Grounding the base of an NPN BJt

[Meta_Alchemy]

Hello,
I made a simple circuit connecting an LED to the collector of an npn transistor. After I applied some 3V, I noticed the LED lit up and very dim of course.

I simply grounded the base and the LED turned off.
When I removed the ground, suddenly the LED turns nice and bright but not to the point of burning and then after a bit it will go back the previous dimly lit LED.

When I hooked up the base to the positive side, I could see amperage increase on the power source and the led will get bright but not as much as when I unhooked it from the ground.



In terms of electron mobility conduction and charge distribution, i.e. capacitance.....What takes places at each of the two junctions base-emitter and base-collector.
1. When the base is floating.
2. When grounding the base
3. When ungrounding the base.
4. Hooking up the base to positive supply.

5. One last question. Why does the LED becomes bright when touching the base or the emitter ( I think ) as I am not remotely part of the circuit. In fact even I move too fast or even get close to these points.

Thank you in advance for your response and especially responses that are put together using simple clear terminologies. It removes all the back and forth.

 

[danadak]

Do you have current limiting R in the base and in the collector/led
branch ?

An ungrounded base is subject to pickup via capacitive coupling. A base grounded
results in a small leakage in collector - base circuit.


Regards, Dana.

 

[Lightium]

Yes, I can be very lazy. Here:

Bipolar junction transistor - Wikipedia

en.wikipedia.org

 

[transconductance]

I am surprised the LED lit at all with the base open. There is some current getting into the base somehow. It doesn't take much and there could be surface leakage through contamination from you handling it.

>When I hooked up the base to the positive side, I could see amperage increase on the power source and the led will get bright but not as much as when I unhooked it from the ground.

Assuming you don't have a base resistor, the BE junction is bypassing a lot of current around the LED. That's why it is dimmer. This is a good way to burn out your transistor! You need to limit the current flowing into the base.

Check this tutorial

 

[crutschow]

Show us your complete circuit schematic with all the part values including the transistor.

 

[audioguru]

I think it is another circuit made with a mess of long tangled wires (antennas) on a breadboard.

 

[alec_t]

The original post was back in July '22. Seems we've lost the TS.

 

[Nigel Goodwin]

Yes, thread locked - moderator.