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Transistor Help

Discussion in 'General Electronics Discussion' started by M4rsH, Mar 5, 2017.

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  1. M4rsH


    Mar 5, 2017
    Hi guys,

    I do not have a whole lot of knowledge concerning electronics so I am turning to the forums for information.
    In my circuit I want to control a servo motor. But I only want to power this servo when I am using it.
    Here comes the transistor in the picture. With a USB supplying 5V to the collector of the transistor, and a raspberry Pi supplying 2.5V to the Base. I thought the Emitter would pass through about 5V, but it is only passing 2V.

    I need to power my server with roughly 5V and be able to control the power to this circuit with my Raspberry Pi.

    Is this the right approach? What am I doing wrong

    Thanks in advance for your help
  2. duke37


    Jan 9, 2011
    The transistor needs about 0.6v between base and emitter. Subtract this from 2.5V and you will get close to what you measure.

    If you wish to use a common ground, you can use a p channel mosfet in the 5V line and drop its gate to turn it on. The resistance of the fet could be much less than 1Ω.
  3. BobK


    Jan 5, 2010
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