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Measure & Beta This simple Beta Tester will test & measure the gain of most bipolar transistors. You simply connect the transistor, press the test button, rotate the knob until the LED lights, and read the gain from the calibrated dial scale. By JOHN CLARKE One of the more popular projects described in SILICON CHIP was the Beta Tester published in the February 1989 issue. It was a simple low-cost unit that worked very well but it did have one drawback - it could not reliably check transistors with a low beta (typically less than 50). Recently, in response to reader queries, we decided to take another look at the circuit to see if this problem coul d be solved. In fact, the solution was quite simple - all we had to do was alter the biasing arrangement around the test transistor and change a few component values. The result is the Beta Tester Mk.2. Th is version can test transistors with a beta as low as 5 which is a big improvement on the original design , especially when it comes to testing low-gain power transistors. 32 SILICON CHIP To make the unit easy to build and to avoid possible confusion, we've re-designed the PC board to accommodate the circuit changes for the Mk.2 version. This board fits into the same case as before so that, outwardly, the appearance of the instrument is unchanged. Of course, if you already have the original version, there's no reason why you can't make the necessary changes to the circuit. All you have to do is remove a few existing components and patch the new values into position on the PC board. Why check transistor beta? Nobody likes putting a dud transistor into circuit - it can cause hours of frustration. A beta tester can quickly indicate whether a transistor is good or bad (although it won't identify leaky transistors), and can thus elimi- nate a potential source of trouble. It's also useful for troubleshooting, as you can quickly remove and check any suspect transistors in a faulty circuit. Apart from providing GO/NO-GO indication, a beta tester is also useful if you want to select a transistor for a critical circuit. Some circuits will operate better with transistors that have a minimum gain figure or may require matched gain transistors for optimum performance (eg, audio amplifiers). With the Beta Tester, selecting transistors for gain is a snack. Finally, you can use the Beta Tester to identify the leads of unknown transistors and for checking whether it is a PNP or NPN device. The way in which you go about this is set out in an accompanying panel. So there are quite a few good reasons for building this handy gadget. Once you do, you'll wonder how you ever managed without it. Long battery life The Beta Tester is battery powered so that it can be used anywhere and has just three simple controls. First, there is a momentary contact pushbutton switch which applies power to the circuit while you do the test. This means that the circuit cannot flatten the battery because you've forgotten to turn it off and so the battery should last a long time. In -addition, there is a slide switch to select between NPN and PNP transistors and a knob with a scale graduated from 5-500 to provide a direct readout of beta. The circuitry is all housed in a small plastic case measuring 130 x 68 x 43mm. By the way, the Beta Tester uses an AC signal to check the test transistor's gain; it is not just a simple DC match transistors with the TesterMk.2 gain test. The frequency of operation is about 3.3kHz. Circuit operation Refer now to Fig.1 which shows the circuit details. There's nothing fancy here; just one 555 timer IC, three low-cost transistors and a few minor parts. Fig.1 can be broken down into four sections: an oscillator (IC1), an amplifier (based on the test transistor), a detector stage (Ql, D3 & D4), and a comparator (Ql, D4 & QZ). The oscillator stage is based on a 555 timer (IC1) which is wired in a somewhat unconventional manner. Normally, the free running configuration uses a timing capacitor which is charged via a resistor from the positive supply rail. In this circuit though, the timing capacitor (.OOZZµF) is alternately charged and discharged by the pin 3 output via a 68kQ resistor. The circuit works like this: at switch-on, pin 3 of IC1 goes high and charges the .OOZZµF capacitor via the 68kQ resistor. Then, when the capacitor voltage reaches 2/3Vcc (ie, 2/3 the supply rail voltage), pin 3 switches low and the capacitor discharges via the 68kQ resistor until it reaches 1/3Vcc. At this point, pin 3 switches high again and so the cycle is i:epeated indefinitely while ever power is applied. The resulting output from IC1 is taken from pin 3 and is a 3.3kHz square wave with an amplitude of close to 9V peak to peak (p-p). This signal is then clipped by diodes D1 & DZ to give a waveform with an amplitude of 1.4V p-p and AC-coupled via a O. lµF capacitor to the following amplifier stage. This stage uses the transistor under test (TUT) in a corn- ... 0.22 16VW+ U• man eniitter amplifier configuration. l To explain, a common emitter amplifier is one in which 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 : either the positive or negative rail of the circuit. The input signal is then applied between the base terminal and ground, while the output signal appears between the collector and ground. NPN/PNP transistors In this circuit, if the test transistor · is an NPN type, its emitter is connected to OV and the collector to the +9V rail via a 5.lkQ resistor. DC bias is prov~ded by the 75kQ and 27kQ . resistors, which are connected to the +9V and OV rails respectively via SZa & SZb. These set the bias to about ' T "l NPN 5.1k .,. 1'/o 9V : C B IC1 555 .., D2 TUT ..I.. D3 1N4148 ~r .,. .,. .,. B NPN .0022+ .,. EOc VIEf/fot.l'oM .,. ~- 1 TRANSISTOR BETA TESTER Fig.t: the circuit uses 3.3kHz oscillator ICt to pulse the base of the test transistor (TUT). This signal is then amplified by the TUT & fed to class-B detector stage Qt, D3 & Q2. When the output from the TUT exceeds 1.8V, Qt, Q2 & Q3 conduct & the LED lights. MARCH 1991 33 All the parts, including the two switches, are mounted on a small PC board. Make sure that the parts are installed so that they do not protrude above the mounting surface of the slider switch. Three PC stakes are used to anchor the pot lugs. 2.4V in the NPN mode to ensure that the transistor operates over its linear region. NPN/PNP selection If the test transistor is a PNP type, slide switch SZ swaps the base and emitter connections, so that the emitter now goes to the +9V rail and the collector goes to ground via the 5.1H2 resistor. SZ also swaps the bias resistors around, so that the base of th e TUT is now biased to 6.6V. This switching arrangement ensures correct bias for both NPN and PNP test transistors. In either case, the 3.3kHz oscillator signal is fed to the base of the TUT via a Z0kQ resistor and series ZMQ potentiometer (VR1). VR1 allows the base current into the TUT to be varied from a minimum of 700nA to a maximum of 70µA. As we shall see, these two figures correspond to beta ISC041 3 1 readings of 500 and 5 respectively. In greater detail, when we plug in a test transistor, the resulting collector current will simply be the base current multiplied by the beta. So, if the transistor has a beta of 500 and the base current is 700nA, the corresponding collector current will be 350µA. Similarly, if the a transistor has a beta of 5 and the base current is 70µA, the collector current will again be 350µA. What if the transistor has a beta of 100? We can again obtain a collector current of 350µA by setting VR1 to give a bas e current of 3.5µA. It follows that the voltage developed across the 5.1kQ collector load resistor will be about 1.8V when 350µA flows through it. Thus, if we have a detector that lights a LED at this 1.8V threshold , the beta of any test transistor can be read off a calibrated scale fitted to VR1 's control knob (provided of course that the beta lies bewteen 5 & 500). Q1, D3 and D4 make up the signal detector stage and this is driven by the TUT via a 0.1µF capacitor. Q1 acts as a class-B detector. It has no DC bias on its base and conducts for positive swings only of the 3.3kHz signal. The negative swings are clipped by D3, to protect the base of Q1. Thus, Q1 acts as an emitter follower for positive swings of the 3.3kHz signal but does not conduct at all for the n egative swings. This is .................. c,A \ 'wi_ __ 9V BATTERY Fig.2 (left): mount the LED so that the top of its lens is about 12mm above the board surface so that it will later protrude through the front panel by the correct amount. The capacitors should all be installed so that they lie parallel with the PC board (see photo). Fig.3 (right) shows the full-size PC pattern. 34 SILICON CHIP RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 No. 2 Value 4-Band Code (5%) 5-Band. Code 75kQ 68kQ 27kQ 20kQ 10kQ 5.1kQ 4.7kQ 2.2kQ 1kQ not applicable blue grey orange gold not applicable not applicable brown black orange gold not applicable yellow violet red gold red red red gold brown black red gold violet green black red brown blue grey black red brown red violet black red brown red black black red brown brown black black red brown green brown black brown brown yellow violet black brown brown red red black brown brown brown black black brown brown The pot lugs are slipped over the PC stakes & soldered as shown in this close-up view. w hy it is called a class-B detector because it only conducts on one half of the signal waveform. The detected signal appears at Ql 's emitter and charges a 0. lµF capacitor via diode D4. This then turns on transistor Q2 which, in turn, drives PNP transistor Q3 and the LED. Ql , D4 and Q2 together perform the comparator function. If the 3.3kHz signal fed to the base of Ql is not 1.8V p-p or more, then Ql, D4 and Q2 will not conduct, Q3 will not be turned on and the LED will not light. So th ese components also perform an important signal level monitoring function - they won't operate if the signal isn 't big enough (ie, until VR1 is adjusted to give 350µA collector curren t for the test transistor) . Self-test function An interesting feature of the circuit is the self-test function which tells you that it is working and that the battery is OK. To test the circuit, you simply select the NPN setting and push the test button. If the circuit is working correctly, the LED will flash momentarily regardless of whether or not a test transistor is connected. This happens because the 0. lµF capacitor connected to Ql 's base charges from the +9V supply rail via the 5. lkQ resistor. This causes a short pulse of more than 6V to occur at Ql's emitter and so D4, Q2 & Q3 briefly conduct and the LED flas h es. This self-test function does not work in the PNP mode because the 5. lkQ resistor is now connected to the 0V line. On the other hand, if you are testing an NPN transistor and the LED will not flash or light at all, then the transistor probably has a short between base and collector. Power for the circuit comes from a 9V battery, which can be either an alkalin e or carbon-zinc type. Either way, the battery sh ould last a year or more with normal use. Construction Construction of the Beta Tester is relatively straightfo rward. All the circu itry, including th e switches and p otentiometer, is mounted on a PC board coded SC0410391 1 and measuring 71 x 61 m m. This in turn is h oused in a compact plastic utility case measuring 130 x 68 x 43mm. Before turning on the soldering iron, carefully inspect the PC board to ensure that there are no op en cir- What is Transistor 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 common emitter amplifier arrangement as shown in Fig.4. A fixed current is fed into the base and the resulting current into the collector terminal then measured. The ratio of the collector current to the base current is then the DC forward gain of the transistor. This 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 10µA and they measure the collector current directly. Our Beta Tester uses an AC signal of 3.3kHz to measure AC beta, which is commonly referred to as the "small signal current gain" or METER T I I ...L.. I I ...L.. Fig.4: the common emitter configuration for an NPN transistor. hfe. Again, a small AC current is fed into the base of the transistor and the AC current in the collector is then measured. The ratio between the two is the AC beta (hfe). In practice, the AC beta of a transistor is generally slightly less than the DC beta. The AC beta also decreases as the signal frequency increases. MARCH 1991 35 How to Test Unknown Transistors Most of us have unmarked transistors lying in the junkbox which could be used in many non-critical circuit applications. With a little practice, the Beta Tester can be quickly used to determine whether a transistor is an NPN or PNP device, and to measure its gain. The first step is to identify the transistor leads. To begin, set the Beta knob to maximum and clip the test leads to the transistor terminals. Test the transistor on both the NPN and PNP settings and systematically swap the test leads until the beta can be measured. There are six different ways to connect the tester to the transistor and two different transistor types (NPN/PNP). This means that, at worst, you will require 12 tests to determine the pinout for a particular transistor (or to discover that it's a dud). To make life easier, here are some basic rules to follow. If you turn a small signal transistor upside down as shown in Fig.5, the leads will ei- cuits or shorts between tracks. You can check thi s by comparing your PC board with the artwork included in this article. Make sure that the holes are correctly drilled out, too. The hole for pushbutton switch Sl should be about 8mm diameter, while the mounting hol es for slide switch S2 should be 2.8mm (7 /64 inches) diameter. r C + B ... ther be in a straight line or triangular arrangement. Further, as shown in Fig.5, the leads will usually run C(ollector), B(ase) and E(mitter) from left to right, but other configurations are possible. Fig. 7: for TO-3 style transistors, the case is the collector, while the emitter & base leads are as shown. VIEWED FROM BELOW 3 cases, the case is the Collector terminal and the Base and Emitter leads are as shown in Fig.7. Reverse Gain Fig.5: possible lead connections for a small signal transistor. For small power transistors in plastic encapsulations such as TO-220 and TO-202, the leads usually run Base, Collector and Emitter as shown in Fig.6. For larger power transistors such as those in metal TOFig.6: the most common lead configuration for TO-220 & TO-202 transistors. BCE Fig.2 shows the parts layout on the PC board. The first step is to install three PC stakes at the mounting points for VRl . This done, install all the low profile components such as the wire links , diodes , resistors and the IC. Be careful with the diodes and IC since they must be oriented exactly as shown on the overlay diagram. The remaining components can Once the pin configuration is discovered , it is simply a matter of turning down 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 a believable reading of Beta. This is because bipolar transistors have a reverse active 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 an NPN or PNP type. now be installed on the board. Make sure that the tops of the capacitors and transistors are less than 8mm above the surface of the PC board, as the entire assembly is mounted on the lid of the case via Sl, S2 & VRl. This will involve bending all the capacitors over so that they lie flat on the PC board or across adjacent parts (see photo) . Also, be sure to use the correct transistor type at each location (see Fig.1) . The momentary contact switch (Sl) is installed by first pushing the lug end of the switch body through the mounting hole and then bending the lugs over sideways. These lugs are then soldered to 400 ·the large pads on either side. The base of the switch should 7 E + + D PNP 100 NPN TEST + L 36 5 _J SILICON CHIP Fig.8 (left) : this full-size artwork can be used as a drilling template for the front panel if the panel is not supplied prepunched. PARTS LIST The PC board is secured to the lid of the case using the mounting screws & nuts for the switches & pot. The battery clamp can be made from scrap aluminium. sit flush with the underside of of the PC board. Mount the 5mm red LED so that the top of its lens is about 12mm above the board surface. This will allow the LED to protrude through the front panel by about 1mm when the board is later installed in the case. The PC board assembly can now be completed by installing the battery snap connector and VRl. Cut VRl's shaft to a suitable length before soldering its lugs to the PC stakes (note: the lugs should be soldered about half way down the PC stakes). Final assembly The board assembly can now be installed in the case. If you are building the unit from a kit, it is likely that the front panel will be supplied prepunched with silkscreen lettering. If not, attach the front panel artwork to the lid of the case and drill out the holes for the test leads, switches, LED and potentiometer. The slider switch requires a rectangular hole and this can be made by first drilling a series of small holes and then filing these to shape. You will also have to drill two holes for the switch mounting screws. The test leads can be made from insulated hookup wire, preferably the multistrand extra-flexible type. Use a different colour for each lead. We suggest red for the collector lead, black for the emitter lead and white for the base lead. Cut each lead about 150mm long and fit an alligator clip with an insu- lated boot to one end. The three leads can then be fed through the holes on the front panel and soldered to their respective points on the PC board. After that, the board assembly can be mounted on the lid of the case and secured using the mounting screws and nuts for S1 , S2 and VRl. The control knob should be installed by first rotating VRl fully clockwise and then tightening the grub screw for an indicated beta reading of 500. Finally, the battery can be secured to the bottom of the case using a clamp fashioned from scrap aluminium. This clamp can be attached to the side of the case using a self-tapping screw. Testing To test the unit, connect the battery, set the slider switch to NPN, and depress the test switch. The LED should briefly flash. If it does, then the circuit is operating from Q1 -Q3. The unit can now be fully tested by checking a known good transistor. To do this , connect the test leads to the transistor, rotate the control knob fully clockwise, select NPN or PNP as appropriate, and press the test switch. Now rotate the control knob until the LED just comes on. The pointer on the knob will now indicate the test transistor's beta on the scale. A word of warning here; when testing high gain transistors, the LED may extinguish again if VRl is now moved to a lower beta setting. This occurs because the test transistor saturates and thus gives false readings. Always 1 plastic case, 130 x 68 x 43mm 1 PC board, code SC04103911, 71 x 61mm 1 front panel label, 127 x 66mm 1 9V battery, Eveready 216 or equivalent 1 snap connector to suit battery 1 battery clamp 1 pointer knob, 30 to 35mm dia. 3 alligator clips 3 PC stakes 1 DPDT slider switch (DSE S2040, Jaycar SS-0821 , Altronics S-2035) 1 momentary contact pushbutton switch (DSE S-1102, Jaycar SP-0710, Altronics S-1102) 3 150mm lengths of hookup wire (different colours) 1 2MQ linear potentiometer Semiconductors 1 555 timer IC {IC1) 2 BC548 transistors (01 ,02) 1 BC558 transistor (03) 4 1N4148 diodes (D1 ,D2,D3) 1 5mm red LED (LED 1) Capacitors 1 0.22µF 16VW PC electrolytic 3 0.1µF metallised polyester 1 .0022µF metallised polyester Resistors (0.25W, 5%) 1 75kQ 1% 1 5.1kQ 1% 1 68kQ 2 4.7kQ 1 27kQ 1% 1 2.2kQ 1 20kQ 1% 1 1kQ 1 10kQ Miscellaneous Tinned copper wire (for links). use the maximum setting where the LED just glows as the correct beta reading. Another problem is that some transistors (eg, BC548s, BC549s & BC559s) have gains of more than 500, which is outside the range of the Beta Tester. However, the unit can still verify that such transistors are working OK (the LED will simply remain on at maximum beta setting). Finally, note that this unit cannot be used to test Darlington transistors. That's because a Darlington transistor will saturate even at minimum base current setting (ie, rotary control at maximum). SC MARCH 1991 37