This has me Confused

[tony3d]

Hi all, I have a very simple circuit here from the Make Electronics book. Basically it's the introduction to transistors. I understand that applying a small voltage to base is allowing the current to flow from the collector o the emitter, but why the two very different voltage drops? If I take a voltage drop between the collector, and the emitter I would think it would be somewhat close to taking the reading from the collector to the negative end of the LED, but it's not even close. The first reading is like .65mV, and the second is like 9.98V.I would think they should very close to the 9.98. Why is it not close seeing how the voltage is flowing between the collector, and emitter either way right?

[awallin]

we probably need a circuit diagram with the voltages you talk about indicated. photos aren't that great for displaying circuits - that's why we like schematics!

[mariush]

If I remember my transistor stuff correctly, it's normal to have about 0.2v drop between collector and emitter, and about 0.4-0.6v at the base. But do get back to us with schematic.

[tony3d]

Trouble is I was getting like 65mV between the the collector end of the 180ohm resistor. That should have read 12v. Found out I had the other end of the resistor plugged into the wrong rail so I wasn't picking up the plus side of the power. It's all good now. I guess that's how you learn.

[geraldjhg]

hi
transistors are turned on with CURRENT it is irrelevant to mesure
the base voltage, it is alwways arounf .6 v (silicon)
so measure the voltage drop in the base resistor and by ohms law
find the current
play around with 1 ma to start with
now for the collector
if you inject 1 ma into the base and the transistor that has a beta of 100
the collecyor is capable of sinking 100 ma and the collector voltage
should be around .2 volt (if not it will get HOT and be working in linear mode)
so limit this collector current to some safe value and play along
disregard small variation in voltage, the just appeared with DMM
and are so dependant on gain and gain depends on temperature etc etc
saludos