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Thread: MOSFET Switch

  1. #1

    Default MOSFET Switch

    Hi all,
    I am stuck on a typical noob question;
    I am using BUZ71 mosfets as switches. One in particular needs to switch +12v to an "Enable" line for a car power amplifier; ie when the picaxe line is high, the line has +12v on it, when the picaxe line is low the line is floating / 0v.

    I have connected a mosfet up as a high side switch but I just get +12v all the time at the drain, no matter what I go with the gate (ie high / low / direct to ground etc.) I know it's a simple problem but where am I going wrong?

  2. #2
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    @bigvalven

    I know nothing about Mosfets, but, these threads might help.

    http://www.picaxeforum.co.uk/showthr...ighlight=BUZ71

    http://www.picaxeforum.co.uk/showthr...ighlight=BUZ71

    There is a lot more info to be found,
    by searching the Forum.

    e

    Actually, the term MOSFET seems to cause
    gnashing of teeth, in some of the posts that I've scanned!

  3. #3

    Default

    Gnash, gnash..
    Seriously though I think the thing that gets some people excitable (not me as I am SO patient and wonderful) is the fact that people sometimes think you can bung any old MOSFET in to relace a mechanical switch or relay in any old application.
    "Oh, it says 50 Amps in the spec" - sadly there is more to it.

    Generally: P-Chans for high side an N-Chans for Low sided switching.
    Yeah, yeah... I know but lets keep it simple.

    So, before a great long para, I must ask a question:

    Is your experience of electronics extensive?
    Do you know (just roughly) the diference between P-chan and N-Chn MOSFETs and PNP and NPN Bipolars. For simple switching they can be comparable.

    For your app (and with the limited info supplied) you could do high-sided switcing with a P-chan MOSFET and stuff. To make it safer you could do it with a P-Chan and a signal NPN bipolar and resistors.
    (I will asume you are familiar with all the suppression and transient caveats).
    Haven't got time to draw or search for a piccy, sorry.

    One tip: have a nerd-around. There are a few sites which explain basic transistor circuits with some very nice diagrams. These should be added to your e-Grimoire.

    You'll be fine. Good luck.

  4. #4

    Default

    Dippy, my knowledge is good (TAFE Diploma in Electronic Engineering, which I am almost ashamed of when I have to come on here to ask a question like this,) somewhat classically we were taught how MOSFETS work in amplifiers etc but never actually used them in a simple switch!

    I am aware of typical caveats when using MOSFETS especially in demanding situations but these will be switching 500mA or so maximum, and typically between minutes and hours cycles so it's hardly a demanding job - as long as they are turned on fully.

    Using a P channel FET on the high side; if it wasn't nearly midnight it probably would have clicked

    I also need to buy some more suitable devices; on a Sunday night it wasn't an option but I can do so today.

  5. #5
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    Default

    .................................................. ......._____________12V
    .................................................. .......|...........|
    .................................................. .......10K........|
    .................................................. .......|...........s
    ......................................____________ |......|__|
    .....................................|C........... ......|---g-|__p-fet
    .picaxe out>-----1K R----|<...npn...................|...|
    .....................................|E........... ..................d
    .....................................gnd.......... ..................|____>your output
    .
    .high on picaxe out grounds npn collector. Pfet is turned on.
    .
    .
    .
    .

  6. #6

    Default

    Jglenn, that's confirmed my suspicions nicely Thanks!

    Also for others wanting to investigate high-side load switching with FETs, I have found this site to sum things up very well:
    http://www.industrialcontroldesignli...owto/199204060

    Thanks for everyone's help.

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