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Simplest variable voltage circuit

I

Infundibulum

Jan 1, 1970
0
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.
 
L

Leon

Jan 1, 1970
0
Just use PWM, it's not a simple passive circuit but it will work.

Leon
 
I

Ian Stirling

Jan 1, 1970
0
Infundibulum said:
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.

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.
 
Infundibulum said:
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.

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
 
R

Robert Scott

Jan 1, 1970
0
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
 
F

Fred Bloggs

Jan 1, 1970
0
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.

View in a fixed-width font
such as Courier.

..
..
..
..
..
..
..
.. .----- DC ----.
.. | MTR |
.. | |
.. | |
.. | _ |
.. --- /| |
.. - .-[100]-+
.. | | / |
.. --- | |/
.. - +-----| TIP31A
.. | | |>
.. | [68] |
.. | | |
.. '-----+-------'
..
..
..
..
 
S

Sjouke Burry

Jan 1, 1970
0
Ian said:
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.
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.
 
Robert said:
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

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
 
F

Fred Bloggs

Jan 1, 1970
0
I think the transistor circuit mentioned would drop too much V.

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] |
.. | /| |
.. --- | |
.. - | |
.. | | |
.. '---------------+-------'
..
..
..
 
E

ehsjr

Jan 1, 1970
0
Infundibulum said:
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.
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 /<----+
\ |
/ [680]
| |
+-----+
|
Gnd

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.

Ed
 
Q

quietguy

Jan 1, 1970
0
A wire wound pot would do the trick - one watt should be OK, but more if
avaialble

David
 
J

JosephKK

Jan 1, 1970
0
quietguy said:
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.
 
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