# Circuit Help Needed!

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#### TjMitlyng

##### New Member
I need to know how to slow down my 2v brush motor to <40 RPM and >20 RPM. I'm using a 9v battery, and im wondering what length of wire, and /or resistors i would need to achieve this. The wire is from a master/slave cord for an old computer, its copper and as thin as hair, do not know its size, or resistance it has. Basically i need to know; How many volts to make a 2v really slow, and how to achieve that voltage with wire length and/or resistor combinations from a 9v.

#### JimB

##### Super Moderator
I need to know how to slow down my 2v brush motor to <40 RPM and >20 RPM.
What is the current speed of the motor?
If it is several 100s of RPM, you need a gearbox.

I'm using a 9v battery,
This sounds like a bad idea.
I assume that you mean a little 9v radio battery.
Those batteries do not have much capacity and will not last long driving a motor.
Also consider that with your proposed scheme you will be wasting at least 7 of the 9 volts heating up a resistor.

and im wondering what length of wire, and /or resistors i would need to achieve this.
As we know nothing about the motor other than that it is normally reted at 2 volts, it is impossible to say.

The wire is from a master/slave cord for an old computer, its copper and as thin as hair, do not know its size, or resistance it has.
And neither does anyone else here.

Basically i need to know; How many volts to make a 2v really slow, and how to achieve that voltage with wire length and/or resistor combinations from a 9v.
I suggest that this will be an impossible task.
If the motor could be slowed down using a resistor in the supply line, it is most likely that the slightest load would stop the motor.

You need a correctly rated motor with a gearbox on the output shaft to do this effectively.
Not a 9v battery and a resistor.

JimB

#### dr pepper

##### Well-Known Member
20 rpm is very very slow for a little dc motor, just not practical.
You could achieve 20 to 40 revs easily with a gearbox to gear down the motor speed and a pwm circuit to control the speed of the motor.
jims right though a pp3 wont last long, better to use 3 or 4 AA's

#### MikeMl

##### Well-Known Member
Even better, run the motor on one 1.5V D-cell

#### 4pyros

##### Well-Known Member
im wondering what length of wire, and /or resistors i would need to achieve this. The wire is from a master/slave cord for an old computer, its copper and as thin as hair, do not know its size, or resistance it has.
Whats up with the wire?
If you don't now the size or resistance, we can't help you use it.
I need to know how to slow down my 2v brush motor to <40 RPM and >20 RPM.
You can't unless its a gear motor.

#### audioguru

##### Well-Known Member
I have a radio controlled model airplane that is propelled by a 3.6V brushed motor with a gearbox driving the propeller. The motor spins very fast.
The single cell Li-Po battery is 120mAh and it flies for 10 minutes. Then the motor, radio and servos total current is only 20mA.

The tiny motor was designed to open and close a CD tray. It is 6mm in diameter and is 14mm long. The brushes or commutator in the motor wears out after running for a few hours.

I have other model airplanes with direct drive brushless motors that last forever.

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#### MrAl

##### Well-Known Member
Hi,

It could very well be possible to run a DC motor at a very slow speed using feedback. Funny though, the feedback might have to be geared UP. But there is a chance that it could overheat the motor too unless it was only to be used in a non continuous way. Many things are rated for low duty cycle so that would just be classed in that group, unless we get lucky and the motor doesnt overheat.
Im not sure if you could use 'normal' simple feedback like the back EMF technique, because the motor would be going so slow that it would only rotate once every three seconds. I dont think that's enough to generate appreciable back EMF. Connected to a gear box that gears the motor UP and then connected to another motor, that second motor would provide enough back EMF but then we had to use a gearbox anyway (although a light duty one now instead of one that has to take the full load torque).

So gearing down is probably the best bet, unless you can move to a different kind of motor like a stepper, and a new drive circuit.

There are a lot of ways to 'gear down'. Pulleys, sprocket/chain, standard gears, worm gears, etc. For very low RPM the best bet is to buy something already made. Check out RobotShop for example.

#### Mr RB

##### Well-Known Member
Yep, or Pololu, they have a good range of dc gearmotors.

Or, since the OP is talking about salvaged wire from an old PC, and old CD driver will usually have a worm drive motor used for the CD drawer. Those are good for about 1 rev/second and can be salvaged from any CD or DVD drive.

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