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MOSFET question

Discussion in 'Electronic Design' started by mc, Jun 1, 2006.

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  1. mc

    mc Guest

    Is there a MOSFET similar to the 2N7000 that switches fully on, reliably,
    with 3 volts on the gate? I'm thinking of something to be driven by a
    low-voltage microcontroller.
     
  2. DJ Delorie

    DJ Delorie Guest

    I was just looking those up yesterday (just research, no experience).
    For example, the BSS84W (although p-channel) from Diodes Inc. Has a
    max Vgs(th) of 2v, and about 1.1 ohms typ "on" at 2.5v (my design
    voltage). Try looking at others in that family.
     
  3. Try the Fairchild NDS331N for up to 1.3 A current. Gate switches at TTL
    levels.

    Mario C.
     
  4. Mac

    Mac Guest

    This topic came up not long ago. Boki was trying to figure how to drive
    some LED's from a low voltage circuit.

    IIRC, there were some decent, cheap candidates aired.

    OK, I just went and looked. Short answer, try the BSS138.

    For further reading, here is the message ID and subject for the first
    post:

    Message-ID: <>
    subject: 2N7000's Vgs

    HTH

    --Mac
     
  5. mc

    mc Guest

    At first sight that doesn't look any different from the 2N7000. The
    manufacturer doesn't publish specifications for Vgs lower than 4.5.

    What I see here is a point of unreliability. You could choose a type that
    *usually* has low Rds at Vgs=3V, but some units of the same type, still
    perfectly within spec, don't.
     
  6. Greg Neff

    Greg Neff Guest

    I use the ON Semi BSS138LT1 for low voltage applications. The data
    sheet specs max rDS(on) of 10 ohms at VGS = 2.75V, from -40c to +85c.

    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  7. Another good N-channel MOSFET that I have been using lately is the IRLML2502. (DigiKey has them for 77 cents.)
    When turned on with 3v on the gate, the resistance from Drain to Source is only .035 ohms! Will allow up to 4 amps
    Drain to Source.
    -Craig
     
  8. mc

    mc Guest

    OK, I just went and looked. Short answer, try the BSS138.
    Thanks, I'll check it out.
     
  9. mc

    mc Guest

    Anything like that in TO-92 or TO-220 so that it can be used on breadboards?
     

  10. The closest thing to a TO-92 I see is the IRLD024PBF in a 4 pin DIP
    package:

    http://www.irf.com/product-info/datasheets/data/irld024pbf.pdf

    IR makes quite a few TO-220AB package logic level N channel mosfets. E.G.

    20 volt .004 ohm IRL3716PBF
    http://www.irf.com/product-info/datasheets/data/irl3716pbf.pdf

    30 volt .003 ohm IRL3713PBF
    http://www.irf.com/product-info/datasheets/data/irl3713pbf.pdf

    40 volt .004 ohm IRL1404PBF
    http://www.irf.com/product-info/datasheets/data/irl1404pbf.pdf

    55 volt .008 ohm IRL2505PBF
    http://www.irf.com/product-info/datasheets/data/irl2505pbf.pdf

    etc.
     
  11. Mac

    Mac Guest

    Well, you didn't specify too clearly what you were trying to do.

    The Vgs(th) max for the BSS138 is 1.5V, and that is with 1mA. The
    min transconductance is 0.12 A/V, so with Vgs = 3V, you could argue
    that that will take you pretty far into the "on" region, depending on what
    you are trying to do.

    Best regards,
    Mac
     
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