That tester looks like the bare minimum. Many DVMs (digital volt meters) today have a transistor hfe test and diode test. I would prefer to use a DVM as I can get more reliable comparison tests. There are many low-cost DVMs today at less than 2x the cost of your example. The idea here is to get a relative comparison. The test does not need to be very accurate, just repeatable by using the same tester under the same conditions. Meaning, you will get the same test result repeatedly with the same device... ie, a result of 101 will be 101 again and again, not 102 or 99. An inexpensive DVM can do this. If you need a transistor with an hfe of 40 to 50, the DVM can help you get a reading within 5% or less, but the readings will be the same % off consistently. The same will be the results of resistance measurements. The repeatability is performed by using the same meter.
To get a closely matched pair of transistors, you need to compare their hfe AND their AC input resistance, little r . Little r is the resistance "seen" across the base to emitter junction in the forward biased direction. To use this for transistor matching, the technician does not need to know the actual little r value, just make a relative measurement for comparison. I have done matching this way many times. Yes it is tedious work. 1 example: I have done this to match substitute transistors in a 1000 watt, 70 volt power amp. (Same transistor part # as originals but were not supplied by the OEM, so 1/10th the cost or less.) It had 10 matched TO3 transistors in its output stage. If they were not matched, the power amp would "fry" the outputs. I always got the "hard-to-fathom" electronic repairs.
Little r can be tested with DC resistance measurements for comparison testing. Using the higher resistance ranges of a DVM (2K to 20M, you'll have to find any high R range that will give a reading) and the "diode" test range (both forward biased and reverse biased) and record your test results in a chart for the samples. Keep the samples physically arranged in the same order as your chart... Sample 1, Sample 2, Sample 3, etc ... The tests are: 1. hfe 2. "high R" (base to emitter = positive on base) (between 500 ohms and 20Meg) 3. "high R" (emitter to base = negative on base) ( should be >1Meg) 4. "diode R" (base to emitter = positive on base) (between 100 ohms and 10K) 5. "diode R" (emitter to base = negative on base) (should be > max reading)
The diode test on a DVM can be used on BJT transistors without harming them as they are a junction of 2 diodes where one junction controls the current in the other junction. How much the base to emitter junction controls the other junction is governed by the AC input resistance, little r.
You will need to buy 10 to 50 transistors of the same run or batch to get enough samples for your tests. The more samples you have, the closer you can get a match.
To avoid this tedious testing, you can use a substitute transistor part # that can be purchased in "matched pairs" or contact a transistor manufacturer for 2 matched pairs of the original part # ( always get yourself a spare pair). Is the original transistor a MPSA42 ?
Datasheet @ http://www.mouser.com/ds/2/149/MPSA42-889941.pdf" rel="nofollow - MPSA42-889941.pdf
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