuse in the fuseholder. Turn the mains switch on and read the resistance between the Active and Neutral pins on the mains plug. Our unit measured 214Ω and we would expect yours to be within 10% of this value. Also check for zero resistance between the Earth pin on the mains plug and the metal shells of the BNC connectors. Once these tests are satisfactory, apply power to the unit and check the ±12V rails from the 3-terminal regulators. Measure the DC voltage at the three PC stakes near the regulators on the main board. With the centre one as earth -12V should be present on the stake nearest the board edge and +12V on the other. Also check the AC voltages from transformer T2. If the AC is not present you have forgotten to solder a transformer pin or else the transformer is faulty. If the DC voltages Fig.5: the parts layout for the front panel board. October 1999  59 Fig.6: follow this diagram to install the mains wiring and to complete the external wiring to the PC boards and rear panel. Note that the two mains transformers have different secondary voltages, so don’t get them mixed up. 60  Silicon Chip Resistor Colour Codes  No.    1    1    1    4    1    1    1    1    1    1    1  13    1    1    1    4    1    1    1    2    1    2    1    5    2    1    2    2    2    1    2 Value 1MΩ 560kΩ 470kΩ 100kΩ 82kΩ 51kΩ 30kΩ 27kΩ 22kΩ 15kΩ 12kΩ 10kΩ 8.2kΩ 7.5kΩ 5.6kΩ 5.1kΩ 3kΩ 2.7kΩ 2.4kΩ 2.2kΩ 2kΩ 1.8kΩ 1.5kΩ 1kΩ 560Ω 510Ω 200Ω 100Ω 51Ω 20Ω 10Ω 4-Band Code (1%) brown black green brown green blue yellow brown yellow violet yellow brown brown black yellow brown grey red orange brown green brown orange brown orange black orange brown red violet orange brown red red orange brown brown green orange brown brown red orange brown brown black orange brown grey red red brown violet green red brown green blue red brown green brown red brown orange black red brown red violet red brown red yellow red brown red red red brown red black red brown brown grey red brown brown green red brown brown black red brown green blue brown brown green brown brown brown red black brown brown brown black brown brown green brown black brown red black black brown brown black black brown 5-Band Code (1%) brown black black yellow brown green blue black orange brown yellow violet black orange brown brown black black orange brown grey red black red brown green brown black red brown orange black black red brown red violet black red brown red red black red brown brown green black red brown brown red black red brown brown black black red brown grey red black brown brown violet green black brown brown green blue black brown brown green brown black brown brown orange black black brown brown red violet black brown brown red yellow black brown brown red red black brown brown red black black brown brown brown grey black brown brown brown green black brown brown brown black black brown brown green blue black black brown green brown black black brown red black black black brown brown black black black brown green brown black gold brown red black black gold brown brown black black gold brown are not right check the diode and capacitor polarities as well as the regulator orientation. Using the curve tracer The front panel board has been laid out so that when all the toggle switches are down (on) you have the safest mode to measure a transistor. The collector DC supply is set to Fig.7: this diagram shows the dimensions of the two Elphantide insulating shields which cover the mains terminals of the power transformers. This photo shows how the front panel is attached to the vertical PC board by fitting it over the switch shafts. October 1999  61 Fig.8 (left): this is the full-size artwork for the front panel. Above: although not shown here, the mains wiring should be secured with cable ties so that if a lead does come adrift, it cannot contact other parts. 62  Silicon Chip Above: the two screws adjacent to the BNC sockets on the rear panel are intended to take CRO clips leads if you don’t have a BNC-to-BNC cable. Table 1: Test Connections Device Collector Polarity Step Polarity BIPOLAR N PN PN P +NPN -PNP + - JF E T N-Channel P-Channel +NPN -PNP + - MOSFET N-Channel P-Channel +NPN -PNP + - Parts List 10V, the collector has the 100Ω load resistor switched in, the steps are set to normal and the polarity is set for an NPN transistor. Both X and Y channels of your oscilloscope must be DC-coupled but because the frequencies being displayed are quite low in frequency, you don’t need a wide bandwidth on any of the channels. The scope’s vertical input should be set to the 0.1 volt/div scale to provide proper collector current readings as indicated by the scale of switch S7. The scope’s horizontal input should be set to the 1V/div scale to provide appropriate collec­tor-emitter voltage readings. If you are using a single channel scope, turn the timebase switch to the X or external position. A two-channel scope with an XY timebase switch position can use one channel as the X channel and the other as the Y channel. Table 1 gives the scope connections and polarities for bipolar and field-effect devices; that table can be modified as required (to match your 1 PC board, 152 x 106mm, code 04110991 1 PC board, 239 x 71mm, code 04110992 1 plastic case, 260 x 190 x 81mm 1 12-0-12V PC-mounting power transformer (T1); Altronics M-7124 or equivalent 1 6-0-6V PC-mounting power transformer (T2); Altronics M-7112 or equivalent 1 DPDT PC-mounting toggle switch (S1) 1 single-pole 12-position PC mounting rotary switch (S2) 2 SPDT PC-mounting toggle switch (S3,S4) 1 single-pole 3-position PCmounting centre-off toggle switch (S5); Altronics S-1332 or equivalent 1 3-pole 4-position PC-mounting rotary switch (S6) 1 2-pole 6-position PC-mounting rotary switch (S7) 1 DPDT panel-mount mains rocker switch (S8) 1 3AG safety fuseholder 1 500mA 3AG fast-blow fuse (F1) 1 500mA M205 fast-blow fuse (F2) 2 M205 PC-mount fuse clips 1 250VAC mains cord with moulded 3-pin plug 1 cordgrip grommet to suit mains cord 2 chassis-mount BNC connectors 3 22mm knobs; Jaycar HK-7022 or equivalent 1 16mm knob; Jaycar HK-7020 or equivalent 1 5kΩ horizontal trimpot (VR1) 1 10kΩ 16mm PC-mounting potentiometer (VR2) 1 red banana socket 1 red banana plug 1 black banana socket 1 black banana plug 1 yellow banana socket 1 yellow banana plug 2 small ferrite beads scope) and attached to the top of the tester as a reference. The step polarity and the collector vol­tage polarity switches should be set to suit the DUT. CAUTION: some of the Curve Tracer’s controls, if set too high, could cause damage to the DUT. Its base current capability is high enough to drive most power transistors to maximum collec­tor current. If the Curve 2 2-way light-duty insulated terminal blocks 4 3mm x 10mm M3 screws 6 3mm M3 nuts 4 3mm toothed washers 4 6g x 6mm self-tapping screws 4 100mm cable ties Hookup wire, tinned copper wire Semiconductors 1 4017 decade counter (IC1) 2 LF412 dual low-offset op amps (IC2, IC3) 1 NE5532 dual op amp (IC4) 2 2N2222 NPN transistors (Q1, Q2) 1 BC639 NPN transistor (Q3) 1 BC640 PNP transistor (Q4) 1 7812 12V regulator (REG1) 1 7912 -12V regulator (REG2) 1 5mm green LED (LED1) 13 1N914,1N4148 small signal diodes (D1-D13) 8 1N4004 power diodes (D14-D21) Capacitors 2 470µF 25VW PC electrolytic 2 10µF 16VW PC electrolytic 3 0.1µF monolithic ceramic 2 27pF ceramic disc Resistors (0.25W, 1%) 1 1MΩ 1 3kΩ 1 560kΩ 1 2.7kΩ 1 470kΩ 1 2.4kΩ 4 100kΩ 2 2.2kΩ 1 82kΩ 1 2kΩ 1 51kΩ 2 1.8kΩ 1 30kΩ 1 1.5kΩ 1 27kΩ 5 1kΩ 1 22kΩ 2 560Ω 1 15kΩ 1 510Ω 1 12kΩ 2 200Ω 13 10kΩ 2 100Ω 1 8.2kΩ 2 51Ω 1 7.5kΩ 1 20Ω 1 5.6kΩ 2 10Ω 4 5.1kΩ Note: all rotary switches require two nuts. Tracer is set to the high collector-supply-voltage range and the 50mA/div range at the same time, the connected transistor can heat up rapidly and could be destroyed. Be sure to always double-check the pinout of all devices and make sure that the correct collector-voltage polarity and base-step polarity are applied to the DUT. SC October 1999  63