Logic level MOSFET circuit, switching something like 5V?

Discussion in 'General Electronics Chat' started by Nicholas, Apr 26, 2016.

  1. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
    Hi all

    Yet another question about logic-level MOSFET circuits. As of now, my circuit is switching 40-ish V very fine,
    but I was wondering if there is any reason why the same circuit shouldn't be able to switch a lower voltage,
    like 5V or 12V...or maybe even 3V. Does it seem odd to use 5V to switch 5V? Well, I thought of doing it since
    my USB controller has the usual 500mA USB limit.

    So, using a logic level MOSFET to switch 5V? Go/no-go?


  2. Marley


    Apr 4, 2016
    Logic-level MOSFET refers to the gate voltage required. Or to be more precise: the threshold voltage. These MOSFETS can be driven from common logic level signals 3.3V to 5V.

    No problem having the gate voltage higher than the drain circuit voltage.

    The only thing you need to consider is the ON resistance and the current you are switching. This will determine the drain - source voltage drop when switched on. Should be fraction of a volt.
  3. Nicholas

    Thread Starter AAC Fanatic!

    Mar 24, 2005
    So it will probably be fine? I am going to light a LED or two with one MOSFET. I know a IRL540
    is overkill for this, but then I can use the same circuit for different voltages, 5V or 40V without
    needing to change anything. Difference is, the 40V application will draw 10 amps or more, the
    5V will draw 0.1A or something like that :)
  4. crutschow


    Mar 14, 2008
    Logic-level refers to the gate-source voltage required to fully turn it on as shown in the data sheet where the Rds(on) value is specified.
    It may be related to the threshold voltage but that voltage doesn't determine whether it's a logic-level type MOSFET (as many seem to assume).
    The threshold voltage is where the MOSFET barely starts to conduct current.
  5. takao21203

    AAC Fanatic!

    Apr 28, 2012
    Fdv301 or fdv303, bss138, 2n2007

    Easy to work with
  6. crutschow


    Mar 14, 2008
    You'll be fine.
    When the MOSFET is fully on, it acts like a low value resistor [as shown for the Rds(on) value in the spec sheet].
    This ON resistance value varies little from zero current up to the MOSFET's maximum current rating.