MOSFET shopping, recomended devices for the experimenter?

Discussion in 'General Electronics Chat' started by Hamlet, Jan 8, 2016.

  1. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    I need a selection of MOSFETS for my kit. I think I understand
    the difference between P & N channel, volts, amps, logic level,
    but what I don't get is when the device is rated for ohms...

    http://www.taydaelectronics.com/t-transistors/fets-mosfets.html

    There seems to be a wide range in ohm ratings, RDSON? From
    high, say 8ohms, to .8 ohms, & very low 9Meg? I don't know enough
    about this to make a selection.

    I'd appreciate any & all recommendations for individual
    devices that I should include in my kit. I'm experimenting,
    and learn best with an iron & breadboard. I expect two or
    three each of perhaps 12 different popular mosfet devices should
    cover 90% of the projects I anticipate building, but that's
    just a guess.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    It depends on your application. For power switching applications, low on resistance is desirable. For most other applications you want resistance because that's how you control current.
    What you stock depends on what you need. Because of the relative cost between MOSFETs and BJTs, I have 1 NMOSFET and 1 PMOSFET on hand, and a dozen or two different BJTs.
     
  3. MrSoftware

    Active Member

    Oct 29, 2013
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  4. bertus

    Administrator

    Apr 5, 2008
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    MrSoftware likes this.
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    That kit has 1 N channel MOSFET (qty 6 2N7000) and no P channel.
     
  6. MrSoftware

    Active Member

    Oct 29, 2013
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    Great point, I didn't realize that. It is heavy on the BJT's isn't it...

    Is there a reason that BJT's might be used more often than MOSFETS in lower power electronics?
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,649
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    Hello,

    Here I have a zip file with 2 excel sheets.
    One for N mosfets and one for P mosfets, all from IRF.

    Bertus
     
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  8. dl324

    Distinguished Member

    Mar 30, 2015
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    Cost comes to mind. Many GP BJTs are a nickel apiece. The only MOSFET that comes close is 2N7000. I haven't found any P channel MOSFETs at that price point.

    I'm careful when I handle MOSFETs, but I've damaged several 2N7000 in the last few weeks. They worked fine and then started exhibiting largish leakage currents. Can't remember the last time I had a damaged BJT...

    The main advantage for MOSFETs is negligible bias current, but the largish and not tightly controlled threshold voltages are often problematic; particularly for low voltage circuits. Just my opinion...
     
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  9. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    The ON resistance of MOSFETs is important in switching applications where, ideally you want zero resistance when ON (some MOSFETs have ON resistance in the low milliohm region).
    Not only does that deliver more voltage to the load but it minimizes dissipation in the MOSFET which might otherwise require a heatsink at high currents.

    One MOSFET variable to note is the minimum gate-source voltage to fully turn the device ON.
    Standard MOSFETs generally require a Vgs of 10V to be fully ON.
    Logic-level type MOSFETs will fully turn on at logic level signals (3 to 5V Vgs depending upon the particular MOSFET).
    Note that this is NOT the Vgs(th) (threshold voltage) which is typically a few volts, it is the Vgs used where the Rds(on) value is specified in the data sheet.

    MOSFETs are generally preferred in power switching applications since they take no steady gate current to be ON (but they do have a significant gate capacitance that must be charged/discharged) and they can have a much lower ON resistance than a BJT.

    The rule of thumb is that BJTs need a base current about 1/10th of the collector current to guarantee them to be fully turn on (saturate) which is independent of the transistor Beta gain (as shown in their data sheets), and that can waste a fair amount of power and require a large base driver at high collector currents.

    BJTs are useful in many lower power switching and amplification applications since they have a low input ON voltage of only about 0.7V and are relatively cheap.

    Just by looking at the application requirements you can usually see whether a MOSFET or a BJT is likely the better device to use.
    Often you will see a mixture of devices in a circuit, where the BJTs handle the low level signals and MOSFETs handle the higher level/output signals.
     
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  10. hp1729

    Well-Known Member

    Nov 23, 2015
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    TO-92 case? 2N7000 or BS170 for N-MOSFET. VP0106 or BS250 for P-MOSFET. Stock maybe 10 P-MOSFETs and 40 or 50 N-MOSFETs. N- run maybe $0.10, P- may run $0.70. I use a lot more N-.

    TO-220? Buy as needed. Too pricey to stock stuff that may go obsolete before you can put it to use. It seems like newer and better devices come out every year.

    Re: RDSON
    Source to Drain resistance. Lower resistance means less voltage lost across the transistor. You will find higher current and higher wattage devices have lower resistance.
     
  11. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    I just dropped $40 on parts, and there wasn't a mosfet amongst them.
    I've been yanking mosfets from old boards, and building a datasheet
    library for them. I have a small collection, so I can try this one, or that
    one, and compare. I wish there was an all around go to mosfet, like
    the TIP series of bjt.

    I hate having an idea, but waiting on parts, or paying $10 in shipping
    for $2 in parts. I've been goofing with power supplies, so I know I need pots,
    0.1ohm power resistors, and TIP35c, and so forth. China is killing the small
    parts business, and say what you will, it's the best way for me to get into the hobby.

    I loved this, thanks for the tip:
    Components Selection Guide
    http://forum.allaboutcircuits.com/threads/components-selection-guide.65137/
     
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