Simplest variable voltage circuit

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

Leon

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

100%.

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 approach.

NT

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

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.

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

View in a fixed-width font such as Courier.

. . . . . . . . .----- DC ----. . | MTR | . | | . | | . | _ | . --- /| | . - .-[100]-+ . | | / | . --- | |/ . - +-----| TIP31A . | | |>

. | [68] | . | | | . '-----+-------' . . . .

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.

NT

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.

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

. . . . . .--------------- DC ----. . | MTR | . | | . | | . | .--[47]-+ . | | | . | | |/ . +------[68]-----+-----| TIP41A . | | |>

. | |_ | . --- |/| | . - [100] | . | /| | . --- | | . - | | . | | | . '---------------+-------' . . .

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] | | +----------+ | [47] | / \\ 500 /

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

David

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.

--
JosephKK