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Question about NPN transitors

DjohnTheShooter

Mar 17, 2016
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Mar 17, 2016
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Hi,

I am very new to electronics and to this forum; haven't got any experience in the area, only read some tutorials online (mainly at www.electronics-tutorials ws) and done some arduino tutorials.

I'm playing around here with an arduino, and I've got an NPN transistor, that I'm trying to understand.

So, I've got this all wired up like this (attached):
schemeit-project.png

When input to the NPN transistor base is 0v, I measure the following voltages:
V->C: 4.3v
C->E 0.7v

I'm not sure why V->C measures as only 4.3v. =\
I thought that when NPN has a positive voltage at Emitter->Base and at Base->Collector, then it would act as an open switch between C->E, and hence V->C voltage would be same as V->E, that is 5v.

My guess is that because I don't actually have positive potential difference at Emitter->Base, the whole NPN has some resistance to it.

So, a couple questions:
- Does transistor introduce resistance on C->E when Emitter->Base has 0 voltage?
- How can I achieve positive potential difference at Emitter->Base when NPN is set up as an open drain? I have no negative voltage supply to bring base below 0, and I don't want to put positive voltage on emitter, because it is supposed to be grounded. Do I even need to do that in order to activate the transistor? After all, I get the current flowing, led is switched on when input is 0.

Some more details:
- Resistor is 470 om
- Transistor is NTE2322
- LED is a 4303F1 (1.85Vf / 10ma)

Thank you for getting all the way down here!
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
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Welcome to EP.
When input to the NPN transistor base is 0v, I measure the following voltages:
V->C: 4.3v
C->E 0.7v
That's not plausible. At Vbe=0V the transistor should be off, Vce=5V, V(V->C)=0V. It Vbe really=0V or simply open circuit?

- Does transistor introduce resistance on C->E when Emitter->Base has 0 voltage?
Not when it is o.k.
- How can I achieve positive potential difference at Emitter->Base when NPN is set up as an open drain? I
By addding a voltage source from base to emitter. Note that you will need a series resistor in the base to limit the current.

The problem you have is that the NTE2322 is a PNP transistor which explains it being on with Vbe=0V. Use a 2N2222 instead.


Read these 2 ressources for more detailed information:
https://www.electronicspoint.com/resources/using-a-bipolar-transistor-to-turn-a-load-on-and-off.30/
After all, I get the current flowing, led is switched on when input is 0.

https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/
 
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