Calculating transistor resistors

Discussion in 'General Electronics Chat' started by Dritech, May 27, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi all,

    I need to use the circuit in the attached diagram where I will be driving the transistor base from a 3.3V GPIO. For the transistor collector I will be connecting a 5V supply instead af 3.3V.
    What equation can I use to determine the resistors values (without assuming resistor values) ?
     
    Last edited: May 27, 2015
  2. crutschow

    Expert

    Mar 14, 2008
    12,988
    3,226
    Nothing is attached.
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Sorry. I updated the post.
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Why you not put the buzzer at the c of bjt?
    And how is the V/I of buzzer?
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    As its an emitter follower; you don't really need those extra resistors - the emitter will follow the base voltage, except that you lose 0.7V in the B/E Vf.

    If you were running at 3.3V the Vbe drop might possibly prevent it working. Even at 5V you could be cutting it fine, in which case a common emitter stage would be better - if your load has to be ground referenced; you'd need a second (PNP) common emitter hanging from Vcc.

    Then it starts getting complicated because the common emiter is inverting, but nothing that can't be sorted out with a few more posts.

    A common emitter does of course require a resistor to limit the current into the base, find out the max current you micro can source and calculate the resistor taking Vout-high and subtractin 0.7 Vbe, and just perform Ohm's law on what's left.
     
    Last edited: May 27, 2015
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,516
    1,246
    If you want the maximum loudness from the buzzer, change the configuration to a common-emitter circuit:
    Eliminate R1 and the connection to GPIO pin 1 (+3.3V).
    Tie the emitter to GPIO pin 6 (GND)
    Connect one end of the buzzer to the collector.
    Connect the other end of the buzzer to +5V.
    Note: the buzzer might have polarity markings. If so, connect the + pin to +5V and the - pin to the collector.

    ak
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    As far as I can see; the TS is asking how to calculate the resistor values AS IF a common emitter were being used, I think they'll have voltage overhead problems with the emitter follower.
     
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