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Q: Identifying Transistor Terminals

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I have some different types of transistors (BJT, PUT) and when I was trying to follow along with a data sheet to identify the terminal layout, it was backwards because it was not the brand specific transistor for the data sheet I was using. So to make life simple for myself, I want to put together a little tester that is as simple as an LED tester - if inserted correctly, the LED lights up.

Using experiment 10 out of the Make: Electronics book as the starting point, the LED should only be lit when the button switch is pressed if the 2N2222 is inserted the right way. I took the transmitter and put it in backwards expecting nothing to happen when I pressed the button switch only to see it lit up again.

I don't have the schematic so I will have to describe it the best I can.

9v Positive - 180 Ohm Resistor - 2N2222 Collector
9v Positive - Switch - 10K Resistor - 2N2222 Base
2N2222 Emitter - 680 Ohm Resistor - LED - 9v Negative

I have replaced the 180R with a potentiometer and if the transistor is placed in one way, the LED gets will go from off to bright at a slow pace. When I put the transistor in backwards, nothing happens for about 1/2 the potentiometer's range and then it will become bright at a rapid pace (depending upon the position of the knob).

My question is why is the 2N2222 working both ways? According to everything I have read up to this point, current should only flow in one direction.

The transistor will work if the emitter and collector connections are exchanged but the gain will be much lower than if they're connected correctly.

You're also lucky that the LED didn't turn on without a base signal when the transistor was connected backwards because the breakdown voltage is lower when the collector and emitter are connected backwards.

A better test would be an oscillator driving a piezo transducer or a small speaker via a buffer amplifier, which won't work when the emitter and collector connections are reversed because the gain will be too low.

The easiest way IMO is to use the diode test of a DMM. The base-emitter and base-collector junctions both behave like diodes, in case of npn, the base is the anode (cathode for pnp). The base-emitter forward voltage drop is slightly higher than the base-collector voltage. No need to use a special tester, just use any DMM.

I know this may sound very basic to you, so please bare with me...

Basic Concept: I would need to build a 555 oscillator circuit, have the output go towards the base of the transistor for the trigger and then have the emitter go to the piezo buzzer?

If you do not mind, I do not want to have the schematic posted for the finished project so I may learn by trial and error. However, any information on what required reading of subject matter would be greatly appreciated since I have only basic beginner books on hand (Make: Electronics and Getting Started in Electronics).

You could also get one of these for US$60


Ya just clip the 3 leads on in any order and press the button, it will tell you which is E B C or G S D for fets or A C for diodes/leds etc..
Also gives you the gain of the transistor, collector test current and any leakage current.


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