Current Flow

[Zalves]

Hello everyone!

I just started to play with electronics, a kids dream that I've finally set to explore.

I have created a circuit with a couple of LEDs a transistor and a switch as a learning experiment. I'm very confused on how the current flows, I'm adding an image to illustrate. On the left I can't understand why the current stops flowing to the left LED.

Very confused...

 

[bertus]

Hello,

The led will have a voltage drop of 1.5 Volts to 3.8 Volts depending on the color.

The B-E voltage of a silicium transistor will be about 0.7 Volts, so the led will go off, as there is not enough voltage drop.

Bertus

 

[danadak]

The graph shows you what the Vbe of a NPN does as is base current turns it on. As you
can see it does not change much once it gets > than its intrinisc threshold. So as previously
stated by bertus an LED in parallel with that junction simply wont conduct.



Regards, Dana.

 

[AnalogKid]

The led will have a voltage drop of 1.5 Volts to 3.8 Volts depending on the color.

Put another way, the LED *requires* a forward voltage across it of greater than 1.5 v to 3.8 V before it will emit light. The forward voltage of the transistor's base-emitter junction clamps the voltage across the LED to only 0.5 V to 0.8 V for a typical small-signal transistor. Thus, the transistor is "on" and the left LED (reference designators!) is off.

ak

 

[Delta Prime]

Thus, the transistor is "on" and the left LED (reference designators!) is off

What reference designator? If I may ask.

 

[AnalogKid]

What reference designator? If I may ask.

There aren't any, making a detailed discussion of the circuit fraught with ambiguity and peril.

ak

 

[Delta Prime]

There aren't any, making a detailed discussion of the circuit fraught with ambiguity and peril

Duh! I was just taking the heat for the TS.
I like your style!
But I always have to remind myself this site is akin to a hospital pediatric emergency room. I feel bad saying that because I lost it as well in my most recent response to posts.It can be rather....

 

[AnalogKid]

Rule #1 in forums - First, answer the question that was asked.

Rule #2 - Bang on and on about reference designators and which way ground symbols point.

ak

 

[Zalves]

Put another way, the LED *requires* a forward voltage across it of greater than 1.5 v to 3.8 V before it will emit light. The forward voltage of the transistor's base-emitter junction clamps the voltage across the LED to only 0.5 V to 0.8 V for a typical small-signal transistor. Thus, the transistor is "on" and the left LED (reference designators!) is off.

ak

Thanks, make more sense. For what I understood, the base of the transistor makes that part of the circuit only have 0.7V

 

[crutschow]

For what I understood, the base of the transistor makes that part of the circuit only have 0.7V

Yes, when the transistor is on, the base-emitter voltage of a silicon transistor is about 0.7V.
Since the LED requires more voltage than that to turn on, it will not conduct any current and will look like an open circuit.

 

[danadak]

Typical LED turn on versus color :




Regards, Dana.

 

[bidrohini]

I can see that you've already got your answer. For more details about how transistors work, you can see here: