How to choose FET for my project.

Discussion in 'General Electronics Chat' started by stoopkid, Jun 9, 2012.

  1. stoopkid

    Thread Starter Member

    Mar 3, 2011
    I need to drive about 1.5A at 5v at PWM frequency for an Arduino. I'm having a bit of a hard time figuring out what mosfet I want. This is what I've come up with so far. The Rds(on) seems low enough to keep my dissipation at only a few mW and it seems to be able to sink the current I need it to but I have a few questions...

    Is the threshold voltage what voltage is required to saturate it? Since it has a 1.8 threshold, do I need to limit the voltage to 1.8? Can I drive the mostfet directly from my arduino?

    What spec decides whether or not I can PWM with it?

    If this one is no good for what I want, could someone suggest a better part and explain why?

  2. stoopkid

    Thread Starter Member

    Mar 3, 2011
    Ok, I don't think that one is logic level... How about either of these two? Do they seem like good choices or is there anything better for a similar price?

    The first leaves me with about 43mW of dissipated power. Is that totally reasonable without a heatsink or should I spend a bit more for the second and get 5mW of dissipation?

    Or is there something else better all together?
  3. crutschow


    Mar 14, 2008
    The Threshold Voltage is the voltage at which the device just starts to turn on (note the measurement current at which they measure that voltage). You need to use the gate-source voltage at which they measure the ON resistance (typically 10V for a standard MOSFET and 5V for a logic-level type).

    The maximum Vgs is usually around 20V and is listed in the Absolute Maximum Rating chart on the data sheet.

    The important parameters for the MOSFET in a switching application such as PWM, is the ON resistance and the gate capacitance (charge).The amount of gate charge you have to charge and discharge determines the speed you can switch the transistor as determined by the amount of gate drive you have (the current output of the gate driver).

    The Arduino output has a relatively small current capability so you may need to add a buffer driver to properly drive the MOSFET. There are MOSFET driver ICs specifically designed for that purpose.