# I need to vary speed of 9V motor...

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by impjester, Oct 7, 2002.

1. ### impjesterNew Member

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I have a 9V motor. I need to be able to adjust the speed of the motor, from 0 volts to the 9V max. I have my 9V battery attached to the motor via alligator clips for now.

I know a potentiometer will work, but I've already burned one out messing around...lol. I can't seem to find a pot that fits my scenario...and the Radio Shack people are of little help. (I also tried a dimmer switch from a ceiling fan, but got no current at all through that.)

I'm kinda new to electronic components, so I don't even know what the specs on the pot meant:

250VDC
power rating of .25W
100K Ohm resistance

Anyone have any ideas? Can I build my own? HELP!

Thanks!

Keith (impjester@hotmail.com)

2. ### pebeMember

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3. ### thecNew Member

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Although the answer is posted, I could explain why you messed up with the potentiometer

The little motor you're running, are using too much current for the potentiometer to handle. I'll explain.

The pot can handle 250VDC (Volts, Direct Current), which is plenty enough. Since you're running 9V (I guess DC), it's no problem.

The problem is the 0.25W limit. Look at it this way. A basic forumla in electronics world say that V*A = W ... if you put in your specs there, you'll get 9V * A = 0.25W, divide both sides with 9, and you'll get A = 0.25/9 = ~28mA.
Now. That means you can run an engine whÃ­ch uses MAXIMUM 27mA at 9V (because of the potentiometer of course). If the engine doesn't have it printed, you could measure it up with a digital meter in serial with the engine and a battery/power supply.

I doubt the engine runs that cheap, hence, you're using too much power for the potentiometer to handle, and you get some smoke coming out from it.

Solution : get a more robust potentiometer (will cost you more and you have to measure just how robust you'll need) or build the clever schematics pebe posted (nice/better solution).

Hope you learned something... that's what we're here for.
//Albert "thec" Sandberg

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5. ### impjesterNew Member

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Wow......how you explained it really helped me a lot. I guess I just need to calculate exactly what I need....although I'm unsure whether those unknown values for the motor are printed on the box or housing. I'll look into that.

As for that schematic.....I'm scared.

<sigh>

Thanks though! I may get this working yet.

Keith