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Measure Tran The Beta Tester is easy to use. You just connect up the transistor, select NPN or PNP, press the pushbutton, and rotate the knob clockwise until the LED goes out. You can measure the gain of any bipolar transistor with this simple Beta Tester. Just connect a transistor, push the button and rotate the knob until the LED goes out. Then read the transistor's Beta off the scale. That's all there is to it. No meter is required and it can he built for around $20. By MALCOLM YOUNG & LEO SIMPSON Nobody likes putting dud transistors into circuit. If you can, it is always a good idea to check your transistors before using them. With this easy-to-use tester it is but a moment's work to check each transistor. By doing so you eliminate one source of uncertainty from . your work - you know that the transistors are OK. Even if you are fairly certain that you don't have any dud transistors there are times when you will want to measure their Beta (DC gain). Some circuits call for transistors with a minimum gain figure and these are easily checked with this Beta Tester. Other circuits call for transistors to be matched to within a certain tolerance, say within ± 5%. Again, this is a snack to do with the Beta Tester. If you are building an audio power amplifier you will get lower crossover distortion, and therefore better sound, if you can match the 20 SILICON CHIP driver and output transistors closely. If you are building a stereo amplifier you will have at least two pairs of driver and output transistors. By using our Beta Tester you can "mix-n-match" the devices for best overall performance. Finally, you can use the Beta Tester to identify the leads of unknown transistors - where the labelling might have been rubbed off or the type number is unknown to you. We set out the method for doing this in a panel accompanying this article. OK. So there you have a number of good reasons to build this handy unit for your electronics workbench. Once you build it up you'll wonder how you ever managed without it. Long battery life The Beta Tester is housed in a compact plastic utility box measuring 130 x 68 x 43mm. Its controls are simple. There is a momentary contact pushbutton which applies power to the circuit while you do the test. This means that the circuit will not flatten the battery because you've forgotten to turn it off. So the battery should last a long time. In addition, there is a slide switch to select NPN or PNP transistors and a knob with a scale graduated from 5 to 500 - the Beta scale. Using the Tester is simple. Rotate the knob fully anticlockwise, connect the three flying leads to the transistor and select the NPN or PNP setting of the slide switch. Now press the pushbutton and the LED (light emitting diode) will light up. Rotate the knob until the LED just goes out. The pointer of the knob will then indicate the Beta of the transistor on the scale. The Beta Tester uses a 2.BkHz signal to test the gain of transistors; it is not just a simple DC gain test. The circuit There is nothing fancy about the circuit components; just one 555 timer IC and a few transistors. However, a closer look will show that there are a number of clever aspects to the circuit (see Fig.1). Fig.1 can be split into four sections: an oscillator, an amplifier, a detector and a comparator. and match transistors Vlith this: sistor Beta Tester pin 2 and a square wave with an amplitude of close to 9 volts peak at pin 3. The frequency of oscillation is about 2.BkHz. There are two advantages of using the 555 oscillator circuit described here, instead of the more usual arrangement. It uses at least one less resistor and it gives an almost exact 50% duty cycle square wave without any need for adjustment or careful selection of the timing resistors. The square wave output from pin 3 is then fed via a tkn resistor and clipped by two diodes, D1 and D2, to give a waveform with an amplitude of 1.2 volts peak. This waveform is then coupled via a O. tµF capacitor to the amplifier stage. This uses the transistor under test, in a simple common emitter amplifier stage. IC1, a 555 timer, is the oscillator stage. Instead of the usual freerunning oscillator configuration with a capacitor being charged from the positive supply rail, this circuit has the .033µF capacitor being charged from the output, pin 3, via a 68k0 resistor. In more detail, IC1 works as follows. The .033µF capacitor is connected between the junction of pins 2 and 6 (connected together) and OV. The capacitor is charged and discharged via the 68k0 resistor connected to pin 3. At switch-on, the voltage at pins 2 and 6 will be OV and the output at pin 3 will be high; ie, close to + 9V. The capacitor will now charge towards 6V (ie, 2/3Vcc). When it reaches that point, the output at pin 3 will switch to OV and the capacitor will then discharge towards + 3V (ie, 1/3Vcc). The output at pin 3 will then switch to + 9V again and the charging cycle will recommence. The result of this cycling will be a sawtooth waveform with an amplitude of 3 volts peak to peak at Common emitter amplifier To explain further, a "common emitter" amplifier is one where the transistor's emitter is common to both the input and output of the amplifier. In its most simple form, the emitter is connected to "ground" which may be the negative or positive rail of the circuit. The input signal is then applied between the transistor's base and ground while the output signal is taken between collector and ground. In our circuit, the emitter of the transistor under test is connected to OV while its collector goes to the + 9V rail via a 1.5kn resistor. DC bias is fed from the collector to the base of the 'Ji'UT (transistor under test) via two series tMn resistors. This describes the connections for an NPN transistor. NPN/PNP selection If the TUT is a PNP type, slide switch S2 swaps the connections to the collector and emitter so that the emitter goes to + 9V while the collector goes to OV. Otherwise the circuit stays the same. The 2.BkHz signal from the 555 oscillator is fed via a 22kn resistor and 2Mn linear potentiometer (wired as a variable resistor) to the base of the TUT. The 2.8kHz signal 0.1 o.1I "l .,. 1M NPN T gy: 0.1 ..L. IC1 555 D1 B .0022I .,. 02 .,.. ELJc VIEWED FROM BELOW ~- NPN .,. .,. .,. 1 TRANSISTOR BETA TESTER Fig.1: a 2.BkHz oscillator (IC1) is used to pulse the base of the transistor under test (TUT). This signal is then amplified by the TUT which drives class-B detector stage Qt. When the positive voltage swings on Q1's base exceed 1.BV, Qt, Q2 and Q3 conduct and the LED lights. FEBRUARY1989 21 parator function. If the positive swings of the 2.BkHz signal fed to the base of Ql are not 1.BV or more, then Ql and Q2 will not conduct, Q3 will not be turned on and the LED won't light. So these transistors perform an important signal level monitoring function - they won't operate if the signal is not big enough. The test function All the parts, including the switch, are mounted on a small PCB. Make sure that the parts are installed so that they do not protrude above the mounting surface of the switch. Three PC pins are used to anchor the pot lugs. is then amplified by the TUT and then fed to the following detector stage via a O.lµF capacitor. Qt, D3 and D4 make up the signal detector stage. Qt acts as a class-B detector. It has no DC bias to its base and it conducts for positive swings of the 2.BkHz signal. Negative swings of the signal are clipped by D3, to protect the base of Qt. In effect, Qt acts as an emitter follower for positive swings of the 2.BkHz signal and does not conduct at all for negative swings. This is why it is called a class-B detector - because it only conducts for half the signal waveform. The detected signal appearing at the emitter of Qt is fed via diode D4 and stored in a O. lµF capacitor. This filtered voltage then turns on NPN transistor Q2 which then turns on PNP transistor Q3 and the LED. Ql, D4 and Q2 perform a corn- In the Beta test procedure described at the start of this article, potentiometer VRl is first set fully anticlockwise which corresponds to its minimum resistance condition (since it is wired as a variable resistor). This means that the maximum amount of 2.BkHz signal is fed to the base of the TUT (transistor under test) and so, providing it is actually working, it can't fail to have a big signal at its collector. This signal will be fed through to Ql and the other transistors and so the LED will inevitably be glowing. Now, to find the Beta of the TUT, we rotate VRl clockwise and this increases its resistance. This progressively reduces the signal to the TUT until, at some point, the signal at its collector will drop below 1.BV peak, or thereabouts. At this point, the LED will go out. The Beta of the transistor under test can then be read off the scale surrounding the potentiometer knob. Fig.2 (left): mount the LED so that the top of its lens is about 15mm above the board surface and don't forget the wire link under S2 on the copper side of the PCB. Note also that all the polyester capacitors should be bent parallel with the PCB. Fig.3 (right) shows the full size board pattern. 22 SILICON CHIP How to Test Unknown Transistors Most of us have come across transistors of the unknown and unmarked variety which usually get relegated to the junk box. The versatility of this instrument can be increased with a certain amount of operator skill and patience. Transistors can at least be identified as NPN or PNP and their Beta measured. With the aid of a data book you might go further and classify transistors into similar groups with a little trial and error. The procedure for such a task begins with identifying the tran_sistor leads. Set the Beta knob to minimum as before and clip the test leads to the transistor terminals. Test the transistor on both PNP and NPN settings and swap the leads systematically until the LED remains on. There are six different ways to connect the Tester to the transistor and two different transistor types (NPN or PNP). This means that, at worst, you will require 1 2 tests to find the particular pin out for a particular transistor (or discover the bl--dy thing doesn't work!). This task is reduced if you remember a number of ground rules. If you turn a small signal transistor upside down as shown in Fig.4, they all have their leads in a straight line or triangle arrangement. Further, as shown in Fig.4, their leads will be C(ollector), B(ase), E(mitter) from right to left in most cases and B(ase), C(ollector), E(mitter) for the remainder. Fig.6: for TO-3 style transistors, the case is the collector while the emitter and base leads are as shown. VIEWED FRDM BELOW Collector, Emitter as shown in Fig.5 . And for larger power transistors such as those in metal TO-3 cases (2N3055, etc), the case is the collector terminal and the base and emitter leads are as shown in Fig .6 . Reverse gain Fig.4: possible lead connections for small signal transistors. With the majority of common small signal transistors (such as BC547, BC557) the base lead is in the middle. For small power transistors in the plastic encapsulations, such as TO-220 and TO-202, the leads usually (but not always) run Base, Fig.5: the most common lead configuration for TO-220 & TO-202 transistors. BCE Once the pin . configuration is discovered then i_ t is simply a matter of turning up the gain control until the LED goes out. Note that there is still a possibility that the collector and emitter leads are reversed even though you have obtained a believable measurement. This is because bipolar transistors have a reverseactive mode of operation as opposed to the normal forward mode of operation . The reverse gain of a transistor is always very much smaller than the forward gain. So take the highest Beta result in figuring out whether a transistor is a NPN or PNP type . Some transistors will have a gain of more than 500, such as some BC548s, BC549s, BC559s etc. Some of these transistors can have a Beta of up to 900 which is well beyond the range of our simple Tester. However, you can still verify that these high gain transistors are working. If the LED lights, they are OK. Self-monitor function Interestingly, the circuit has its own self monitoring function which tells you that it is working properly and that the battery is not flat. Push the NPN/PNP slide button to the NPN setting and then push the button. Regardless of whether there is a transistor under test or not the LED should light momentarily. This happens because when power is first applied, the 1.5k0 resistor connected to the + 9V rail (for the NPN condition) charges the An insulated wire link must be installed on the copper side of the PCB between two of the slide switch terminals (see Fig.2). The completed PCB assembly is secured to the lid of the case using locking nuts on the pot and pushbutton switch collars. FEBRUARY1989 23 PARTS LIST 1 plastic utility box, 130 x 68 x 43mm (Altronics H-0153 or equivalent) 1 PCB, code SC04102891 , 72 x 61mm 1 9V battery, Eveready 21 6 or equivalent 1 snap connector to suit battery 1 clamp to suit battery 1 pointer knob, 30 to 35mm in diameter 3 alligator clips 3 PC pins 1 DPDT slide switch plus mounting screws (DSE S-2040, Jaycar SS-0821) 1 momentary contact pushbutton switch (DSE S-1102, Jaycar SP-0710, Altronics S-11 02) 0.1µF capacitor connected to the base of Ql. This causes a short pulse of more than 6V to occur at the emitter of Ql. So D4 conducts, as does Q2 and Q3 and the LED flashes briefly. This self test function does not work in the PNP mode because the 1.5k0 resistor is connected to the OV line. On the other hand, if you have an NPN transistor under test and the LED will not flash or light at all, then the transistor under test probably has a short between base and collector. r C + Semiconductors 1 1 2 4 555 timer IC BC558 PNP transistor BC548 NPN transistors 1N4148, 1N914 signal diodes 1 5mm red LED Capacitors 5 O. 1µF metallised polyester (greencap) 1 .033µF greencap Resistors (0.25W, 5%) 2 x 1MO, 1 x 68k!:l, 1 x 22k!:l, 1 x 1 OkO, 2 x 4 . 7k0, 1 x 2.2k0, 1 x 1.5k0, 2 x 1 kO, 1 x 2MO linear potentiometer Miscellaneous Insulated hookup wire, solder Power for the circuit is provided by a 9V battery which can be an Eveready Energiser type 5 2 2 alkaline battery for long life or an Eveready 916 carbon zinc type which will have a lower initial cost. Either way, we estimate that the battery should last a year or more with normal use. Construction As already noted, our Beta Tester is housed in a compact plastic utility box measuring 130 x 68 x 43mm (Altronics Cat H-0153 or equivalent). All the circuit,ry is B mounted on a printed circuit board measuring 72 x 61mm (code SC04102891}. Construction of the Beta Tester is relatively straightforward but before you turn on the soldering iron you should inspect the PC board to ensure the tracks are all etched properly and that there are no open circuits or shorts between tracks. You can check this by carefully comparing your PC board pattern with the artwork included in this article. Make sure that all the holes are drilled out too. Enlarge the hole for pushbutton switch Sl by first drilling a pilot hole of around 1-2mm. The hole for S1 should be around 8mm. The mounting holes for the lugs of the slide switch S2 should also be enlarged to 2.8mm (7/64 inches). . Three PC pins are used for the connections to the 2MO potentiometer VR 1. These should be mounted first as these are inevitably the hardest components to mount. Install all the low profile components next; ie, the resistors and the diodes. The slide switch we used is a particular panel mount type, Altronics Cat. S-2035 or Jaycar SS-0821. You can also use the Dick Smith Electronics type S-2040. Whichever switch is used, make sure that its lugs will fit into the PC board. Note that there is an insulated wire link on the copper side of the board 7 E + + D PNP 100 NPN TEST + L _J Fig.7: here is a full size reproduction of the front panel artwork. 24 SILICON CHIP under this switch. The momentary contact pushbutton switch we used is readily available. You can use Dick Smith Cat. S-1102, Altronics S-1060 or Jaycar SP-0710. The capacitors must be installed so that their height above the board does not exceed 10mm since the entire PC board assembly is mounted to the lid of the case using the fittings of S1 , SZ and VRl. Similarly, the transistors should be mounted so that their overall height does not exceed 10mm. Mount the LED, a 5mm red type, so that the top of its lens is about 15mm above the board surface. This will allow the LED to protrude from the front panel by the required amount. Wire in a snap connector for a 9V battery. Three test leads need to be fitted to the board but they should be left off until after it has been tested and installed on the lid of the case. This side-on view shows how the PCB is secured to the lid of the case using locking nuts on the pot and pushbutton switch collars. What is Beta? There are a number of ways of testing the gain of a transistor. The most common method is to connect the transistor in a commonemitter amplifier arrangement as shown in Fig.8. A fixed current is fed into the base and the resulting current into the collector terminal is measured. The ratio of the collector current to the base current is then the DC forward gain of the transistor. It is commonly known as DC current gain , hFE or DC Beta. Most digital multimeters with a Beta measuring facility perform the above test. They use a base current of typically 1 OµA and they measure the collector current directly. Our Beta Tester uses an AC signal of 2.8kHz to measure AC Beta, which is also commonly METER T I I ..L.. I I ...L.. Fig.8: the common emitter configuration for an NPN transistor. referred to as the "small signal current gain" or hte· Again a small AC current is fed into the base of the transistor and AC current in the collector is then measured. The ratio between the two is the AC Beta. In practice, the AC Beta of a transistor is generally slightly less than the DC Beta. AC Beta also decreases as the signal frequency increases Checking it out Check your soldering and installation of the components carefully. Compare the board with the PC component diagram of Fig.2. Now connect the battery and switch S2 to NPN. When you depress the test button, the LED should briefly flash, as described above. This tests Ql to Q3 but does not test the 555 timer, ICl. To test ICl, connect a short jumper lead between pin 3 (the junction of the lkn and 68k0 resistors] and the junction of the 22k0 resistor and VRl. Connect another jumper lead between the base and collector lead connections for the TUT. Connecting the two jumper leads couples the 2.BkHz signal from !Cl to the input of Ql, via VRl and a O.lµF capacitor. Now, with the pushbutton pressed and VRl fully anticlockwise, the LED should light. Rotating VRl clockwise by about 30 degrees will then put out the LED. The Tester is now ready to be placed into the plastic case. Attach the front panel artwork to the lid of the case and drill out the holes where marked. If you are building from a kitset it is likely that the front panel will be supplied screen printed and drilled so these steps won't be necessary. Now you need to make up three flying leads; these will be the transistor test leads. Use three different colours of insulated hookup wire, preferably of the multistrand extra flexible type. We suggest red for the collector lead, black for the emitter lead and white for the base lead. Cut the three leads about 150mm long and solder an alligator clip to one end of each. Fit each alligator clip with an insulating boot. The leads should then be poked through the appropriate holes in the front panel and then soldered to their respective points on the PC board. Secure the battery in the base of the case to stop it from rattling around. You can use a piece of double sided foam backed tape for this purpose. Alternatively, for a more secure job, make up a battery clamp from scrap aluminium. Connect up the battery snap, attach the lid assembly to the case and you are in business. ~ FEBRUARY1989 25