# Transistor-LED Circuit Design Question

I am using a transistor to drive an LED with a GPIO pin of a microcontroller.

Ic is specified to be 25mA

I designed the circuit so that Ic can be as high as 100mA before the transistor itself acts as a current-limiter, and then I decided to use a resistor to limit the current through the LED (just felt safer to me - idk is this dumb?).

Now I am confused on how to do KVL on the output loop to solve for the value of the current-limiting resistor. Do I just assume Vce is very small?

• Ic = 100mA, Ib = Ic/B = 1mA, Rb = (3.3-0.7)/1mA = 2k6 ..... With this you will limit Ic to 100mA max. It's very temperature and Beta dependent, but for Led should be enough if you measure the exact Beta of transistor you use.
– user208862
Nov 20, 2021 at 2:18
• Normally for saturated operation a beta of 10-25 is used to ensure the transistor is saturated. A Vce of 200mV is a good place to start for calculating the series resistor. Using a resistor as you are doing is the recommended method, not dumb at all. Nov 20, 2021 at 2:19
• Yes, Vce is simply negligible there, 0.2V, once you give enough IB, check the TR datasheet, nothing special.
– jay
Nov 20, 2021 at 3:02