I'm not sure if this is the right place to ask.
How can you tell the difference between NPN and PNP transistors? They both look the same. I have several that came in a grab bag but can't tell them apart!
Please Help!
SubMicro
You have to read their reference on the case. And then google a little to find some datasheet which will tell you if they are PNP or NPN
You can also use a multimeter to determine npn vs pnp
Lefty
I generally get the "NPN vs PNP" thing, but otherwise I am quite lost as to how people select specific packages. I am often seeing project boards where someone says "use a 7LQ34b" and someone else says "nah, a 65cx0 is good enough."
A grab-bag I bought has three or four DIFFERENT part numbers with different pinouts for each of the eight varieties in the baggie. Four transistor variants in this bag are labeled "general purpose," which I think basically defeats the purpose.
There seems to be a few variables involved:
- voltage range at the collector,
- voltage range at the base,
- voltage required to trigger at the base,
- how much current it can withstand,
- how much heatsink is stapled to the back of it.
There are probably a lot more, seeing as there are as many variations on the transistor as there are transistors in a Cray XMP supercomputer.
It would be nice to see a bit of middle ground tutorial on transistors from the hobby perspective. Something between "a transistor is like a spigot valve on your water faucet" and "the forward voltage must cascade by a factor of sqrt(Q)/pi in the complex plane, or vreemination may occur."
Ultimately, I want to feel secure when I walk down the transistor aisle, and know that I can find a suitable variety of transistor given my needs, and not given a huge digikey catalogue of part number indices.
Well there is no getting around the understanding of the basic parameters of specific transistors. A lot depends on the application it's going to be used in. Say if it's for analog RF amplification, then it's speed or frequency response is a key specification. Important specs are max collector current, max emitter/collector voltage, as if either of these specs are exceeded in a circuit the device will be damaged. The gain (amplification factor) can be important, again depending on the circuit application.
Certainly now a days one can probably get by with less then a dozen different types of NPN & PNP transistor that would meet most hobbyist's applications. But lets not forget FET transistors, they are a better choice for some applications.
So to summerize, if the vast range of specifications were not important or at least useful, they would not manufacture so many different models. 
I have frequently tried to come to grips with the wide variety of transistors.
Forgetting the PNP/NPN thing for a moment, you have to decide what kinds of circuits you are building. Audio Amps? RF Amps? Switching?
With Amps, we care about gain and frequency but with switching we mostly care about current capability.
If you are using a transistor to power a relay, just about anything will work ok. The frequency doesn't matter because we will switch maybe 10 times/sec (not 10 million/sec) and gain doesn't matter because we simply drive it via a resistor to turn it on. Depending on the application, many of the specs of a transistor do not matter.
Me, I very seldom build RF amps, even though I have a ham radio license. I mostly want transistors as switches or some sort of gating function. In those cases, anything that seems to fit usually works aok.
Well thanks for all the replies but, I'm still just as lost as before! And how can you read the part#'s they are so small! :-?
SubMicro
ah! That I do know the answer.
I went to a store called Princess Auto and they sold a small magnifying monocle that sits on the desk on 3 little legs. I think it was meant to check for bad money. I think I paid $3 or $4 dollars for it. Not as precise as a jewelers glass but the same idea. It makes the smallest print easy to see. I can easily read the numbers off SMT chip resistors that are so small I can barely see the resistor itself.
Thanks I'll see if I can find one. You got a part#?
I have attached 2 pics. The silver thing is what I use. The eye piece comes out. Tonight I ripped apart an old digital camera. The lens seems to work well doing the same thing. The box say 8005764 but I would simply ask the staff if they have something to view part numbers on TINY things.
:)Thank you the pictures really help me to know you were talking about. Again thanks! ;D
In looking for the part numbers it helps to know the format for transistors, most are 2nxxxx (2n3904, 2n3906, 2n2222) BCxxx, MPFxxx, PNxxxx (basically the same as 2N series).
There are usually some extra numbers on there that signify some combination of lot number/ production run, temperature rating or case type.
Don't overlook the lighting around your work area. Lots of good quality light can make it much easier to read small parts.
I put eight 4 foot fluorescent tubes over my work bench, and then use two lamps on spring arms for task lighting (one of them has a 4 inch magnifier lens beside the bulb, which helps).
It's quite an improvement over working in a dark corner 
How about a device with three crocodile clips on it. You connect up your transistor, one lead to each of the clips any clip.
Then you push a button and it tells you if it is a PNP or NPN, and what clip is connected to the collector, emitter and base. Then it tells you the gain of the device.
Would such a device be interesting? I made one last summer but it doesn't use an Arduino, it uses a USB to I2C interface and some processing code. Is it worth documenting this and putting it out as a web page?
Well for testing transistors I found a B&K precision semiconductor tester on E-bay a while back. Really fine peice of equipment, identifies npn/pnp and figures out the EBC pinout, gain and other cool stuff. Below is a PDF link of the manual (including schematic) for those interested in how it works. It probably wouldn't be too difficult to turn a Arduino into a semiconductor test what with it's A/D inputs PWM outputs, etc. Might make an interesting group project.
http://bama.edebris.com/download/b&k/530/B&K%20530%20Semiconductor%20Tester.pdf
Lefty
Grumpy that would be great little thing to have around. Please post it.
Grumpy, that sounds awesome.
Going one step further with the "ease of use" department, a little push-insert breadboard might be better than alligator clips for small parts. But maybe not required. Maybe I'm slow but I just discovered the usefulness of sticking a bit of male header into a breadboard so I had posts to probe a whole row of columns easily.
OK sorry for the delay but I have just posted it at:-
http://www.thebox.myzen.co.uk/Hardware/Transistor_Tester.html
It took some writing up and finishing off but I hope that it will be useful. It uses six ICs but there are some hints on how to make it with even fewer ICs. It's just that when I designed it these were the only ICs I had to hand.
Very sharp, Grumpy. Your analysis of the problem (all the permutations of switches, etc.) really helps show off the design process. And the care in power-up condition too.
I like your I2C header adapter. Most of the I2C I've done, I have just wasted the next two analog pins to do the +V,GND, but that leaves the pullups to the slave (like a Wii Nunchuck).
Thanks Grumpy, thats exactly what I need. I wish I had your mad skills! 