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  1. #1
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    pnp mosfet

    Hi all,

    I wish to be able to switch 5VDC using a mosfet. What i want to do is to be able to control the supply of 5VDC to a particular part of the circuit through an I/O from a microcontroller.

    I am more familier with transistor with very little experience in FET. Can anyone recommend me common mosfet used in switching? The expected current to the isolated circuit is estimated to be around 400mA.

    Is there actually any advantages in using a Mosfet as compared to using a typical pnp configuration? The only i can think of now is the voltage drop inherent in transistor.

    How much more expensive is a typical mosfet as compared to a normal switching transistor?

    Thanks and have a good day!

    Chicken_feet

    •   Alt17th July 2006, 13:08

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    how to use a mosfet

    Keep in mind that a MOSFET has a gate threshold voltage, so in some cases 5V application can not switch them ON, besides that similar conditions apply: heat= U x I ..
    For your application I would suggest the very popular 2N7000-series with the Vgs<3V ..
    http://www.fairchildsemi.com/ds/2N/2N7000.pdf
    Pricewise they will be in the same region as BJTs ..

    Regards,
    IanP


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    pnp mosfet switch

    Hi,

    You actually need a P-channel MOSFET to do the job.
    But as IanP said it's the -Vgs voltage of the FET that
    will decide if it will work or not, you could try a 2N7060.
    If it doesn't work use a PNP darlington transistor as a
    replacement for the FET.
    Another solution is the use of a switchable 5 volt
    regulator like the PQ05R01.

    on1aag.


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    2n7000 switch

    Hi all,

    Thanks for your infor.

    on1aag, you mention about P-channel mosfet, but the datasheet provided is a n-channel. Any other p-channel recommendation?

    Any good circuit tutorial for fet?

    Actually, i don't quite understand on the Vgs thing. Anyway, will do the necessary read up. Any recommendation on a higher current mosfet? Most probably i may need around 500mA. The 2N7002 can only take a max of ~250mA continuous...

    Lastly, is it really necessary for me to use a mosfet to do what i need to do, instead of using a PNP transistor? Is the extra work neccessary since i can always use a pnp and do it in a short time?

    Thanks for all your infor.

    Have a nice day.

    chicken_feet



    •   Alt18th July 2006, 05:41

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    how to use mosfet

    If you need simple switch on/switch off function both, BJTs and MOSFETS, wll do similar job ..
    The small inconvenience while driving BJTs is the base current, but in the most cases this is not incovenience at all ..
    Unlike BJT drive circuits which require a base resistor to control the base current, a MOSFET drive circuit is designed to connect the gate directly to a voltage bus or supply with no intervening resistance ..

    The Vgs stuff is in some cases important, as if Vgson > 5V, 5V systems will not be able to turn them ON (open the Drain-Source channel) ..

    More on MOSFETS you can read at:
    http://www.powerdesigners.com/InfoWe...s/mosfets.shtm
    or
    http://www.eetasia.com/ARTICLES/2000...ICD_PD_AN2.PDF
    to name just a few ..

    Lastly, is it really necessary for me to use a mosfet to do what i need to do, instead of using a PNP transistor? Is the extra work neccessary since i can always use a pnp and do it in a short time?
    Use PNP ..

    Regards,
    IanP



    •   Alt18th July 2006, 06:38

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    how to use a mosfet as a switch

    Hi Ian,

    Thanks for your many information.

    I have checked out the datasheet you point to me 2N7000 mosfet. Yes, now i understand what you mean by the Vgs voltage.

    Upon further checking shows that mosfet may not be suitable for my current case...

    The Drain-Source Resistance (Rds) when 'on' is very high! The resistance are at least in the range of 2 to 3 Ohm. If the Drain current is 400mA that would means a Vds of 0.8V to 1.2V, and that is based upon a very conservative estimation...

    PNP may have this problem also. I have just checked a general pnp (BC857) and it shows that it has a drop of Vce = 0.65V based on Ic of 100mA when 'on'. Any popular pnp that has Ic(max) of around 500mA but has a low Vce(on) drop?

    I wanted to drive the pnp/mosfet using the output of a 74hc574(374) latch. I am hoping to be able to control(on/off) the power to several independent modules(circuitry). The drop in voltage makes it more complicated. I guess I will have to use some dc to dc converter with some shutdown capability.

    Thank you for all.

    chicken_feet


    - the problem is that i have yet to find one dc-dc converter that can provide more than 400mA at 3.3V. any recommendations?



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    mosfet pnp

    Assorted small-signal PNPs Ic>0.5A:
    ZTX550, ZTX751, ZTX753, BD136, BD137, BD140 ..

    P-Channel MOSFETs:
    IRF9530N (Rdson<0.2Ω), IRF9520 (<0.6Ω) ..

    As dc-dc step-up, check out MAX710/711:
    http://www.reinerziegler.de/GPS/regler.pdf
    or LTC3426:
    http://www.linear.com/pc/productDeta...31,C1061,P8430

    Regards,
    IanP


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    mosfet pnp switch

    sorry, use the wrong term. it should be Regulator with shutdown capability. Thanks Ian.



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    how to use mosfet as a switch

    Try something like the LM2574N-3.3 ..
    http://www.onsemi.com/PowerSolutions...id=LM2574N-3.3
    It has dedicated On/Off input .. see picture below ..

    Regards,
    IanP



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    pnp mosfet applications

    Greetings Everybody,

    With regards to this FET:
    "For your application I would suggest the very popular 2N7000-series with the Vgs<3V ..
    h**p://www.fairchildsemi.com/ds/2N/2N7000.pdf "

    I believe for 500mA drain current you need to drive the Gate with more than 4V, that comes from the graph "Figure 10. Gate Charge Characteristics" in above mentioned 2N7000-datasheet.
    3V threshold is for Id=1mA.

    nike



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    ztx 751 fet

    To: nikeplato

    2N7000 or 2N7002 is not going to work, internal resistance in ON state is much to high. 4 Ohm max 7 Ohm !!!!. Secondly it can only handle some 200mA MAX.

    Better go for a Si2309DS P Type MOSFET or Si2308DS N Type MOSFET can handle 1 - 2 A !!!. and some 0.2 - 0.3 Ohm ON Resistance. Vth = 2V.

    regards,

    Paul.



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    2n7002 mosfet switch using 3v pic

    Hi Paul,

    Thanks for your answer but I wasn't going to use that one for anything, I just quoted IanP, and made an observation that we should probably take into consideration the Gate Charge Characteristics, when looking for the Gate threshold voltage, I didn;t look to other specs.

    nike



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