Post by thread:[EE:] Simple Transistor

I recall buying 2N2222 transistors years ago under the Radio Shack label and finding that they were not up to expectations, If I recall correctly, the connections did not agree with the label and even when connected correctly the gain was attrociously low. Maybe they are still storing up a batch of suspect ones. There is a simple way to identify the transistor connections by testing. If you try to test a transistor with a digital multimerter on the ohms ranges, it will read open circuit, because it does not have enough voltage across its leads to overcome the standoff voltage of the transistor junctions, however the meter can be used on the diode test range to test the transistor, and the meter reading will be the voltage drop across the junction. . A NPN transistor when tested with a multimeter on the diode test range will give a reading of a diode between base and emitter, with the anode of the diode towards the base, and a forward voltage of 0.6-0.7 volts. When tested between anode and base the result will again be a diode with the anode towards the base. This means that in operation, the collector current is passing to the base through a diode in the blocking direction and the emitter current passes through a diode in the conducting direction. It is because the two junctions are in extremely close proximity inside the transistor that an interaction occurs allowing current to flow through the reverse biassed collector junction, and at a value of beta (or HFE) times the base current In the case of PNP transistors the test results will be similar but the diode polarities are reversed. To identify the base lead, find the one which gives the same reading, like a diode when tested from either of the other two leads. When tested with the diode test range of a multimeter, the voltage across either junction when conducting in the forward direction is between 0.6 and 0.7 volts Having identified which lead is the base it is now only necessary to sort out which of the remaining two leads is the emitter and the other has to be the collector. When connected in the non conduction direction, the base- enitter junction behaves like a zener diode with a breakdown voltage of between 5 and 6 volts, whereas the collector junction has a much higher breakdown voltage (more than 100 volts for a 2N2222A) . Connect the base lead of the transistor to the negative of a 9 volt battery, and connect the positive of the battery through a 4.7K resistor to one of the remaing leads of the transistor, with the other transistor lead left disconnected, so that you test only one junction at a time . Now measure the voltage across the transistor junction (ie between the base lead and the transistor end of the 4.7K resistor) If the voltage reading is about 6 volts, (which is the zener breakdown voltage of the emitter-base junction) it means that the junction being tested is the emitter. If the voltage reading is around 9 volts, (which is the battery voltage), you are testing the the collector junction of the transistor. If the voltage reading is close to zero, you probably have the circuit wrongly connected, or the transistor is internally short circuited. If the reading is about 0.7 volts, you probably have the battery connections reversed so that you are measuring the forward conducting voltage instead of the reverse blocking voltage Once the first reading is taken, check the results by testing the other junction. If you are testing a PNP transistor, just reverse the connections to the battery, then follow the instructions above In use, you can interchange the emitter and collector leads of a transistor and it will still work, but the gain is extremely low, sometimes even less than 1, and from the above you will see that in its reversed connection it cannot develop more than 6 volts of signal between its "collector" and base Regards, Harry {Original Message removed}

> Having identified which lead is the base it is now only necessary to sort > out which of the remaining two leads is the emitter and the other has to be > the collector. Another method. Arguably less risky. In the following R can be an eg 1 megohm resistor OR a spit wetted finger :-) Assume an NPN. Swap polarities for PNP. Given XYB connection where X & Y are emitter and collector OR the opposite and B is base. Place meter on about 100 kohm range. Range used MAY vary - choose one which works best for you. Connect positive lead of meter to X Connect negative lead to Y Connect R from X (positive) to B Note meter reading Swap meter leads on X & Y Connect R from Y (positive) to B Note meter reading. The lowest reading (by far) will result with positive lead on collector. With a little practice you can do this with a multimeter on ohms range and a licked finger. This works because, in both cases you are applying forward bias to the base while positive is also applied to the suspected collector. When the transistor is 'correctly" connected the transistor has far more beta than when reverse connected so more current flow so lower resistance. This is an essentially fail proof test. Do be careful that wet finger does not vary greatly in resistance between tests :-) If you cant tell the difference then it probably doesn't matter (but you always can). Or you could use a boring old multimeter transistor tester. RM -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu