Reducing voltage on electric motor

[tristanlee85]

I've got this electric motor from a K'NEX toy kit I found lying around. It was power by 2 AA batteries (~3V DC). I would like top use this motor on a 12V car circuit, but when I wire it to 12V, it spins way too fast. How can I reduce the voltage significantly? I want it very very slow, possibly slow enough so it will take 5 seconds for the motor to make 1 revolution. I tried using a 470ohm resistor, but then the motor wouldn't even spin. Any ideas?

 

[Mike Dyble]

How much current does the motor draw when run at 3v

 

[tristanlee85]

~ 0.125A

 

[Mike Dyble]

470 ohm is much to big it will only give about 25mA at 12v Try some resistors in the 50 - 100 ohm range to start with, need to be at least 2 watt rating. If you run a 3v motor at 12v it will not last very long! Depending on what you are trying to acheive you may be better with a variable voltage 3 terminal regulator such as an LM317 or equivilent, which you can then set it to give the speed you want. The trouble with using resistors in this way is that speed will be very dependent on the load on the motor.

 

[tristanlee85]

Well, this is a pretty small motor. Do you have any other opinions on a more powerful motor? What I have set up is a aliminum rod. It's connected to my license plate that is under the bumpuer of my car. I am trying to make a "flip-plate" as I may call it for show purposes. I plan on using a relay and a DPST momentary switch to control the polarity of the current going into the motor which desides on whether the plat flips up or down. I need a motor that will be strong enough to hold in position with the bounce that may occur with the plate from bumps. Something that will only turn the voltage is applied to it?

Is there even such thing?

 

[FoxyRick]

Hi,

There is no way you will get a 5-second revolution out of a toy motor, unless you add a gearbox to it. The motor will go from not spinning to spinning a lot faster than 5 sec/rev. as you increase current. Even PWM won't help at such a slow speed.

Adding a gearbox will also increase the torque and hold, so it should be strong enough for your application. A resistor is the simple way to go for voltage drop, but it will be tricky to calculate an exact value, just experiment.

An alternative idea is two solonoids, commonly available in 12V. These just do a push/pull action with a lever to the plate to flip it.

Cheers,
FoxyRick.

 

[tristanlee85]

Where can I find solonoids that work on 12V? I know how they work becuase I have did research on car "pop-door" kits that use solonoids. This is a project I would like to complete because I've yet to see it done in my eyes, but I'm not sure where to get solonoids.

 

[Mike Dyble]

Farnell in Leeds in the UK have a reasonable selection, (not sure where you are based) otherwise it's a trip to the local car breakers to try and find something.

 

[tristanlee85]

Ok, so, with a 12V pull solenoid, when I supply 12v to it, will the solenoid always pull as long as there is power going through it, or can it only pull for so long>

 

[rauol]

8) You can use a stand alone audio casett mechanism which has a dc motor and gear system to reduce speed. with the play button you get low speed and with the fst forward button you get high speed.

 

[rozerf]

U may use voltage regulator to reduce the voltage with high efficiency.