Author Topic: Potentiometer: Help needed  (Read 3146 times)

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Offline Voidugu

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Potentiometer: Help needed
« on: July 12, 2013, 06:21:07 am »
I got a 10 K pot connected to 5v and ground. The potentiometer will be used as a voltage divider. I want to be able to utilize as much as possible of the potentiometer's rotation to vary the voltage output of the potentiometer between two voltages (Assuming that the rotor of the potentiometer can go from 0 to 180 degrees, i would like 0 degrees to be 2.5 volts and 180 degrees to be 2.8 volts).

Can you people provide me with a circuit that will allow me to do that and also allow me to change those boundaries easily?

Cheers :)
 

Offline CodyShaw

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Re: Potentiometer: Help needed
« Reply #1 on: July 12, 2013, 06:49:37 am »
The quick and dirty way: Assume the potentiometer is 0 ohms, and set up a resistor divider on both sides of the pot to get 2.8 volts.

http://www.wolframalpha.com/input/?i=5*%28x%2B10000%29%2F%282*x%2B10000%29+%3D+2.8.

Use 3 resistors. one 36.6k, then the pot, then another 36.6k in series. The output will be before the pot, after the first resistor.

The calculations are easy, because you know the two resistors on either side of the pot must be the same to get the 2.5 volt division when the potentiometer is 0 ohms!

Edit: More mathy proof that this should work.

http://www.wolframalpha.com/input/?i=5*%2836666+%2B+x%29%2F%2836666%2B36666%2Bx%29+for+x+%3D+0+to+10000

This is all using simple voltage divider math, if you are not well versed in that side of electronics:

https://en.wikipedia.org/wiki/Voltage_divider

It gets trickier as you change your boundaries, as your initial condition of assuming the surrounding resistors are the same is no longer true (if you don't start at 2.5 volts). Simply changing those resistors will do the trick though (make sure to run calculations first)!
« Last Edit: July 12, 2013, 06:54:50 am by CodyShaw »
Candidate for Bachelor of Applied Science, Electrical Engineering, University of Waterloo, Waterloo, ON, Sept. 2011 – Present
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Offline Rerouter

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Re: Potentiometer: Help needed
« Reply #2 on: July 12, 2013, 06:51:59 am »
assuming the thing your setting with the center tap has a really high input impedence (close to a meg)

you add a resistor before and after the pot to drop the voltages to get your range, the only difficulty is you saying a value at 180 degrees, where most pots are 270 degrees, but i'll assume its linearthis means you want 0.45V across the pot, which gives us a current of 45uA

from this you want your lower resistance to make up 2.5V which will be 55.55K
and you want your upper resistance to chew up 2.05V which will be 45.55K
 

Offline CodyShaw

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Re: Potentiometer: Help needed
« Reply #3 on: July 12, 2013, 06:58:00 am »
assuming the thing your setting with the center tap has a really high input impedence (close to a meg)

you add a resistor before and after the pot to drop the voltages to get your range, the only difficulty is you saying a value at 180 degrees, where most pots are 270 degrees, but i'll assume its linearthis means you want 0.45V across the pot, which gives us a current of 45uA

from this you want your lower resistance to make up 2.5V which will be 55.55K
and you want your upper resistance to chew up 2.05V which will be 45.55K

Mmm, I didn't realize pots don't go from 0 to 180...

I just assumed he wanted to use the full range.
Candidate for Bachelor of Applied Science, Electrical Engineering, University of Waterloo, Waterloo, ON, Sept. 2011 – Present
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Offline Voidugu

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Re: Potentiometer: Help needed
« Reply #4 on: July 12, 2013, 10:37:12 pm »
I don't really know the full swing of a potentiometer in degrees. What i meant to say is the full range. I gave a theoretical example with a potentiometer able to swing from 0 to 180 degrees (its full range)  to get my point better understood.

Thank you both very much
 

Offline CodyShaw

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Re: Potentiometer: Help needed
« Reply #5 on: July 12, 2013, 11:31:20 pm »
I don't really know the full swing of a potentiometer in degrees. What i meant to say is the full range. I gave a theoretical example with a potentiometer able to swing from 0 to 180 degrees (its full range)  to get my point better understood.

Thank you both very much

Ah, no problems then, my setup should work. Test it out!
Candidate for Bachelor of Applied Science, Electrical Engineering, University of Waterloo, Waterloo, ON, Sept. 2011 – Present
3A Electrical Engineering
 


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