Mosfet gurus help needed to find appropriate mosfet! - Page 1
Forum section: electronics / beginners

author: Fliz
author_rank: Contributor
timestamp: July 12, 2016, 05:22:12 pm
content: Hello fellas,so I would need help selecting a proper mosfet for this project:I would like to make mosfet switch for flashlight that would allow me to avoid using high resistance wire for remote pressure switch.using standard remote switch adds significant length of pretty thin wire so resistance increases and that makes quite inefficient current path, also those wires and pressure pads are not realy built for currents like 6+Amps.Idea is to make small mosfet switch circuit and then just run thin wire to remote e-switch that would be used to turn mosfet on and off or should I say open and closedRequirements: max voltage that goes thru the switch is 4.35V; max planed amps: up to 15Anow this is important, I plan on powering that mosfet with 3V coin cell so it must be fully open (let thru 4.2V 12A) on under 3V (ideally 2V), it should also have small resistance so that the small coin cell could be used for a long time without need to replace it too often, other than that, SOIC package so I could solder with ordinary soldering iron...Maybe I forgot something, not a skilled EH (electronic hobbyist) hereThanks a bunch

author: Signal32
author_rank: Frequent Contributor
timestamp: July 12, 2016, 05:30:42 pm
content: The circuit should be no more complex than this:http://brunningsoftware.co.uk/Pictures/U-fig10.jpg(you don't need R2)No need to have coin cells to power the MOSFET, you can use the internal battery of the flashlight.You should also have ESD protection for your MOSFET gate.How much room do you have for the MOSFET inside the flashlight ?Do you intend to make a PCB ? Most MOSFET's will work in this situation, what is the voltage of the flashlight battery ?

author: Fliz
author_rank: Contributor
timestamp: July 12, 2016, 05:57:35 pm
content: Falshlight uses lithium ion 18650 battery = 4.35V max (later I plan on making same thing for flashlight that uses 2 x 18650 battery so 8.7V max but for now ~4.2V it is).If I understand correctly, if no coin cell is used then full circuit consists from flashlight driver -> battery - > long wire to pressure pad with e-buttons (that are not designed for 4,2-4,35V @ 10A) then back thure second wire -> to the mosfet -> thrue mosfet to the battery -> flashlight driver = closed circuit.Or, if the mosfet is connected directly to the battery that means flashlight driver also must be turned on (without turning the LED on) and that significantly complicates driver circuit which I would like to avoid at all costs.Yes, I plan on making PDB, available room is about 16mm dia, 7-8 mm in height...

author: Signal32
author_rank: Frequent Contributor
timestamp: July 12, 2016, 07:34:16 pm
content: Quote from: Fliz on July 12, 2016, 05:57:35 pmI don't understand what you're saying there.You have the exact circuit in the image attached. The remote wires won't be conducting much current.Here is just an example of a MSFET that you can use:https://www.sparkfun.com/products/10213

author: Fliz
author_rank: Contributor
timestamp: July 12, 2016, 11:11:52 pm
content: well, I mentioned I am not that skilled so there's always a chance I dont know what I am talking aboutI will try to get my hands on those fets and try them out, see what happens.