Home Brew Transistor Diode Tester

Not just blinking lights and weird noises .

Henry L Bowman Rt. Box A-190 Parsons TN 38363

Although Tm not a radio amateur, I enjoy reading each new issue of 73 Amateur Radio Today. Unlike other electronic magazines, which offer construction projects with silly blinking lights and weird noises. 7_i provides useful projects that are well worth the time and effort to build. I buiit J. Frank Brumbaugh's Capacity Meter (March 1996) and have found il to be one of my most useful lest sets.

I have always needed a better method

■mi to identify and test transistors, but I didn't want to invest a lot of money in commercial test equipment, I decided to build my own tester The test set described here is designed for oui-of-cir-clijt testing, uses two ICs (only I if you use the dual 556 timer), and can be built in an afternoon. It will automatical]v m identify transistors for NPN or PNP t\pe

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Power Transistor Tester Homebrew

Speaker

Fig. The transistor/diode test set schematic. S2 positions: f I \ diode test: (2) transistor test!identify; f3) freeze iasi pulse.

73 Amateur Radio Today * May 1997 69

Fig. The transistor/diode test set schematic. S2 positions: f I \ diode test: (2) transistor test!identify; f3) freeze iasi pulse.

73 Amateur Radio Today * May 1997 69

as well as give you a relative indication of gain. In audio mode, unmarked transistor leads can he identified. Diodes also can he checked for shorts, opens and polarity.

Circuit description

Fig. 1 provides ihe schematic diagram of the transistor tester set. IC1 and IC2 are both 555 timers and their outputs arc capable of soureing or sinking up to 150 milIjamps of current. ICI is connected as an asiable multivibrator and the output clock rate is dependent upon the values of CI, R2 and the setting of selector switch S2. The output clock from ICI is connected to the trigger input of IC2 and also connected to one side of variable rheostat VRL IC2 is wired as an inverter and simply provides a complementary Output, of ICIt to the other side of VRI. When the selector switch is in position 1 or 2, the I>C voltage on the two 555 timer oulput pins is continually reversing in polarity and similar to a*flip-flop in operation. When the selector switch is moved to position the RC network Of ICI is disconnected so that the last output voltage pulse remains constant.

The wiper arm of VR I provides an adjustable voltage* in series with LED I and LED2, to an oscillator circuit, which depends upon the transistor under test for oscillation. If a known good NPN transistor is correctly connected to leads E, B and C and the selector switch is set for position 2, the NPN LED should light for approximately one second, remain off for one second and repeat the cycle, During the pulse period that the LED is lit, the transistor will oscillate as indicated by a tone from the speaker or an output indication on the meter, depending upon the setting of switch S3 If the NPN LED lights on one pulse and the PNP LED lights on the next pulse, then the transistor should be considered faulty. R6 provides current limiting to the LEDs, if a shorted transistor were inserted. VR2 provides tone pitch control and also has an effect on transistor gain.

Circuit construction

I built the circuit on perfboard using two 8-pin wine wrap IC sockets to mount ihe 555 timers. ira low wattage soldering iron is used, wiring can be soldered directly to the IC pins, eliminating the 70 73 Amateur Radio Today * May 1997

use of sockets. Ail resistors, capacitors, transformer T1 and the 9-volt battery were mounted on the perfboard; the remainder of the components were mounted on the faceplate. Parts locations arc not critical. For the test set faceplate, I cut out a five- by seven-inch panel from scrap aluminum and drilled out holes for the pots, switches, meter, LEDs and test leads. Four 1/8-inch holes were drilled in the midsection to allow audio from the speaker The aluminum panel just fit a small metal enclosure I had in my junk box. A plastic cabinet with aluminum faceplatc is available from Radio Shack™ and gives any homemade test set a professional look. I used color-coded clip-on test leads for the emitter, base and collector connections. These leads have alligator clips on both ends and are also availahle from Radio Shack, I cut ihe clips off at one end and soldered the cut ends to the proper components on the perfboard and faceplate, then installed a rubber grom-met in the faceplate hole for the test leads, and then ran all three leads through ihe same hole. 1 placed a small nylon cable tie around the test leads on Ihe inner side of the faceplate for strain relief; you may prefer to use a larger faceplate and mount several types of transistor sockets, wired in parallel to provide test connections,

I salvaged the meter from a discarded stereo amplifier, but any 150-200 micro-amp meter should work nicely. I have also had good fuck with 50 microvolt full-scale meters for audio level usc\ The 50-microvolt meter is intended to be paralleled with a high-current shunt bar. and telephone companies usually replace these meters when increasing their power distribution capacity. If you know someone who works with telephone switching equipment, they may donate one that was removed from their power boards after an upgrade. If you are unahie to locate a 3-position selector switch, you can use an SPDT switch to change the pulse rate, and add an SPST switch in series, between CI and pins fi and 2 of ICI, to open the RC network and stop the pulse.

The LED indicators LED2 and LED3 should be capable of 50 milliamps of forward current in case a shorted transistor is connected for testing. If you intend to use LEDs found in your junk box, and are unsure of their current capacity, be sure to use red, instead of yellow or green, because they are usually {but not always) rated for higher current During normal operation, with a transistor connected for test, the total test set current drain is approximately 30 rnilliamps.

Testing

After assembly is completed and the test set is connected to a 9-volt battery, place the selector switch in position 2 and turn power switch SI on, LED! should light, indicating power has been turned on. losing an analog voltmeter, place the voltmeter leads across the collector and emitter test leads. With the gain potentiometer (VR!) turned fully clockwise (all resistance out), you should see the voltmeter indicate approximately 6 volts for a second, then a polarity reversal during ihe next second, then the cycle repealed over again. If no voltage is observed, move the voltmeter black lead to ground (negative) and connect the red lead to pin 3 of ICI. If a pulsing voltage is observed, move to pin 3 of IC2 and also check for voltage, Recheck ail wiring and replace ICs if necessary. After the problem is located and corrected, return to the collector and emitter test points and recheck for alternating voltage. Change the selector switch S2 to position 3. The voltage should remain constant from the last pulse, and you may have to reverse your meter leads to verify. Change the selector switch S2 to position 1 and verify that collector-emitter voltage drops to approximately 2-3 volts. This is because of the rapid reversals (100 eps) that are occurring; the meter is only providing an average reading.

Operation

Connect a known good NPN- or PNP-type transistor to the three test leads, turn switch SI to position 2 and turn on ihe power. The LED indicating the correct transistor type should lighl for approximately I-second intervals and the second LED should remain unlit. If switch S3 is in the "audio" position, an audible tone should be heard from the speaker each time the LED lights. To "freeze" the pulse, switch S2 to position 3 just as the LED lights, Adjusting VR2 will change the pitch of the tone and also have an effect on gain. Throw switch S3

Parts List

101, IC2

NE555 IC timer or NTE955M

(2) 8-pin IC socket*

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