# Weird voltage measurements on transistor

Just as an experiment, I decided to put together the following "circuit":

I then went ahead and measured Vbe, Vbc and Vec and this yielded the following results:

• Vbe = 0.602 V
• Vbc = 0.589 V
• Vec = 0.013 V

Here is the question: As you can see from the circuit above, the Collector is not connected to anything so shouldn't Vbc be an absolute zero? Why am I measuring 0.589 V?

Thanks.

EDIT: I wanted to be more specific regarding what is really confusing me, I am not sure if this will make a difference or not but here it goes:

First let’s take the circuit mentioned above and replace the transistor with the diode equivalent analogy. I realize that replacing a transistor with two diodes is not truly equivalent but I just want to make a point.

Looking at the circuit above, it is clear that the diode representing the collector pin is doing absolutely nothing. As a matter of fact, it should be perfectly fine to just remove that diode and the circuit should behave exactly the same right?

What this reveals is that the collector plays no part whatsoever in this circuit (it should be like it is not there at all) and yet, somehow, I am getting a Vbc voltage. Is that crazy or what?

Of course, the fact is that the collector IS playing a role here, this is the only logical explanation for the reason there is a voltage across Vbc. But why is this happening? Is the diode analogy of a transistor not accurate?

• There's something wrong there. Are you measuring the 'base' voltage from the source side of the resistor? – Spehro Pefhany Mar 13 '15 at 3:48
• @Spehro Pefhany: I am measuring the voltage directly on the pins of the transistor. I have updated the original question to reflect that fact. – T555 Mar 13 '15 at 3:50
• Well- if you have Vbe = 5.89V you've either got the wrong kind of transistor (PNP or MOSFET or JFET or whatever rather than NPN BJT) , the transistor is damaged, or it is connected to the wrong pins! There are no other options I can think of. Vbe should be about 0.7V for reasonable base current. – Spehro Pefhany Mar 13 '15 at 4:28
• @Spehro Pefhany: Sorry, my decimal points were on the wrong place (I fixed it now) and did't notice until you mentioned voltage across base / emitter. Still, that does't explain why the voltage across the Base and Collector is 0.589 V instead of 0 right? Or am I still missing something? – T555 Mar 13 '15 at 4:40
• When the transistor is saturated (fully 'on') the C-E voltage will be very low (13mV in your case, with no current flowing). Since the voltages all have to add up (to zero around a loop taking direction into account- that's fundamental principle named after Gustav Kirchhoff), you must have almost the same voltage from base to collector as from base to emitter. That make sense? – Spehro Pefhany Mar 13 '15 at 4:52