Transistor as a switch (uC output interface)

Discussion in 'Homework Help' started by minesadab, Sep 29, 2009.

  1. minesadab

    Thread Starter New Member

    Sep 29, 2009
    2
    0
    Hi, I am designing a really simple circuit to switch a relay using a uController, and I was wondering if anyone could tell me if there is anything wrong with the design I've attached?

    All the designs I've seen on the web, which use an NPN transistor as a switch, have the emitter connected to ground with the load "above" the transistor, meaning that a resistor is necessary between the uC and the transistor's base.

    Are there any advantages in that configuration, or can I use the attached design instead and use less components? (in the design, the uC is powered from the same 5V source as the relay coil)
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    This will work. The transistor never saturates so will drop at least 0.65V.
    On a 5V supply maximum relay volts will be 4.35V. Most small 5V relays will pull-in at this voltage.

    Is a resistor a big deal?
     
  3. minesadab

    Thread Starter New Member

    Sep 29, 2009
    2
    0
    JDT, I see it now - so the main reason I'd use the more common configuration is to get the full 5V across the load.

    Thanks very much for your help.
     
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