# Help me in calculating base resistance for transistor

Discussion in 'Power Electronics' started by rahulb, Oct 20, 2019.

1. ### Audioguru

2,605
586
Sep 24, 2016
Most 5mm LEDs have their brightness and forward voltage rated at a current of only 20mA. Some of mine have a maximum allowed continuous current of 30mA.
Four 2.2V LEDs in series use 8.8V then the current-limiting resistor will use the remaining 3.2V, then for 20mA the resistor will be 3.2V/20mA= 160 ohms.
But the forward voltage is not a fixed voltage, it is a range of voltages so the actual forward voltage of your green LEDs might be 1.8V then four of them use 7.2V and the 160 ohm resistor with the remaining 4.8V across it would limit the current to 4.8V/160 ohms= 30mA which might cause the LEDs to fail soon.

Your red LEDs might have a forward voltage of only 1.8V and four will need a current-limiting resistor of 220 to 240 ohms.

2. ### rahulb

45
0
Mar 14, 2018
I have calculated resistor values using 2.1 v as forward voltage for green and yellow leds and 2 v for red leds.

Also, the current flowing to leds will be 10 mA.

About the mosfet, I am thinking to use stp55nf06 mosfet as it is easily available for me. I hope this is a good choice for this project.

thanks

3. ### Audioguru

2,605
586
Sep 24, 2016
If a transistor saturates well turned on) then its voltage between collector and emitter is 0.2V. Four 2.1V LEDs in series use 8.4V.
With your 12V power supply then the 180 ohm LED series resistor has a voltage of 12V - 8.4V - 0.2V= 3.4V. Then ohm's Law calculates the current to be 3.4V/180 ohms= 18.9mA. How did you calculate only 10mA?

The 50A huge Mosfet you selected is over-kill but it will work fine.

4. ### rahulb

45
0
Mar 14, 2018
no, I chose 330 ohm for green and yellow, 390 ohm for red.

180 ohm was due to some wrong calculation earlier. sorry.  