Base Collector or Base Emitter ?

Thread Starter

N11778

Joined Dec 4, 2015
174
It's easy to find the base of an unmarked transistor with a meter or C kt Tracer.
Is there a way to tell the (base collector) from the (base emitter).

Found the way, I think. To reliably pick out the C.B. and E.B.
I have a Handy dandy Heathkit Capacitor checker IT-28.
It tests for 2 microamps leakage in capacitors up to 600 volts.
I use it to test the reverse breakdown voltage of the two junctions.
From my testing and looking at many transistor specifications the Emitter Collector
breakdown voltage is always at least twice as high as the Emitter Base.
 
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#12

Joined Nov 30, 2010
18,076
Usually on diode test,the B/E is 0.68 to 0.7V, the B/C is lower like 0.625V..
That seems like a direct contradiction of my experience. Maybe my memory is faulty or you measured different kinds of transistors. The bottom line is: There is a small difference in the DC voltage per current of the two junctions when measured in the forward (conducting) region.
 

WBahn

Joined Mar 31, 2012
24,684
The emitter is generally much more heavily doped than the collector. As a result, the B-E depletion region is wider than that of the B-C region and thus a higher barrier potential is created in the B-E junction resulting in a higher forward voltage drop.
 

WBahn

Joined Mar 31, 2012
24,684
The other (or another) way to make the determination is to set up a simple circuit and check the current gain. The gain if you have collector and emitter swapped is almost always a lot less than the correct way. You can do this with a single circuit (no need to physically try both orientations) since the wrong orientation will result in the transistor going into saturation while the correct orientation will keep it in the active region. This can be detected readily with a couple of voltage measurements.
 

WBahn

Joined Mar 31, 2012
24,684
I don't see any indication of what transistor Dordodynov used. If he used something other than a 2N3904 that might well explain it. Also, even if they are the same device the particular models in the two libraries might not agree, at least as far as the parameters that are important in reverse operation like this.

What I have an issue with is that the base voltage in the left sim seems too high. I would expect it to be closer to 700 mV. Here's my reasoning: In that circuit the current in the top resistor should be a few hundred times as much as the voltage in the bottom resistor and, thus, has a few hundred times as much voltage drop. In either case the Vbe can be assumed to be roughly 0.7 V (or 0.6 V or anything in that ballpark). So the voltage drop across the bottom resistor on the left in his sim is about 150 mV while the voltage drop across the top one is about 4.15 V; but this is only a ratio of about 28. Perhaps the model he is using is for a power transistor.

With a 2N3904 I would expect the voltage across the top resistor to be something around 4.3 V and the voltage drop across the bottom one to be something like 20 mV (a gain of about 200), placing the base of the transistor at about 700 mV give or take.
 
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cmartinez

Joined Jan 17, 2007
6,461
And speaking of reverse operation. I had never seen a transistor connected like that before, that's why I was curious and decided to sim it. Is there a practical (real world) circuit out there that would require using an NPN transistor like that?
 

WBahn

Joined Mar 31, 2012
24,684
And speaking of reverse operation. I had never seen a transistor connected like that before, that's why I was curious and decided to sim it. Is there a practical (real world) circuit out there that would require using an NPN transistor like that?
Uh ... read the first post in the thread. The TS is asking about how to distinguish the emitter from the collector in a transistor for which you don't know the pinout but after you have done the simple checks to determine whether it is an NPN or a PNP and which pin the base is.
 

WBahn

Joined Mar 31, 2012
24,684
So I just did the sim (using a 2n3904) and in the forward direction got a base voltage of 694 mV and a collector voltage of 708 mV. So in this configuration the base current is low enough that the voltage drop across the base resistor is only 14 mV (so 20 mV wasn't a bad estimate at all). Thus it looks very much like a diode-connected transistor. The calculated current gain is 307.

In the reverse connection, the base voltage is 697 mV, so virtually identical, but the "collector" voltage is 1418 mV, very much NOT like a diode-connected transistor. The calculated current gain here is 5.
 

Bordodynov

Joined May 20, 2015
2,386
The purpose of my drawing is to help with a simple circuit to reliably distinguish between the emitter and the collector.
 

WBahn

Joined Mar 31, 2012
24,684
The purpose of my drawing is to help with a simple circuit to reliably distinguish between the emitter and the collector.
It's a circuit that works well for that purpose, the sim results are just a bit perplexing. I suspect that you are just using the default NPN transistor model. I have no idea whether that model is intended to produce very realistic results and, if so, what kind of transistor it is supposed to resemble, or if it's just intended to allow of quick-and-dirty functional sims.
 

Bordodynov

Joined May 20, 2015
2,386
Standard NPN model BF=100 BR=1 and Vbe~Vbc~0.7
Vc=0.7+(5-0.7)/(BF+2)=0.74
Ve=0.7+(5-0.7)/(BR+2)=2.13
 

WBahn

Joined Mar 31, 2012
24,684
Standard NPN model BF=100 BR=1 and Vbe~Vbc~0.7
Vc=0.7+(5-0.7)/(BF+2)=0.74
Ve=0.7+(5-0.7)/(BR+2)=2.13
But notice how those numbers don't match your sim results for the normal orientation.

If BF = 100, then to get a collector voltage of 852 mV the base voltage would have to be 810 mV.

If Vbe = 0.7V, then to get a collector voltage of 852 mV the BF would need to be 27.
 

R!f@@

Joined Apr 2, 2009
9,647
All my life I have tested transistors.
Emitter drop is higher than collector.
That is how I have seen it.

I need to be proven wrong if any one disagrees.
 

RichardO

Joined May 4, 2013
2,273
And speaking of reverse operation. I had never seen a transistor connected like that before, that's why I was curious and decided to sim it. Is there a practical (real world) circuit out there that would require using an NPN transistor like that?
Sometime a transistor is used a zener diode. The base is shorted to to the collector and the base to emitter is about 6 volts when reverse biased.
 
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