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Simplest variable voltage circuit

Discussion in 'Electronic Design' started by Infundibulum, Dec 23, 2005.

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  1. Infundibulum

    Infundibulum Guest

    I am looking for the simplest possible passive circuit for providing a
    variable voltage of 1.5 vdc to 3 vdc to a small pump motor which requires a
    few hundred milliamps. I am using 2 AA 1.5 volt batteries in series as a
    supply. The impedance of the motor is so low that series resistors do not
    work. Just looking for a way to vary the voltage of the source. Thanks.
  2. Leon

    Leon Guest

    Just use PWM, it's not a simple passive circuit but it will work.

  3. Ian Stirling

    Ian Stirling Guest

    Of course series resistors work - you just need to choose them

    I'd be looking for a power variable resistor in the 1-2 ohm range.

    A switch to swap the batteries between a series and parallel arrangement
    would work.

    If you mean smallest physically, then PWM would probably work.
  4. Guest

    Series resistance is the simplest, and yes it will work. It would drop
    1.5v @ guess 150mA = 10 ohms. Drawback with resistance is on low speed
    your batteries are giving 3v, and motor eating 1.5, so youve got only
    50% efficiency. Diodes are another way to drop V.

    A switched mode regulator would improve your efficiency, though not to

    A switch that connects the motors to either one battery or 2 woulod be
    a 100% efficient controller. Mechanically make the switch connect M to
    batt A or B alternately, this is easily done with a multiposition
    rotary switch. No electronic solution can match the efficiency of this

  5. Robert Scott

    Robert Scott Guest

    Yes, a properly chosen resistor will work. For a given load condition
    you can adjust the resistor to achieve any given motor speed. But the
    advantage of a low-impedance adjustable voltage source is improved
    speed regulation in the face of changing loads. Using a resistor to
    drop the voltage, when the load increases, the increased current will
    drop more voltage, resulting in much lower speed. Using a hard
    voltage source will try harder to keep the speed constant when the
    loading changes. But if you don't mind fiddling with the resistors
    when the load changes, then a variable resistor will work just fine.

    -Robert Scott
    Ypsilanti, Michigan
  6. Fred Bloggs

    Fred Bloggs Guest

    View in a fixed-width font
    such as Courier.

    .. .----- DC ----.
    .. | MTR |
    .. | |
    .. | |
    .. | _ |
    .. --- /| |
    .. - .-[100]-+
    .. | | / |
    .. --- | |/
    .. - +-----| TIP31A
    .. | | |>
    .. | [68] |
    .. | | |
    .. '-----+-------'
  7. Sjouke Burry

    Sjouke Burry Guest

    Motors dont like series resistors, you get problems with the
    stall current of the motor ,so with a resistor for making it
    run at say 25 percent speed, it might not be willing to start.
    Supply it with a regulated voltage, and over a large range
    it will draw watever current it needs,to start moving.
    I have tested motors(ironless core) ,which ran at .5 volt to
    40 volt, a range of 1 to 80. You wil not get that with a
    series resistor.
  8. Guest

    Thats true in the general case. The OP is powering a pump, which I
    presume wil have approx the same load curve as a fan, ie power
    proportional to speed cubed. In this case the load issue you mention
    wont happen.

    I think the transistor circuit mentioned would drop too much V.

  9. Fred Bloggs

    Fred Bloggs Guest

    This one is not too bad for a Radio Shack solution, drops maybe 0.1V.
    View in a fixed-width font such as

    .. .--------------- DC ----.
    .. | MTR |
    .. | |
    .. | |
    .. | .--[47]-+
    .. | | |
    .. | | |/
    .. +------[68]-----+-----| TIP41A
    .. | | |>
    .. | |_ |
    .. --- |/| |
    .. - [100] |
    .. | /| |
    .. --- | |
    .. - | |
    .. | | |
    .. '---------------+-------'
  10. ehsjr

    ehsjr Guest

    As far as I can tell, all of the answers presume you will
    stay with the 2 AA cells as a supply. If that is the case,
    I'd choose the answer posted by Fred Bloggs. (His stuff
    is always great). I'll give you a different answer, that
    uses a wall wart to save on batteries.

    Get a wall wart supply of say 9 or 12 volts. Then do this:

    + -----in| LM317 |out---+--->
    ------- |
    Adj |
    | [240]
    | |
    500 /<----+
    \ |
    / [680]
    | |

    Put the LM317 on a heatsink. At 300 mA with a 12 volt input
    and a 1.5 volt output, it will need to dissipate over 3 watts.

  11. quietguy

    quietguy Guest

    A wire wound pot would do the trick - one watt should be OK, but more if

  12. JosephKK

    JosephKK Guest

    Assuming you wish to hook up a few readily available components rather than
    build a modified SMPS; i suggest you try a small hobby Engine Speed
    Controller (ESC), use three cells (4.5 V) so that the ESC works and any
    simple cheap potentiometer. Ask if you are interested.
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