Please help powering LEFs with a MOSFET  - Page 1
Forum section: electronics / beginners

author: Njackste
author_rank: Contributor
timestamp: May 15, 2023, 11:29:58 pm
content: I'm designing a circuit, where a touch ic (at42qt1012-tshr) is used to turn on 6 high power Samsung LEDs (2.7V 65mA) so the output of the ic goes to the gate of the MOSFET, the source goes to ground and I'm confused on if the drain goes to the anode or cathode of the LEDs)Also for calculating the resistor, will it be powered by the MOSFET 20V 4.2A with the LEDs in series meaning a 59R 2W resistor or the power supply 5V 1A with the LEDs in parallel meaning a 6.8R 2W resistorThanks

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 15, 2023, 11:54:43 pm
content: The LED cathode would go the drain and the anode to V+ (Positive power rail).If you run the LEDs in parallel from a 5V supply, you should have individual resistors on each LED. (36 ohms each for apx 65mA per LED) Total current = 390mAYou don't want to put a bunch of LEDs directly in parallel because one will hog more than it's share of current and burn out.If you use a 20V supply and LEDs in series, they will only draw 65mA from the 20V rail. Not 4.2A

author: Njackste
author_rank: Contributor
timestamp: May 16, 2023, 12:05:13 am
content: Thanks so much for the help, I can't believe I made a mistake of putting only 1 resistor for a parallel circuit but in haven't done this stuff in a while.Also my supply is fixed to 5V 1A because I'm powering from USB and I meant the drain-source from my MOSFET was 20V 4.2A.So how does it work if your LEDs are powered by the supply voltage and then the MOSFET which outputs 20V 4.2A at drain will be going to the cathode?Will it not make a difference because the current can only go one way on the LEDs.Lastly does it matter for this scenario if the resistor is on the anode or cathode of each LEDThanks so much for the help

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 16, 2023, 12:37:59 am
content: QuoteNo, it doesn't matter at all.Quote from: Njackste on May 16, 2023, 12:05:13 amI think you are confusing the MOSFET's specification (That it can handle up to 20V @ 4.2A) with what will happen with a 5V supply. See diagram:

author: Njackste
author_rank: Contributor
timestamp: May 16, 2023, 12:42:54 am
content: I understand now for the LEDs, Thanks so much.But if I were to use this MOSFET (IRLML2502TRPBF) in a completely different circuit in order to increase the voltage or current what would it output, there is so many values in the datasheet that I just don't get

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 16, 2023, 03:52:45 am
content: A FET cannot increase voltage or currentall by itself.All it can do is switch/control the current through it.To increase voltages or currents you'd need additional components like inductors, capacitors, ICs, etc, to create a switching power supply. How that supply is designed, in addition to the specs of IRLML2502TRPBF, would determine what the output would be. Your choice of FET would definitely put a max limit of the power that could be outputted by such a supply, but there's no simple answer to that.A better way, would be to decide at what you want to do, and then choose the circuit and components based on that.

author: Njackste
author_rank: Contributor
timestamp: May 16, 2023, 01:54:09 pm
content: Thanks so much