Help me find a mosfet - Page 1
Forum section: electronics / beginners

author: Gurpreet
author_rank: Contributor
timestamp: September 05, 2016, 10:06:20 pm
content: Hello Guys,Currently I am working on a low voltage system of 3.3V and I cannot go higher than this because i want to minimise the power consumption. I have to drive a RED led and vary its intensity. I was thinking of using DAC of my MCU to vary voltage on the gate of a mosfet and drive it in the ohmic region to vary the amount of current flowing in the LED. I could not find any mosfet that can run with a Vds of 3.3V. The minimum Vds that I could find is 5V.Has anyone encountered this situation? If someone has used such a mosfet please share with me.If you guys have any other ideas to tackle this problem then again plz share.Thanks..

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2016, 10:14:12 pm
content: Why not use PWM to vary the brightness of the LED?There are plenty of MOSFETs which can switch with a gate voltage of 3.3V but if you're only driving one LED, can't you simply connect it directly to the output of the MCU?

author: Gurpreet
author_rank: Contributor
timestamp: September 05, 2016, 10:28:06 pm
content: Its a high intensity LED so MCU wont be able to provide enough current to it. I cannot use PWM be cause I need continuous light output which I want to use for my light sensor.Yes there are many mosfet which can switch on 3.3V but i want a mosfet which can run with "Vds" of 3.3V across it.Thanks for your reply though.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 06, 2016, 03:46:09 am
content: Quote from: Gurpreet on September 05, 2016, 10:06:20 pmPerhaps you are misinterpreting the mosfet data sheets.I just put together the simplest circuit on my breadboard, using a red LED as high-side load, a IRF3205 mosfet, a 10k gate pulldown resistor and a 10k gate trimpot to vary Vgs. Using 3.3 volts from a regulated supply, the circuit works fine to control the LED brightness from "off" to relatively full brightness, depending on the setting of the trimpot.This is a horrible way to dim an LED though. Since the mosfet is working in its linear range it will be dissipating lots of power, and the IRF3205 isn't fully turned "on" at a Vgs of 3.3V (hence I needed no current-limiting resistor for the LED because the mosfet's Rds is limiting the current). Perhaps a different (logic level) mosfet will turn on more fully at Vgs 3.3V and so might drive your high-current LED better.You would be better off using PWM and a low-pass filter.

author: arlipscomb
author_rank: Contributor
timestamp: September 06, 2016, 05:15:06 pm
content: If your goal is to minimize power consumption then you don't want to be heating up the MOSFET by keeping it in the linear range. Something like a BSS816NWH6327XTSA1 (just something I found searching Mouser) looks like it might do what you want. I am pretty new at this myself so I am not sure I am looking at the right parameters.

author: tatus1969
author_rank: Super Contributor
timestamp: September 06, 2016, 07:51:45 pm
content: Quote from: Gurpreet on September 05, 2016, 10:06:20 pmLook for small-signal MOSFETs. But doing like you describe is not very wise, as the Vgs of a MOSFET is highly temperature dependent, and also has a high production spread. Any two transistors will likely produce different intensities at the same gate voltage. You may think of making the transistor an adjustable current source by placing a resistor between source and ground. But that again reduces your voltage budget. I think you're better off running your IO port in PWM style, vary the duty cycle, and use a resistor to control 100% current set point. That's the classic way to implement intensity control with a microcontroller.

author: tatus1969
author_rank: Super Contributor
timestamp: September 06, 2016, 07:55:14 pm
content: Quote from: Gurpreet on September 05, 2016, 10:28:06 pmYoun can PWM that external MOSFET. If you choose a high enough PWM frequency your light sensor will not see that, it'll integrate over a certain amount of time. Look if you can find a bandwidth information of your sensor.

author: Zero999
author_rank: Super Contributor
timestamp: September 06, 2016, 10:12:52 pm
content: Use a constant current sink and a filter to get a voltage from a PWM signal.