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Do you have a boxful of unknown transistors or a transistor circuit that’s not working properly? This simple tester will indicate whether a transistor is working or not & tell you whether it is an NPN or PNP type. By DARREN YATES Build an in-circuit transistor tester I F YOU’VE built a few projects, then the odds are that you’ll have a fair collection of transistors in your junk­box. You will probably have a good range of types as well, ranging from small signal to high power devices. After a while, it’s easy to forget which ones are good and which are suspect. And if you’ve bought one of the semiconduc­tor “grab bags” that some retailers offer, you’ll undoubtedly have trouble determining which are NPN and which are PNP types –unless, of course, you have the appropriate data books. That’s where this simple Transistor Tester comes in handy. It can test both 10 small signal and power transistors and will indicate whether the device is an NPN or PNP type. Basically, it tells you whether a device is “go” or “no-go” and can indicate the nature of a fault – it cannot determine the lead configura­tion or tell you anything about the gain. In addition, the project can be used to test transistors that are already in circuit. So if you have an AM radio, an amplifier or some other device that’s not working, this project will prove invaluable for troubleshooting. You don’t even have to bother pulling the transistors out of circuit to test them. The test results are indicated by two LEDs mounted side-by-side on the Silicon Chip’s Electronics TestBench front panel. If nothing is connected to the test leads, both LEDs flash rapidly. However, if a working device is connected, then one of the LEDs will go out, depending on whether the device is an NPN or a PNP type. If the transistor is faulty, the result will depend on the nature of the fault. Both LEDs will flash if there is a base-emitter short, while both LEDs will go out if there is a short between collector and emitter. A chart on the front panel shows what the results mean. Circuit diagram Let’s now take a look at the circuit diagram - see Fig.1. It’s based on tran- signals on the collectors of these two transistors are complementary, their voltage levels will be out-of-phase; ie, when one is high, the other is low. This causes both LEDs to flash alternately when power is applied, provided no TUT is connected. NPN test transistor Let’s now see what happens when we connect a working NPN transistor as the TUT. There are two conditions to consider. The first is when Q1’s collector is low and Q2’s collector is high. In this case, the NPN TUT is biased on and so current flows through D3, D4 and the collector-emitter junction of the TUT. This means that there will be about 1.2V across D3 and D4, which is too low to Fig.1: transistors Q1 & Q2 form a 5Hz keep LED 2 on. multivibrator which alternately switches Thus, LED 2 will go out when the collector & emitter terminals of the the test transistor is con­ducting. TUT high & low. If the device is good, one LED 1 will also be off during of the LEDs will alternately flash on & off. this time, since it will be reverse biased. sistors Q1 and Q2 which are wired to Now let’s consider what happens operate as a standard astable multi­ when Q1’s collector goes high and vibrator. The frequency of oscillation Q2’s collector goes low. In this case, is set to about 5Hz by the associated the TUT will be biased off and so LED 100kΩ resistors and 1µF capacitors. 1 will be on. At the same time, LED 2 will be reverse biased and so will As a result, a 5Hz square-wave is produced at Q1’s collec­tor while a sec- remain off. ond 5Hz waveform of opposite phase Thus, if a working NPN transistor appears at Q2’s collector. Q1 drives is used as the TUT, LED 1 will flash the emitter of the transistor under test on and off at a 5Hz rate, while LED 2 (TUT), while Q2 drives the base of the will be off at all times. TUT via a 1kΩ resistor. The collector PNP test transistor of the TUT is driven via diode array D1-D4. For a working PNP transistor, the Note that these are universal inputs; opposite occurs. When Q1’s collector ie, both NPN and PNP devices connect is low and Q2’s collector is high, the to the same EBC test points without TUT will be biased off and LED 2 will any need for switching. light. LED 1 will be reverse biased The two LEDs are connected in re- during this time and will be off. verse-parallel between the collectors When the collectors subsequently of Q1 and Q2. Because the 5Hz output change state, the TUT will be biased S1 1k 1uF 1k K Q2 D4 LED2 D2 C 100k 1k 100k 1uF TO 9V BATTERY Q1 LED1 A D1 D3 TO B TEST CLIPS E Fig.2: install the parts on the PC board as shown here. The LEDs are mounted about 15mm proud of the board & clip into two bezels on the front panel. PARTS LIST 1 plastic case, 82 x 54 x 30mm 1 PC board, code 04109931, 51 x 37mm 1 self-adhesive front panel label, 49 x 79mm 1 SPDT toggle switch (S1) 1 9V battery 1 9V battery clip lead 2 LED bezels 1 150mm length of black hookup wire 1 150mm length of yellow hookup wire 1 150mm length of blue hook-up wire 3 small hook clips Semiconductors 2 BC548 NPN transistors (Q1,Q2) 2 5mm green LEDs (LED1,LED2) 4 1N4148, 1N914 diodes (D1-D4) Capacitors 2 1µF 16VW PC electrolytic Resistors (0.25W, 1%) 2 100kΩ 3 1kΩ on and current will flow through the transistor, this time via diodes D1 and D2. LED 2 will now be biased off, while LED 1 will remain off due to the low voltage across it. This voltage will be equal to the voltage across the two diodes plus the saturation voltage of the transistor (ie, a little over 1.2V). Thus, when a good PNP device is used as the TUT, LED 1 goes out and LED 2 flashes. Crook devices What if you connect a TUT with a collector-emitter short? Regardless of whether it’s an NPN or a PNP device, neither LED will light because the current will alternately flow through each of the series diode pair. This means that only about 1.2V will be developed across the LEDs, which is insufficient to turn them on. If the base-emitter junction of the TUT is shorted, then the transistor will be unable to turn on and current will flow through the 1kΩ base resistor. Both LEDs will continue to flash since the voltage developed across this 1kΩ resistor is suffi­cient to allow them to operate. Silicon Chip’s Electronics TestBench  11 C B E + + TRANSISTOR TESTER + + NPN PNP CE SHORT BE SHORT ● ● ● ● ● ● ● ● LEDON LEDOFF ● ● + OFF + ON + Fig.4: this full-size artwork can be used as a drilling template for the front panel. Make sure that all polarised parts are correctly oriented & note particularly that D1 & D2 face in the opposite direction to D3 & D4. The battery clip must be modified slightly to allow the battery assembly to fit inside the case – see text. Power for the circuit is derived from a 9V battery. Construction Since there are only a few devices in the In-Circuit Tran­ sistor Tester, the construction is a breeze. All the components are installed on a single PC board measuring 51 x 37mm and coded 04109931. Fig.2 shows where the parts go on the PC board. You can mount the parts in any order but make sure that the diodes, LEDs, transistors and electrolytic capacitors are the right way around. The two LEDs should be mounted so that their tops are about 15mm above the surface of the board, so that they later protrude through two bezels mounted on the front panel. You can easily identify the LED leads since the anode lead will be the longer of the two. The board can now be mounted inside a small plastic utility case. First, attach the adhesive label to the lid, then use it as a template to drill out the 12 holes for the LED bezels and the on/off switch. In each case, it’s best to drill a small pilot hole first and then carefully ream the hole out to the correct size. Three small holes are also drilled in one end of the case to take the flying Base, Emitter and Collector leads for the TUT. This done, the on/off switch and LED bezels can be mounted and the Fig.3: this is the full-size etching pattern for the PC board. Silicon Chip’s Electronics TestBench wiring to the PC board completed. Use different colours for the test leads and feed them through the holes in the end of the case before making the connections to the PC board. The PC board is held in position by clipping the two LEDs into the bezels. The battery clip will have to be modified to allow the battery assembly to fit inside the case. This involves removing the plastic cover from the clip and soldering the leads onto the sides of the clip eyelets. Finally, the three test leads must be fitted with hook-type test clips or alligator clips. Alligator clips were fitted to the prototype but you will find that small hook clips are easier to use. As soon as you switch on, you should find that both LEDs flash at a rapid rate. To test the circuit, you’ll need two working transistors – one an NPN device and the other a PNP. Check that only the lefthand LED flashes when you connect the NPN device and that the righthand LED flashes for the PNP device. If both LEDs stay on or both go out and you are certain that the transistors are OK, check that the two LEDs are correctly oriented. Finally, we should mention that the In-Circuit Transistor Tester does not work well with Darlington transistors. This is because they have a higher saturation voltage than normal transistors and so both LEDs will simply go dim SC for a working device.