Yeah, you really need something that can check the AC gain, to distinguish between the collector and emitter leads. I like the find by Studiot:Sure you can use an ohmeter to check the base emitter junction. Now tell me how you will tell it from the base collector junction. This method will tell you the difference between NPN and PNP but it won't likely tell you which is the collector and which is the emitter.
But it is not always that straightforward and an electonics/electrical major should known the pitfalls.Pretty straightforward.
Yes but I didn't use an Ohmmeter to test my junction. I understand now that the ohmmeter uses external voltage supply to drive current through resistor and measure the resistance.But it is not always that straightforward and an electonics/electrical major should known the pitfalls.
Yes, I seem to remember you have recently bought or borrowed a digital multimeter.
Older type Analog meters, with pointers respond to current not voltage.
We make a voltmeter out of them by putting a high value resistor in series with them.
We make an ohmmeter by putting a ressitor either in series or paralle with the unknown and measuring the current through the resistor. This yields in a highly non linear scale.
In order to measure higher value resistors we need a relatively high supply voltage (my old meter had 22.5 volts.)
This high voltage was more than enough to turn on (bias on in the forward direction or sometimes breakdown in the reverse) semiconductor junctions.
Further the colours of the terminals and leads of these older type meters were reversed ie the black lead produced a positive voltage on the black lead.
So you cannot use this type of meter to measure in resistors incircuit as you will switch or destroy junctions. Electronic ohmeters were introduced that applied about 0,2 volts to overcome this.
Modern digital meters respond to voltage
So we make current meters by measuring the voltage across a known resistor with them.
We measure resistance by either passing a constant current through the resistor and measuring the voltage or using a feedback amplifier or by other means.
Whichever we do, the red lead should now be positive and around 0.2 volts only applied to the unknown resistor.
This means that we can no longer use an ohmmeter to test for forward and reverse resistance on a junction as we simply will not switch it on.
So modern meters have a special setting which applies sufficient current limited voltage to turn on the junction and presents the voltage across it as a reading in the forward direction.
Isn't that what I said?But I have special feature on my multimeter, called diode test, designed specially for diodes. I don't think it sends large currents through test leads because its designed for diodes.