charging supercap through transistor

Discussion in 'Power Electronics' started by mikewax, Oct 3, 2016.

  1. mikewax

    Thread Starter New Member

    Apr 11, 2016
    9
    0
    this may seem like a really dumb question, but the transistor dynamics just make my head spin.

    i've wired it up like this:

    5v-----12ohms------npn------1Fcap------G

    the npn is switched from an I/O pin like this:

    pin(5v)-------1kohm-------base

    according to an online transistor calculator, i can't get more than about 90mA through the transistor (2n2222a) with this circuit. i need like 4 times that much current and i need to be able to switch it on and off from the arduino. does anyone know about this?

    thanks, mike
     
  2. #12

    Expert

    Nov 30, 2010
    16,250
    6,746
    It takes two.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    If you use a logic-level MOSFET transistor as a switch, your current is limited by only the 12Ω resistor instead of the base current for the transistor.
     
  4. mikewax

    Thread Starter New Member

    Apr 11, 2016
    9
    0
    talk about senior moments, i had a whole senior day. THANK YOU expert and wayneh, i'll order the MOSFET and use the stacked BJTs in the meantime. now i can get back to work. :)))

    one more thing though. if i gate the MOSFET with an analog output pin(0-5v), can i thottle the gate-source current up and down? i'll write a routine that will read the capacitor voltage through one pin, calculate dV/dt, and then increment/decrement the voltage on the gate pin untill the dV/dt is equal to some constant value. sound like a plan?
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,771
    1,103
    The FET current changes dramatically with only a small change in gate voltage around the Vgs(threshold) value, so you would need a very fine resolution D/A output to give adequate current control. A bjt might be a better option than a FET for that purpose.
     
  6. mikewax

    Thread Starter New Member

    Apr 11, 2016
    9
    0
    OK i'll try it thanks. :)
     
Loading...