Electronics > Beginners

How to choose the base resistor value for the BJT in a LED on/off circuit?

(1/7) > >>

Aakash:
Can anybody explain, how do I set the value of resistor R1 here? why 1k is chosen specifically?

Zero999:
The transistor needs sufficient base drive to saturate, i.e. turn hard on. A general rule of thumb is the base current should be about 1/10 of the collector current, hence why the base resistor is around ten times the value, of the LED's current limiting resistor.

rstofer:
Compute the collector current as Vcc - VceSat - 2V (assumed LED forward voltage) / 100 Ohms = (5 - 0.2 - 2) / 100 = 28 mA
Using the 1/10 current idea, calculate the base resistor as (Vcc - 0.7) / 0.0028 = 1536 Ohms so a 1K resistor will easily pump enough current into the base.

Now it's your turn:  build the circuit on a breadboard and measure the currents/voltages and see if my math comes close.

Aakash:
Appreciate your response, but may I know why does the base current need to be 1/10th of the collector current. what if it's different?

Zero999:

--- Quote from: Aakash on December 03, 2021, 09:56:20 pm ---Appreciate your response, but may I know why does the base current need to be 1/10th of the collector current. what if it's different?

--- End quote ---
It's a general rule of thumb. The base current required to saturate a transistor varies greatly. It varies with the temperature and from one part to another, even if they have the same part number. Here's a link to an explanation I gave in another thread, to save me repeating it.