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simple mosfet switch

Discussion in 'General Electronics Discussion' started by mcgilles, Feb 20, 2012.

  1. mcgilles


    Feb 20, 2012
    hello, glad I found this forum!

    I am working on a project and need some assistance.
    I want to build a simple MOSFET switch to switch on a high current (up to 5A) load from a micrcontroller (arduino) 5V logic outputs. here are my specs and requirements:

    standard 5V logic outputs from the microcontroller to my switch, 0V off, 5V on.
    I must switch the positive, not the negative, so I have to use a P channel mosfet
    for saftey reasons, I need my outputs to fail off, the microcontroller fails to off, and I need my output to fail to off (ie, positive trigger, positive output) so my plan is to run the arduino digital output through an 74HC04 inverter, then connect that to the gate of the MOSFET.
    the load runs off a different voltage than the microcontroller, I need to switch approximately +12 to +15VDC.
    I do not want the circuit dependent on the load current, I want to make transistor outputs capable of operating devices at 12-15VDC drawing anywhere from a few milliamps up to 5A of current.

    is doing this just as simple as using a logic level P channel MOSFET with a drain/source rated for >5A (I want one rated for 10 just in case) and 15+ volts?
    then making the connections: arduino output to inverter input, inverter output to MOSFET gate. MOSFET drain to the +15V/5A output to have the load connected, and the MOSFET source connected directly to the +15V source? is it really that easy?
    do I need resistors anywhere like on the gate? my understanding is that a logic level mosfet can be connected directly up to the IC's output is that correct?

    something like the 12V pic sample on this web site:
    except couldn't I do away with the NPN using the inverter instead? are the resistors still needed?

    can someone please help me design this circuit? and I'd really appreciate some help with selecting a MOSFET that will do the job. I'd prefer a TO-220 form factor device.

    the above is what I really need help with but another related question:
    these outputs may also be powered by other sources. if the transistor output were on or off and the output received +12-15V from some other source would that cause any problems? if so would just inserting a properly rated diode on between the transistor and the load solve the problem preventing any backfeeding of voltage from other sources? I'd rather avoid that if I don't need it to prevent the voltage drop, but if its needed I can add it.

    thank you in advance!
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    One of the easiest ways of selecting a mosfet is to go to the digikey web site and search for "mosfet". You'll get a series of headings, and you typically pick the one with the most matches, in this case "FETs - single" under "discrete semiconductor Products".

    You will then get a parametric search, and you can search for devices having the characteristics you want. For example, you might first search for P channel, then perhaps an appropriate current rating (be generous, pick those which can handle at least twice the current you need and select all the way up to something ridiculous (use shift-click t select a range)), and also the package (watch out TO-220 can be described in a number of ways (use ctrl-click to select a collection of values))

    Keep selecting more and more stuff (maybe things like Vds(max)) until you get a relatively small number of devices listed.

    Then I typically sort them by price and look at the cheapest ones.

    You can also select by "in stock" and ROHS if that's important to you. I generally don't for the first search because if I find a good device, I can always shop around to see if anyone else has it.
  3. GonzoEngineer


    Dec 2, 2011
    Do you guy's have Digikey in Australia?
  4. mcgilles


    Feb 20, 2012
    thanks! following that method I came up with something that would probably work, but $4.37 each for the cheapest one that's in stock!

    I found this one earlier that's much less expensive, logic level P channel mosfet, 24A

    I imagine both would work.

    How about the circuit? would something like this be what I need (I hope I'm allowed to do this)

    or is it still possible to use a logic level mostfet in my situation without the extra NPN? I'm perfectly fine with the NPN if that's what it takes to make it work, I just don't want to blow up too many $1.30 transistors trying to figure it out.

    I don't have any parts handy to try the circuit above or I'd just risk it.
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