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 ----- From: "Matthew Brush" To: Sent: Sunday, June 06, 2004 5:00 AM Subject: [EE:] Simple Transistor Question Hi all, Something has been bothering me for a while. I know there was recently a question posted about transistor pinouts, but I was still left with mixed answers on this. I have attached a (crappy) drawing I quickly made in photoshop to go with this question. The package of 2N2222 transistors I bought from RadioShack has pinouts on the back which seem to go against what actually works for me, and is also the opposite of how Eagle Layout wires the transistor (from a schematic->make board). Eagle wires it the way that is in the picture, which is also how I've been wiring it all along. It seems these transistor will work in either direction, but as I'm soon to be attempting an audio circuit (as opposed to the usual turning on a relay or whatever), I want to make sure I have it right. Is this circuit in the attached GIF correct, labelled properly and whatnot? I would assume that the collector "collects" electricity and when the base is positive, the emmitter "emits" electricity towards ground. But it seems like I'm getting confused going from the schematic to the actual circuit. Thanks for any clarification anyone can offer as to why i see these standard NPN transistors wired differently (ie backwards) sometimes and not others. Peace MJ Brush -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu