Samuel my apologies, been very busy, and looked for the diagram of my tester but no found so had to re-draw it.Firstly your tester. As far as I can see without actually building it, it should work. It will light one or the other LED depending if it's an NPN or PNP transistor, and will show short or open transistors, and diodes.What it does not give you is the transistor gain, which is where my transistor comes in. It will also tell you if your transistor is short or open, but will also give you a gain indication, and can be used for matching transistors if you need that.As you can see below, it's just a 1.5V battery with a meter in series, through a switch to reverse the polarity (for NPN/PNP transistors. There is a variable resistor to bias the transistor and you calibrate the variable resistor (use a pointer knob and a scale) for various gains. A digital multimeter will be good enough for calibrating your scale. The theory is that if your meter is say 100 uA, and the transistor gain is 10, you'll need 10uA through the variable resistor to supply the base current to give 100uA of collector current. You just turn the variable resistor till the meter reads the right current, and read the gain off the scale. The other tow resistors are really jsut for protection agains high currents and can be omitted if you wish.It's pretty simple but pretty useful. It is not too accurate a low gains (such as you'd find with power transistors) but will still match them quite well. With normal silicon transistors with a gain of 50+, it's pretty accurate. With germanium transistors and darlington transistors it is not accurate, but still tests them ok. You need to use a fairly rugged meter because with a high gain transistor it will whack against the end stop if your gain resistor is set too low. An added refinement is to use a variable resistor (pot) with a switch on it - if you get any current through the meter with the switch off, either you have a diode or the transistor is wired wrong. You can test diodes by wiring them one way then the other way across the C and E points, one way will whack the meter over, the other there should be no current.The little horizontal meters that you used to get form radios for tuning indication are good for this, they are really rugged. I've given resistor values for various gains and various meter sensitivities in the table.
Sorry, its come out a bit small, one of the GRRRRR things about this site.....
