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Posted by Lisandro Pin on July 4, 2005, 3:19 am
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I need to drive a small DC motor (the general kind used in toys and
such), at very low speeds - below 50 rpm, perhaps. It won't be needing
a lot of torque, since it will only be holding a small mirror. I'm
rather inexperienced with these devices, so, is it possible to drive
the motor directly at these speeds without a gearbox (perhaps with a
simple 555-based PWM driver, or a voltage regulator)? The simpler the
better.
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Posted by mlw on July 4, 2005, 8:16 am
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Lisandro Pin wrote:
> I need to drive a small DC motor (the general kind used in toys and
> such), at very low speeds - below 50 rpm, perhaps. It won't be needing
> a lot of torque, since it will only be holding a small mirror. I'm
> rather inexperienced with these devices, so, is it possible to drive
> the motor directly at these speeds without a gearbox (perhaps with a
> simple 555-based PWM driver, or a voltage regulator)? The simpler the
> better.
Depending on your current requirements and precision, you could probably do
it with a number of passive components and a few opamps.
The voltage at the terminals of a motor (assuming a current source is
driving the motor) represents the back EMF of the motor. A perminent magnet
motor will generate electricity when turned. Just because you are powering
it, doesn't mean it ceases to be a generator.
Using (assuming low current) use a 2N2222 transistor and an opamp to create
a variable current supply to drive the motor.
Using an opamp and a few other parts, read the voltage off the back of the
motor, this will be the speed feedback.
Now, design a comparitor circuit that takes the input from the speed
feedback and an adjustment potentiometer. (probably 10K between power and
ground, or something.) Feed the output of this circuit to the current
source that drives the motor.
I'm not sure that the back EMF of the motor will be enough to really do this
circuit, you may need to use a simple integrated motor gearbox. 50rpm is
VERY SLOW.
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Posted by on July 4, 2005, 11:18 am
Please log in for more thread options No, you can't do it without some sort of speed reduction device. But
there are choices in addition to a gear box; such as a belt, or a wheel
on the motor rotating against the mirror platform.
Joe Dunfee
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Posted by Lisandro Pin on July 4, 2005, 11:47 am
Please log in for more thread options The problem is i don't have a lot of space to work with - my other
choice was a small stepper motor salvaged from old junk like floppy
drives. I'll check the wheel idea though, thanks!
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Posted by Gordon McComb on July 4, 2005, 1:00 pm
Please log in for more thread options Lisandro Pin wrote:
>
> The problem is i don't have a lot of space to work with - my other
> choice was a small stepper motor salvaged from old junk like floppy
> drives. I'll check the wheel idea though, thanks!
These toy motors are very ineffecient and crudely made. You can feel the
coarseness of their armatures by manually totating the shaft. As another
poster mentioned, small R/C servo motors will allow for quite slow
movement. If these motors need to be driven continuously you can modify
the servo for this (they usually turn only about 180 degrees max), but
keep in mind that the modification isn't as easy with the really small
micro and mini servos out there.
-- Gordon
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> such), at very low speeds - below 50 rpm, perhaps. It won't be needing
> a lot of torque, since it will only be holding a small mirror. I'm
> rather inexperienced with these devices, so, is it possible to drive
> the motor directly at these speeds without a gearbox (perhaps with a
> simple 555-based PWM driver, or a voltage regulator)? The simpler the
> better.