Switchable const curr source

Discussion in 'General Electronics Chat' started by atferrari, Jan 19, 2014.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    2,689
    801
    Switchable constant current source for a green LED.

    Switching is commanded with logic signals +5V and 0V and constant current is around 15 to 20 mA.

    Before I buy components, given the circuit (attached) could I go away if Vcc is just 5V, replacing the MOSFET with a 2N7000?

    http://www.alldatasheet.com/datasheet-pdf/pdf/32720/TRSYS/2N7000.html
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,451
    1,068
    Here is how I would do it. Note that the emitter follower is a constant current source and switch in one. This will work provided the forward voltage drop of your Green LED is 3.2V or less.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
    5,040
    783
    2N7000 VGS = 4.5 V, ID = 75 mA,Rds typ=1.8Ω, max=5.3Ω
    Vds = I * Rds_max
    = 75mA*5.3Ω
    = 0.4V
    So if your Load is <75mA then using 2N7000 is ok.
    Otherwise you have to find some other logic level mosfet and has a low Rds.
     
Loading...