Tutorial: LED switching circuit using N-Channel(BSS123) MOSFE

I want to design LED switching circuit using N-Channel(BSS123-Datasheet) MOSFET. Please find the supply details in the image.

  1. What formula can be used to calculate the value of R1?
  2. How VDS voltage impact here? How can find that? What is the formula for that?
    ZWX5X

Welcome to the community !
I guess this is your home work :slight_smile: and it’s not really related to any particle devices :grin:
But let’s start.
Firstly to calculate your R1 value you have to know the Ohm law and understand it 100%
secondly the value of Vds can be close to Vcc (transistor closed ) or (when the transistor is open) can be calculated base on the parameter
Rds-on which can be found in datasheet here


Let’s back to Ohm law so we know that I=U/R so the R=U/I so your final equation should look like this R1=Vcc-(Rds-on * I)/I
Rds-on * I is your Vds where transistor is open Finally
R1=3300mv-(Rds-on*70mA)/70mA
You just have to find out the Rds-on from the table :wink:

Please note I changed 3.3V to 3300mV as this is really important and is easier then changing mA to A :+1:
Best,
Arek

2 Likes

Hi @DeionAnnel123

Welcome to the Particle forum :slight_smile:

I can second what @dreamER said. Good info. Of course you can try a m ore agile approach like I do it, haha.

If you are looking for R1 I suppose you want to reduce the voltage over your LED? Which colour LED are you using, they have different operating Voltages. Having said that, you should be safe with roughly 2.5V - 3V over the LED. Usually 200Ω - 330Ω is a safe choice, permitting your RDSon resistance on the MOSFET it not too high.

Your circuit seems fairly straight forward, I am quite sure this should work :wink:

Regards, Friedl!