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Efficiency of a transistor?

  1. Jan 30, 2012 #1

    I've just been playing around with a 2n3904 transistor and 3V coin battery. I noticed when powering an LED with a transistor switch the LED is significantly dimmer than when powering the LED directly.

    Why is this? Is it because 0.7 volts is siphoned off just to turn the transistor on?
  2. jcsd
  3. Jan 30, 2012 #2
    The saturation voltage. Even if you drive the base hard, their is a 0.3V drop across the CE of the transistor so only get 2.7V instead of 3V. If you want to get rid of this, get a MOSFET with low rds.
  4. Jan 31, 2012 #3
    Thank you for the reply.

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