I bought a 12v motor for the focus system, but this little demon is too fast! I think it should be easy to slow it down, but how?
I couldn´t find any reducer wheels yet, so I think some kind of circuit should help.

http://www.solorb.com/elect/solarcirc/pwm1/


Or just put a Rheostat in the 12 volt line.

Here's how to figure out the value of the Rheostat and a little description of it when used for a fan, but it would work with any motor.

First off, rheostats (or rheostat-like devices) allow you to infinitely vary the speed of a fan between it's maximum speed and some arbitrary minimum speed determined by the fan's specifications. In order to accomplish this, a rheostat acts as a variable resistor. In other words, the rheostat is essentially a valve for electrical current, impeding the electrons from reaching the fan or device. The excess voltage is usually dissipated in the form of heat. The terms rheostat and potentiometer refer to the same type of device, differing mostly in terms of arrangement of the terminals.

Regardless of the name you give them, a variable resistor is what you get. Since this business of reducing a fan's spin rate is what's pertinent, we care mostly about two factors: Capacity and Ohm rating.

The capacity of a rheostat is given in Watts. This number tells you how much juice the device will take before some type of failure is imminent. Since most rheostats or pots are constructed using a wiper that contacts a semiconducting trace of some sort, the interface between the wiper and substrate becomes the weakest link. This is an inherent reason why most rheostats and pots are relatively low wattage devices. In order to find out how much wattage your 12 volt fan draws or produces, multiply the voltage (Volts) times the current draw (Amps).

Watts = Volts x Amps

Example: Watts = 12 Volts x 500mA x (1Amp/1000mA)

Watts = 6

In this example, a 12 volt fan draws 500 milliamps. Since the formula requires Amps, we make the conversion to Amps before multiplying. I point this out since most fans are labeled in milliamps (mA) rather than Amps. To simplify further, just hang a decimal point in front of mA to get the corresponding Amps for the calculation.

Don't try to confuse yourself with overcomplicating things. Yes, a fan that is hooked up to a rheostat won't be receiving the full 12 volts, so the wattage at lower voltages will be less, but this is putting the cart before the horse. Use the full 12 volts when determining the wattage of your fan.

Once you have determined the wattage or power of the fan, you are halfway there. The next step is determining just how much resistance to add to the circuit to give you the range of reduction you require. This is not quite so straightforward, but it can be accomplished.

In general, each fan will have three voltages associated with it: A starting voltage, a sustain voltage and a full voltage (or rated voltage). Each fan differs somewhat in these values, and can usually be ascertained from the manufacturer. The starting voltage is the minimum voltage required to get the fan starting initially from a dead stop. The sustain voltage is the minimum voltage required to keep the fan spinning at it's slowest rate. Note that the sustain voltage will always be less than the starting voltage , since the starting voltage must overcome the inertia and static friction of the motor's bearings. The full voltage or rated voltage is the full 12 volts in virtually every computer case application. Most fans will run at higher voltages, at the cost of longevity.

Ideally, you would like the rheostat to control the fan between its maximum voltage (high) and its sustain voltage (low). In other words, you would like to be able to use the full range of the rheostat to control the fan. It is not imperative that the fan be able to start itself when the rheostat is turned fully down, since we can turn the fan up to start it and then turn it down all the way after it is spinning. If we limit the rheostat to the starting voltage at minimum turn, we are not getting the full range of RPM's out of the fan and rheostat.

A rheostat, as discussed earlier, is simply a variable resistor. It will deliver a full amount of voltage (almost no resistance) at maximum turn, and some smaller voltage at minimum turn depending on the Ohm Rating of the rheostat. The voltage delivered across a resistor is based upon Ohm's Law.

Ohm's Law: Volts = Amps x Ohms

In other words, the amount of voltage delivered across a circuit is proportional to the current draw (in Amps) of the device and the resistance encountered (in Ohms).

If we rearrange this equation using some simple algebra, we can say that

Ohms = Volts / Amps

What we really care about is not the starting voltage and ending voltage, but the Voltage Drop encountered between the two. In order to figure out what kind of resistance value we need, we need to consider the amount the voltage drops, and what kind of resistance is required to make this happen. So again rewriting the equation, we get:

Ohms = (VoltageMax - VoltageMin) / Amps

This is the "magic formula" that so many of you have asked me for. You may recognize this as the same equation used for figuring the resistor needed for an LED. Same concept, really. Here we go:

Let's say that the manufacturer of the fan I used as an example earlier states that his fan draws 500mA, and the sustain voltage is 7 volts. So plugging into the equation above we get:

Ohms = (12Volts - 7Volts)/.5Amps

Ohms = 10

There you have it. The voltage drop is what we are concerned with, and a resistance value of 10 Ohms should give us that voltage drop. In this example, with this fictitious fan, a rheostat that is rated for 10 Ohms and 6 Watts would give us the range we need at the power requirement for reliability.

Unfortunately, this isn't so cut and dried. Rheostats don't come in 31 flavors, sad to say. Invariably, a compromise will have to be made when shopping for rheostats. Either the Ohm rating will be wrong, or the power requirement will be too small, or most often, both. This is an inherent limitation that we have to live with unless we resort to a voltage regulator which is somewhat more involved. In shopping for components, I have found what I believe to be the best all around rheostat for most fans used in the computer case modding arena, and that is a 5 watt, 100 Ohm unit from Jameco (part number 140513). This particular rheostat is both a compromise in resistance and power handling capacity, but it will satisfactorily adjust most fans with the exception of the bigger 120mm fans like the YStechs's and Sunon's (the exception is the Panaflo 120mm fan that draws only 170ma and delivers about 77cfm, and is very quiet).

Jonjandran, I think I´ve drawn somewhere in the middle of the river....

Someone told me that a potentiometer should do the trick... or not?

No. You'd probably burn up the pot in a heartbeat.

How are you powering it now?
Is it a small permanent magnet motor?
You using any type of gear reduction?
Any idea how much current it draws?
Approx size of motor.......

A variable resistor is another name for a potentiometer, also called a pot. A trimpot would not work, because they're very low wattage devices, but a 1W pot ought to do for a typical 12V fan easily. A 2W would definitely handle it.

You should be able to pick up asuitable part at Radio Shack, or equivalent.

He could just supply it lower voltage...right?

Yes but you do that with either:
1.Rheostat
2.Variable resistor (potentiometer)

They both add resistance to the circuit to lower the voltage and slow down the motor.

Ok...yeah...or a different power supply.

Deaths method is much simplier. Most people use a wall AC to DC convertor that has adjustable output voltages. If not using one, you can get one a walmart for not much more than buying the pot/r-stat at radio shack. That way no circuit and connection need to be made.

most of those AC/DC converters have voltage ranges from 3v to 12v, in 1.5volt steps .....

Well, a trimpot with an LM317 vreg would also supply variable voltage.

The problem is that many 12V fans won't start on less than 9V. Most will start with 7V (which is a somewhat popular PC mod, take the potential difference between the 5V and 12V rails) but very few will start with 5V. Once started, though they'll RUN on 5V. With a PC, as long as the CPU and PSU fans work, the other case fans are just a nicety. With a projector the fans are mandatory.

what kind of motor is it? from a fan? a slot car? all these suggestions would work but the simple way is less voltage = less speed, if you are using this for a focus mech see how little voltage it will take to get it started, than once you find the right speed get a voltage supply for that voltage, than to get the motor to go the other way you can reverse the polarity. a rheostat is what is in a slot car controller, it changes the resistance and lowers the voltage delivered to the motor which varries the speed, it also disipates the heat created when you change the resistance. of course you could look around for a gearbox to reduce the speed mechanically.

If having enough torque is an issue, then going the gearbox route would be better.

A geared motor would be a good choice. I cant imagine a high speed motor going 0.5 to 1 rpm without stalling. An AC clock motor might work.
Do you know the parameters that you need?

Has anyone thought about modifying a second hand electric car aerial? You would have to modify it so that the distance would be shorter maybe just cut a piece of the end. You also might need to make it run slower.
Just a thought