# Voltages to keep my circuit running at 200mA with NMOSFETS

Im trying to build a circuit to send information using IR led, using pulse signals my leds (TSAL6400) operates at 1.35V, and their maximum pulse current is rated 200mA

After struggling a bit with BJT transistors, and realizing that since I'll send the data using RPI (Gpio is 3.3V, 16mA), I switched to NMOS transistors. I found a local dealer with RFP30N06LE

Vgs(th)= 1-2V (min,max)
Rds(on)=0.047Ohm


My circuit (due to other limitations, and the fact I need to decide on which branch of leds to send the command on, each branch is a different room in my home)

As I demonstrated in this sketch, each of the branches represents a room, and I want to each time send a command to a specified room. So, for instance, to send to the second branch from the left, I will turn the M2 on ("1" = 3.3V of my gpio at 16mA max), and will send the pulses by turning on and off M5 In order to reach 200mA I need a total voltage drop of 2.3V over my resistors and transistors. If R1=11.5Ohm, for M1-M4: Vds=0V, Vgs=3.3V Since Vds=0V will keep the transistor closed (no current flow), I assume I need to raise the voltage just a bit, but not sure how to make it appropriate for case with two transistors, one after the other. If I will use Rtot=8.5Ohm + 1.5Ohm + 1.5Ohm, and placing R1 where it is already, R2=1.5Ohm before M5 and another one after. Is that the correct way? This way: Vgs(M2)=3V Vds(M2)=0.3V Vgs(M5)=3V Vds(M5)=0.3V

Is that correct?

You do not need the extra resistors around M5, the circuit will work just as it is, but with R1 11.5 ohms.

Vgs(th) for those MOSFET's is between 1V and 2V, so you just use 2V which is the worst case in this circuit. The RPI outputs 3.3V so it can easily turn on M5. When M5 turns on its drain will be almost 0V, which means the source of M1 to M4 will also be 0V. The RPI can easily turn on M1 to M4 because the 3.3V output from the GPIO exceeds the Vgs(th). When M5 is turned off then it doesn't matter what happens with M1 to M4 because no current can flow in any case.

In addition I suggest that you do not need M5 for the circuit to work. You can send the IR data to one MOSFET M1 to M4, while keeping the other MOSFET's turned off.