how to select suitable mosfet for switching

Discussion in 'General Electronics Chat' started by athulascc, Jan 31, 2015.

  1. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    here image shows my micro controller based project.
    I need to drive large display so I have used uln2803 Ic.
    also I need to multi plexing.
    So I have used common anode to multiplex , but there I can't use relay.

    I selected BC327 but I understood that I can't turn off it using 5v micro controller output because emmiter in 12V.

    I need mosfet somthing really same as relay. when I give 5V it should turn on or off and for 0V it should do the inverse (on or off).
    it's maximum current should be about 500mA because display needs 250mA when it fully lit.
    Capture.JPG

    any one can help me.

    please suggest me a mosfet.
    if there is IC which has 4 switches like I mentioned also very good because I need 4 mosfets.

    thank you
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
  3. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    500mA. max. if it is more also about 350mA fine
     
  4. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    yes I know I can use PFET. but I am not sure how. I am going to draw pcb and handover to manufactures. so I need your help
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    For that, you dont need a PFET. A simple PNP switching transistor will do. Follow the link to see what else you need...
     
  6. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    I need to do switching. So I like to use PFET (because I think they are good in high speed switching)
     
  7. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    I saw the link. there you have used level shifter. will that circuits works fine for high speed switching. I mean switching speed should be enough past then eye can't see any flashing

    isn't there any simple circuit like one FET ?
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Why do you think you need high speed switching for multiplexing a display?
    Do you understand how to follow a posted link?

    Click on this
     
  9. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    [​IMG]
    will this work?
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Think about this: If you use a PFET, its source is tied to +12V.

    What gate voltage is required to turn it off?

    What gate voltage is required to turn it on?
     
  11. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    I am new so please forgive me if I have done any mistake.
    I click on the link and check that design.
    it is looks good.
    but supply is 24V
    I think it can be adjusted.
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    W
    Why do you think you need a power device like the IRF530 to do the level shifting?

    What is the threshold voltage of a IRF530?
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Yes, and I am trying to get you to learn something instead of just handing you a ready-made answer...
    Yes it can.
     
    Last edited: Jan 31, 2015
  14. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
    ok.
    I will use that design.
    please confirm this for my knowledge.

    there 2n3904 is the level shifter.
    darlinton pair provides the sufficient current
    am I correct?

    is there any IC we can buy directly for this?
     
  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Since you are switching only 500mA, a simple PNP like a 2n3906 will work.
     
    • 137.gif
      137.gif
      File size:
      35.6 KB
      Views:
      43
  16. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
  17. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,779
    1,103
    ? Don't think so. It's rated for 200mA :(.
     
  18. athulascc

    Thread Starter Member

    Aug 15, 2014
    86
    1
  19. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    I dont like the emitter follower. That circuit is bad because the Ib =>Ic/10 is not even close to being full-filled...

    Look at this sim. Note that this driver cannot supply 350mA.

    137a.gif

    Here is a fixed version, but look at the power dissipation in Q2

    137b.gif

    Back to the one I recommend: All of the base currents have the right ratios with respect to the collector currents. I selected the appropriate PNP for sourcing 350mA.

    137c.gif
     
    Last edited: Jan 31, 2015
  20. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    Typical display multiplex strobing is about 500Hz - you don't need fast for that.
     
Loading...