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Why use a transistor to drive a motor?

Discussion in 'Electronic Basics' started by Martin Cote, Jun 26, 2007.

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  1. Martin Cote

    Martin Cote Guest

    Hi, I was looking at the following circuit:

    and I was wondering why the transistor is used in the first place. From
    what I can see, it is used to amplify the current in order to drive the

    Since the motor seems to require 470mA to run, why not simply use a
    current limiting resistor instead? With a 5V battery and a 10 ohm
    resistor, the current should be around 500mA which should be able to
    drive the motor, no?

    What am I missing?

    Thanks all,
  2. Eeyore

    Eeyore Guest

    No. All the voltage ( and hence the power) would be across the resistor wouldn't
    it ?

    The transistor isn't limiting the current. It's acting as a switch. It can be
    driven from logic level outputs.
  3. 10 ohms will draw 500ma from 5V, but what voltage does the motor need?
    Motors usually do better with a low-resistance constant-voltage source.
    The current will be high at start, then drop as the motor comes up to
    speed and develops back EMF. There are exceptions; some motors are
    designed to apply constant torque to a load whether it's turning or not,
    but those are usually AC induction motors.
  4. DJ Delorie

    DJ Delorie Guest

    Which is enough to fry most logic circuits. The transistor lets a
    small current (2.15 mA) control a larger current (470 mA) in cases
    where the control current is simply not powerful enough.

    Most TTL/CMOS outputs are only rated for a few milliamps.

    The "hfe" spec is the amplification factor - in this case, 300 means
    that a 470mA load needs (470/300 = ) at least 1.57 mA of input
    current. At 2.15 mA input, the load can draw up to 645 mA.
    Approximately. Good to leave some margin too.
  5. JeffM

    JeffM Guest

    The concept of a "relay" is well established in this field.
    As a semiconductor has no moving parts,
    it is often better than an electromechanical relay
    at using a low-level signal
    to get something with greater power requirements to run.

    You are also ASSuMEing that *you* always get to decide
    on the nature of the originating source used to turn something on .
    We often have to tie into someone else's existing design.
  6. Martin Cote

    Martin Cote Guest

    Ok. As I understand it, the transistor provides a way to give more
    current without adding resistance in the circuit, therefore, keeping the
    potential 'intact'.

    Is this correct?

  7. No.

    If whatever you have that turns on and off can only output 5mA, then it
    can never supply enough current for that motor that needs 10mA. A resistor
    will not do it, since a resistor can only limit current.

    The transistor switches the current. It's an amplifier, taking the low
    current on/off signal from (presumably) the IC, and providing enough
    current to the motor.

    Any limiting resistor in series with the motor at that point is to limit
    current to the motor. But that's because the transistor can pull a lot
    of current through the motor, unlike the output of the IC.

  8. Eeyore

    Eeyore Guest


    The transistor when used as a switch here is either off ( no base current) or on (
    base current applied ). It works like an on-off switch controlled by the input to the
    base resistor. So a logic 1 from typical digital lelectronics ( e.g. a microprocessor)
    will cause the motor to run and a logic 0 will turn it off.

  9. Martin Cote

    Martin Cote Guest

    Got it.
    Thanks all for your answers!
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