# help : control direction of 12v dc motor using potential resistor

Discussion in 'Homework Help' started by Iraways, Mar 5, 2011.

1. ### Iraways Thread Starter New Member

Nov 16, 2008
3
0
i've been searching the web but still no solution

my objective is to control the direction of 12v dc motor using potential resistor

let say i use 100k ohm potential resistor

if i adjust the pot to 0 ohm ==> motor turn right
if i adjust the pot to 100K ohm ==> motor turn left
if i adjust the pot to 50k ohm ==> motor stop

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,791
771
What world are you from buddy.

You talk of impossible things.
To do that you need more than a pot.

3. ### Heavydoody Active Member

Jul 31, 2009
140
11
How about an H bridge made up of transistors? They would have to be biased such that two were in cutoff and two were in saturation when the pot was at 0, and vice versa at 100k. I am not sure if this will work, and I am not sure how the stop function might work. Its just one idea.

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,852
967
If you had +5 volts on one end of the rheostat and -5 volts on the other end, you would find a 'zero' voltage point in the middle.

give you any ideas?

5. ### mjhilger Member

Feb 28, 2011
119
16
You will certainly need more than a potentiometer to control the motor. The motor would surely require more current than the pot will supply. So you need a circuit of some type to drive your motor. Is this a home work question or the need for a real world solution? If it is a homework thought problem, then it is just to get you to think about a configuration which might allow control of the motor with the 3 positions you indicated (though 100K will never work).
If however, it is for a real world solution, you should probably post in one of the other areas, as it will require transistors and bias circuits or at least an IC and a few transistors. H bridge as has been suggested and PWM using a 555 or something would be a start.

Dec 26, 2010
2,147
300
You really do not want to try doing this with just a pot. Yes, theoretically it could be done, using some big expensive wire-wound pot rated at many watts in a bridge configuration with two power supplies. You would not use 100kΩ, by the way, more like 33Ω for a small low-voltage motor.

That would work, but it would be horribly inefficient. It would also cost a lot. Big pots like that do exist, but they are quite expensive and not that readily available. Take the advice you are being given. Use a small pot to control something like an H bridge.

7. ### Iraways Thread Starter New Member

Nov 16, 2008
3
0
thanks a lot guys

i interested about the H bridge idea.
by refer to this circuit :
how to make 1 potential resistor turn 'ON' and 'OFF' the transistors of the H bridge??

so,assume i using 100k pot
if i adjust the pot between 0k ohm - 49k ohm ==> motor keep turning right (don't care about the speed)
if i adjust the pot between 51k ohm - 100K ohm ==> motor keep turning left (don't care about the speed)
if i adjust the pot at 50k ohm ==> motor stop

Last edited: Mar 9, 2011