# Current Flow

#### Zalves

Jul 26, 2023
2
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

Moderator
Nov 8, 2019
3,268
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

Feb 19, 2021
725

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.

Last edited:

#### AnalogKid

Jun 10, 2015
2,862
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

Jul 29, 2020
1,807
Thus, the transistor is "on" and the left LED (reference designators!) is off
What reference designator? If I may ask.

#### AnalogKid

Jun 10, 2015
2,862
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

Jul 29, 2020
1,807
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.
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....

Last edited:

#### AnalogKid

Jun 10, 2015
2,862
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

Jul 26, 2023
2
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

May 7, 2021
834
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.

Feb 19, 2021
725
Typical LED turn on versus color :

Regards, Dana.

Feb 1, 2023
164

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