Saturday, February 24, 2018

Testing an unknown transistor

I mounted a transistor in the black box. The object is to test it and see what we can tell from a simple test.I will need an ohmmeter,voltmeter, resistor from 500 to 1500 ohms and a battery ( or power supply).
The ohmmeter MUST be 1ma or less. My meter is 2KOhm per volt or 500ua so I just use the highest range. The lower range will shunt the meter and use more current. I will also need to know the meter polarity. When using a multi meter for voltage the red lead is positive but most ohmmeters will have the red negative. (it's the difference between sourcing and sinking. The volt meter reads an external source while the ohmmeter is powered by a built in battery.)

I hooked the red to black and black to red as you can see the polity is correct so my red lead is negative. I will show the test and a table of readings.

This is my black box component. The tilted lead is #1.

If you examine the table you will see when term 2 is negative I get the same readings to terminals 1 and 3 so the device is PNP and term 2 is the base. To make the test I connect the meter between term 1 and term 2 with the negative on term 2. I will put a resistor between term 2 and the positive power supply with the negative on term 3. The simpler statement  is forward bias 1 and 2 with the ohmmeter and reverse bias term 2 and 3 with a power source feeding through a resistor. In my test I am using a 1.2K resistor and 3 volts power supply. So now drum roll and display the test results......................
 As you can see the meter goes below ZERO Ohms. Mr Gernsback says use a 500 ohm resistor and a 6 volt battery. I used the 1.2K because i have a drawer full within reach. The reading went negative befor I reached 3.0 volts too. This transistor could be exceptionally strong and the next may need a little more potential but I have tested a half dozen with this setup and had the same results.

 Now to analyze the results. What does it tell me? This test tells me something about the way the transistor was manufactured. The meter going below zero indicates an alpha greater than 1. Point contact transistors have an alpha greater than 1. The reading will drop with a micro alloy transistor but not past zero. Modern silicon transistors may so a flicker as the power is applied and removed but will not drop very much. So what's in the box is a point contact PNP transistor.
The test jig schematic.

You can do a quick test with some test leads or breadboard. If you plan to test several it may be worth making a little test board. I should point out the Ohmmeter is forward biasing the Emitter - base junction and the power supply is reverse biasing the base - collector junction.


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