Switchable const curr source

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

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    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
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    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
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    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.
     
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