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Low cost transi & Mosfet tester base cur­rent from the DMM test circuit may be less than it should be, another source of inaccuracy. Another drawback involves power transistors. These typical­ ly require much more base current than small signal transistors and so beta tests of a power transistor using a DMM can often give misleading results. On the other hand, many of the top brand digital multimet­ers do not have a transistor test facility at all and this is where the SILICON CHIP transistor tester comes into its own. Plug this adaptor into your multimeter and measure the beta of power transistors, small signal types and small signal Darlingtons. In this case, the reading on the DMM indicates that the transistor has a beta of 81. Transistor gain This handy tester is designed to plug into a digital multimeter to provide an accurate measurement of transistor beta, to values up to 50,000 & more. You can use it to test small signal, power & Darlington transistors &, as a bonus, it will also check Mosfets. If you need to use transistors from your junk box for your projects, it is a good idea to test them before soldering them into circuit. Actually, this is a good idea even if you have just purchased the transistors because it can stop you from soldering the wrong type into circuit. But now that many digital multimet­ers incorporate a simple transistor tester, why would you want to build this adaptor? Well, there are several drawbacks to 68  Silicon Chip the typical “transis­tor test” facility in most digital multimeters. First, most will not measure transistor gains in excess of 1000. Most ordinary transistors have a beta of less than 1000 but many Darlington transistors have a beta far in excess of 1000 – up to 50,000 or more, in some cases. Also the fact that Darlington transistors have a base-emitter voltage drop of 1.2V or more and they incor­ porate internal base-emitter resistors means that the You can use the tester to match transistors for gain or to decide whether an unknown device is a Darlington (very high gain) or a standard transistor. You can also find out the transistor pin-outs by trying all connection possibilities until a valid gain measurement is found. Similarly, you can determine whether the device is NPN or PNP by finding the polarity which gives a gain reading. Mosfets are used extensively in SILICON CHIP circuits these days and testing them can be difficult. With this tester, you can obtain valuable information about the condition of a Mosfet. The test is not a gm measurement but it will give a good indication of Mosfet gain. The tester is housed in a small plastic case. Three flying leads with alligator clips are clipped to the device to be test­ed. On the underside of the case are two banana plugs which insert directly into the “VΩ” and “common” inputs. Main Features • Measure s beta fr om 1 to • Plugs dir over 50,0 ectly into 00 a digital • Measure multimete s NPN a r for beta nd PNP • Tests N-t readings transisto ype and rs P -t y • Two test pe Mosfe ts ba • High beta se currents: 10µA an d1 a • Battery o ccuracy and resolutio mA n at mea perated sured cu • Suitable rrent for high im pedance • Short in (>1 istor r for DMMs dication By JOHN CLARKE 0MΩ) mu ltimeters C +9V 1 There are two toggle switches; one is the NPN (N-type)/ PNP (P-type) switch to select the device polarity and the other is the 3-position range switch. The digital multimeter is turned on and a DC range selected, normally 2V to start. Then you press the button and the meter gives a reading. To convert the reading to beta, just take the reading in millivolts. For example, if you are on the 2V range and the reading is 0.695V or 695mV, the transistor beta is 695. Alternatively, if the 200mV DC range has been selected and the reading is 115mV, then the beta is 115. Power is consumed only while the Test button is pressed. If you want to hold the reading on your multimeter, press the “hold” button if it has one. That is how we stored the reading for the setup shown in the photograph accompanying this article. 1mA E1 TRANSISTOR UNDER TEST B R1 Q1 B2 Q2 D1 R2 E E Fig.1: this is the basic beta test setup with a fixed current supplied to the base of the transistor. If 100mV appears across the 1Ω resistor, the collector current is 100mA & the beta is 100. 1k NPN DARLINGTON Fig.2: typical Darlington power transistors have internal baseemitter resistors which means that a minimum base current of about 1mA is required to turn them on. Most beta testers in DMMs cannot supply this much base current. SHORT LED1  R2 C1 9V C2 R1 CURRENT SOURCE Multiplier switch The 3-position multiplier toggle switch needs some explana­tion. The position marked “X1 POWER” is used for testing power transistors and power Darlingtons. The other two settings are used for small signal transistors. The centre position marked “X1” gives a result as described; ie, the reading in mV is the beta. When on the “X100” setting, the readings are multiplied by 100 to give the actual result. This position is intended for small signal Darlington transistors which can typically have a beta of 30,000 or more. Mosfets are measured in a similar B C SWITCH B B TO MULTIMETER C TRANSISTOR UNDER E TEST SWITCH A PULSE GENERATOR Fig.3: this circuit shows the principle of operation of the Beta Tester. The current source is shunted to ground by switch A. When switch A opens, the current source drives the base of the tran­sistor & a voltage proportional to the collector current is developed across R1. Switch B & capacitor C2 form a “sample and hold” circuit which stores the voltage developed across R1 so that it can be read as a DC voltage by the multimeter. May 1995  69 SHORT LED1 1k TEST S1 A  K 120  1W 470 16VW +9V NPN (N-TYPE) 470 16VW S3a 4x1N4148 9V D3 D1 D4 D2 REF1 LM334Z 47  S2: 1 : x1 POWER 2 : x1 3 : x100 SMALL SIGNAL V+ 330k 7 1k 6 4 8 IC1 7555 2 3 IC2a 4053 S2b 1 2 V+ V- 2 3 1 0.1 100  IC2b 6.8k 16 by B 10 B 14 a ay 13 TO METER bx 2 100  15 b S2a 1 11 A ax 3 1 R 68  10 16VW PNP (P-TYPE) +V 10 16VW C DEVICE UNDER E TEST +9V S3b PNP NPN 6,7,8 0.1 A K R VV+ VIEWED FROM BELOW TRANSISTOR BETA AND MOSFET TESTER Fig.4: the circuit of the Beta Tester uses a 7555 astable mul­tivibrator (IC1) & a 4053 analog switch (IC2) to shunt the base current to the transistor. manner to power transis­tors. A good Mosfet will give a very high gain reading. If a device being tested has a short between collector and emitter, the “Short” LED will light. The LED will also light when the wrong polarity is selected for Mosfet and Darlington transis­tors. Test method Fig.1 shows the method of gain testing used in the circuit. The transistor under test is connected in a common emitter con­figuration with a 1Ω resistor for the collector load and a 1mA current source for the base drive. A transistor with a gain of 10 will produce a 10mV drop across the resistor. However, there are a few problems with this circuit. First­ly, for high gain transistors, a high current will be drawn from the supply and secondly, some transistors will not handle the The PC board is mounted on the lid of the case & secured to it using the switch nuts. Adjust the LED leads so that it just protrudes through the lid after it is placed in position. 70  Silicon Chip Pulse testing Because we cannot reduce the base current we need to modify the circuit in some other way to curb the excess current which will otherwise be drawn by high-gain transistors. Fig.3 shows how this is done by pulsing the base current with a short duty cycle. By having a long period between each base current pulse to the transistor, the average collector current can be reduced to only a few milliamps. Capacitor C1 lowers the supply impedance so that it can more easily deliver the required high current pulses. Switch A is normally held closed by the pulse generator and thereby shunts the current source to ground, preventing the transistor from turning on. When switch A opens, the current source drives the base of the transistor and a voltage propor­tional to the collector current is developed across R1. Switch B and capacitor C2 form a “sample and hold” circuit which stores the voltage developed D1 D3 S2 C IC2 4053 D4 D2 1 1 10uF 0.1 1k 470uF 120  1W 470uF 100  1 6.8k IC1 7555 10uF TO B DEVICE UNDER E TEST LED1 A K 0.1 TO 9V BATTERY 68  REF1 100  S1 NC NO C 47  330k 1k collector current without self-destructing. Simply reducing the base current and increasing the collec­tor resistor will drop the current but will not solve the prob­lem. This is because we need the 1mA base current to drive power transistors. Fig.2 shows the internal arrangement of power Dar­lington transistors. This entails two transistors with the emit­ter of the first transistor connected to the base of the second transistor. In addition, they also include base-emitter resis­tors. Resistor R1 can be as low as 1kΩ while R2 is generally smaller again. Since we must develop about 0.7V across the base and E1 of Q1 before transistor Q2 will switch on, the base cur­rent into Q1 must be at least 700µA. TO MULTIMETER S3 Fig.5: follow this parts layout diagram when installing the parts on the PC board. Note particularly the orienta­tion of the contacts on switch S1 – see text. across R1 so that it can be read as a DC voltage by the multimeter. Hence, when switch A opens, switch B closes and “samples” the resultant collector voltage. Resistor R2 is included for short circuit protection. If a transistor is connected incorrectly or if the collector and emitter leads are shorted together, excess current will otherwise flow. LED1 indicates whenever a short is present and also lights briefly each time the “TEST” button is pressed. The type of measurement used in B E C BC5xx BC3xx PLASTIC SIDE BCE "POWER" E C B E BC6xx B C "POWER" GD S MOSFET "POWER" E C (CASE) B "POWER" Fig.6: typical pin-outs for various case styles of transistor. the beta tester gives us the DC gain or hFE for the transistor. Mosfet devices are tested in a similar manner to transistors. The current source will charge up the gate to switch on the Mosfet and a voltage propor­tional to the Drain current will appear across resistor R1. Circuit operation The complete circuit for the Beta Tester is shown in Fig.4. IC1 is a 7555 CMOS timer connected as an astable multivibra­tor set to run at about 43Hz by the resistors and capacitor connected to pins 6 & 7. Its pulse train output at pin 3 is high for 23ms and low for 70µs. Pin 3 of IC1 controls IC2, a 4053 triple 2-channel demulti­ plexer. In our circuit we are using the 4053 as a 2-pole switch, with IC2a closed when IC2b is open, and vice versa. IC2a is used to alternately shunt the base current to the transistor under test, while IC2b is the sample-and-hold switch. A crucial part of the circuit is the 2-pole toggle switch, S3. S3a & and RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 2 1 2 1 1 1 Value 330kΩ 6.8kΩ 1kΩ 120Ω 100Ω 68Ω 47Ω 1Ω 4-Band Code (1%) orange orange yellow brown blue grey red brown brown black red brown brown red brown brown brown black brown brown blue grey black brown yellow violet black brown brown black gold gold 5-Band Code (1%) orange orange black orange brown blue grey black brown brown brown black black brown brown brown red black black brown brown black black black brown blue grey black gold brown yellow violet black gold brown brown black black silver brown May 1995  71 The banana plugs are mounted close to the end of the case & with a spacing of 19.5mm. Alternatively, set them at the spacing to match your multimeter. Fig.7 at right shows the full-size etching pattern for the PC board. S3b reverse the supply polarity to the transistor under test so that NPN and PNP devices can be tested. Since REF1, an LM334Z constant current source which supplies the base current, is a polarised device, a bridge rectifier consist­ ing of diodes D1-D4 ensures that it is correctly polarised, regardless of whether NPN or PNP devices are being tested. REF1 has its constant current programmed by the resistance connected between its R and V- pins. This is varied using 2-pole 3-position toggle switch S2. This is actually a “2-posi­ tion, centre-off” switch which is connected to vary both the base current and the collector load resistor for the device under test. Position 1 of S2a connects a 68Ω resistor in parallel with a 6.8kΩ resistor to provide a 1mA base current to the transistor under test. In position 2, the “centre-off” position, the 68kΩ resistor by itself sets the base current to 10µA. Positions 1 and 3 of S2b switch a 1Ω resistor in parallel with 100Ω, while the “centre off” position 2 leaves the 100Ω resistor by itself. Hence, for power transistors and small signal Darling­tons, the collector load resistor is 1Ω (shunted by 100Ω) while for small signal transistors the collector load is 100Ω. Power for the circuit is derived from a 9V battery which is applied via pushbutton S1 to S3 via a 120Ω resistor. This supply is decoupled with two parallel 470µF capacitors which provide the peak currents required. When S1 is open, the supply rail is discharged using the normally closed contact to prevent any voltage remaining on the circuit when the switch is released. When the switch is pressed, the 470µF capacitors are initially discharged and so LED1 lights momentarily. This provides a good indication of battery condition at the beginning of each test. Construction The Beta Tester is housed in a plastic utility case measuring 130 x 67 x 43mm. All the circuitry mounts on a PC board coded 04306951 and measuring 92 x 61mm. This is secured to the lid by the three switches. You can begin the construction by inserting PC stakes at the external wiring points. This done, install the resistors, links and diodes, followed by the capacitors and lastly, the integrated circuits. Make sure that the semiconductors and electrolytic capacitors are correctly polarised. The PC board is attached to the lid of the case and held in place by the nuts of the switches. Note that the LED lead length needs to be adjusted so that the lens of the LED just protrudes from the front panel. 72  Silicon Chip the E and C terminals and with S2 in the x1 power position check that LED1 lights. Now affix the Dynamark label to the front panel and drill the holes for switches S1-S3 and LED1. The four corner holes in SHORT the lid should also be drilled + out. One end of the case re­ quires separate holes for the three test leads which are fitted P-TYPE with crocodile clips. X1 POWER PNP Drill 3mm holes for the banana plugs so that they are + X1 + mounted as close to the end of the case as possible, 19.5mm X100 N-TYPE apart. The battery can be held NPN in place with a metal clamp or with Velcro®. You will need to remove the TRANSISTOR BETA & internal ribs of the case so there MOSFET TESTER is sufficient clearance for the PC board. You can do this job easily with a sharp chisel. + Now connect up wires on the board for the base, emitter and TEST collector test leads and for the banana plugs. Attach the PC board to the front panel by firstly placing a single nut on each switch bush about 5mm down from the top and then securing the panel with a second nut Fig.8: this full-size front-panel artwork for on each switch bush. The LED the Transistor Beta & Mosfet Tester can be should be adjusted in height so used as a drilling template for the case lid. that it sits correctly in the front panel hole. Next, the switches can be installed. Attach the meter output wires to Note that pushbutton switch S1 must the banana plugs and pass the B, C be oriented in a particular way. You and E wires through the holes in the will find that its three contacts are case. Terminate these wires to the labelled C (common), NO (normally alligator or easyhook clips. Fit the open; ie, when not pressed) and NC lid assembly into the case, attach (normally closed). The contact posi- the screws and the tester is ready tions should match the labelling on for service. the copper pattern side of the board (ie, NC contact toward the edge of Measurements the board). Use the centre-off switch Fig.6 shows typical pin-outs for for S2. various case styles of transistor. Use Finally, LED1 is inserted so that it this to help with identifying the correct sits at the same height as the switch pin arrangement. When testing small bushes. Do not cut its leads to length signal transistors, use the x1 and x100 yet, so that it can be set to the correct small signal setting for S2. height in the front panel later on. There will be some differences between readings on each range for Initial tests a given device under test. This is beAttach the battery clip leads to the cause transistor gain varies with base PC board and apply power. Connect current. a multimeter between the negative Mosfet “gain” values should be in battery lead and pin 8 of IC1 and check the region of 1000 or more and should that there is about +8V present when be tested on the x1 power position. S1 is pressed. Similarly, check for a The gate will only be pulled to about similar voltage on pin 16 of IC2. Short +6.5V due to the voltage drop across CBE DGS βΕΤΑ PARTS LIST 1 PC board, code 04306951, 92 x 61mm 1 plastic case, 130 x 67 x 43mm 1 front panel label, 64 x 124mm 1 9V 216 battery & battery clip 1 SPDT momentary pushbutton PC board mounting switch (S1) 1 DPDT centre-off PC mount toggle switch (S2) 1 DPDT PC-mount toggle switch (S3) 7 PC stakes 2 banana plugs 2 3mm x 10mm screws & nuts 3 crocodile clips 1 50mm length of green hookup wire 1 50mm length of red hookup wire 1 100mm length of black hookup wire 1 100mm length of blue hookup wire 1 100mm length of yellow hookup wire 1 100mm length of 0.8mm diameter enamelled copper wire Semiconductors 1 7555, TLC555CN or LMC555CN timer (IC1) 1 4053 triple 2-channel demultiplexer (IC2) 1 LM334Z current source (REF1) 4 1N4148, 1N914 signal diodes (D1-D4) 1 3mm red LED (LED1) Capacitors 2 470µF 16VW PC electrolytic 2 10µF 16VW PC electrolytic 2 0.1µF MKT polyester Resistors (0.25W 1%) 1 330kΩ 2 100Ω 1 6.8kΩ 1 68Ω 2 1kΩ 1 47Ω 1 120Ω 1W 1 1Ω REF1 and the bridge rectifier which is usually not sufficient to turn a Mosfet fully on. Consequently, the Mosfet will be operating in the linear region. Note that the polarity indication on the multimeter will differ, depending on the setting of the NPN/PNP switch SC (S3). May 1995  73