Hello, i'm a beginner in eletrônics and i'm having a few problems with a basic schematics, i'm trying to connect a transistor, i'm trying to connect it to my arduino but the led won't power up in the simulator, but if i connect it on the protoboard it goes just fine ... I don't know what is wrong. the sourcers in the schematics are of 5 and 10 V

 

hi C,
Welcome to AAC.
Try connecting the LED and the 1K in the transistor collector, connect the emitter to 0V.
Let us know how it goes.
E

 

Your circuit is an emitter follower, so the emitter voltage across the resistor-LED can be no more than the input base voltage minus about 0.7V.

 

You did not say the specified forward voltage or color of the LED. A red one is about 2V and a white, blue or modern bright green one is about 3.2V. Since your transistor is an emitter-follower, not as a common-emitter switch then the 4.7k base resistor is not needed.

With 5V into the 4.7k resistor then the base voltage will be a little less than 5V, maybe 4.8V. Then if a 3.2V LED is used, it and the 1k resistor get 4.8V - 0.7v= 4.1V and the current in the LED will be only (4.1V - 3.2V)/1k= 0.9mA which will be fairly dim.

 

You can drive an LED in the emitetr, although that is not the best way to do so,
as Eric points out.

Here is a sim driving in emitter. 3 1N4148s were used to sim a 2V LED. As you can see
LED current is not very good.



Regards, Dana.

 

Here is collector drive circuit. You can see current a little higher.





Regards, Dana.

 

Were you intending to use a PNP transister instead of an NPN?

[edit]... and have the emitter/collector switched[/edit]

 

but the led won't power up in the simulator, but if i connect it on the protoboard it goes just fine ... I don't know what is wrong.

Are you certain you're breadboarding the circuit as shown in the schematic? What is the voltage being applied to the base resistor.

For the way you've drawn the circuit, you're trying to switch the LED high side. For that, you'd use a PNP or P channel MOSFET. We use NPN transistors to drive loads low side.

 

Hi,
This TS has not been on AAC since his one and only post Dec 9th 2019.
E