# “Fine tuning” a potentiometer

I am in the middle of a quandary at work. We have wire feeders that obviously control the feed rate of wire through a welder. We have need of putting these units up out of the way and what I did to accomplish this is to move the 10K pot from the feeder unit to the hand-held "whip" Because the knob on the new pot (also 10K) is now considerably smaller it is very difficult to fine tune the wire speed. The usable range for wire speed is approx 30%-60% as indicated on the original dial.

How could I solve this problem? Change the value of the "new" pot in the hand-held unit? Parallel? Dropping resistor across tab 1 and tab 2? Am at a loss.

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If I understand you correctly, you want to adjust things so the range only covers between ~30% and ~60% of the current range.

Let's say the output voltage from the pot wiper is currently 0-10V, here is a circuit to alter it to 3.3V to 6.6V (so 33% and 66% of original range):

The 10kΩ pot is represented by the two resistors in the dotted rectangle.

Output simulation with pot swept from 0 to 10kΩ (x-axis, output voltage on y-axis):

You can make it even easier to fine tune by using a 10-turn pot.

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Why do you need the parallel resistor? Even without it the response ought to be linear. – Nick Johnson Feb 20 '13 at 10:22
Thanks for the help. Was looking at options. – Tim Bo Feb 20 '13 at 11:26
@NickJohnson - You don't if you use 10k resistors for above and below, I was half asleep here, for some reason I had in mind the total resistance had to be 10kOhm (bad excuse, been up all night :-) ). As you know, the parallel resistor together with the pot resistance makes the sweep from 1 / (1/5e3 + 1/10e3) = 3.333kOhms to match with the upper and lower shown. I'll simplify it, thanks. – Oli Glaser Feb 20 '13 at 15:32
Thank you. Look forward to that – Tim Bo Feb 21 '13 at 23:07
So if I understand correctly. In order to obtain the 33-66% ratio in the handle pot I would place a 10K resistor in line on lug 1 and lug 3 of the pot and leave the wiper as it is, Correct? – Tim Bo Feb 23 '13 at 0:06

Two potmeters an option? Replace the single potmeter with a 'Coаrse' 10k potmeter in series with a 'Fine' 1k potmeter. The 'coarse' can be put a bit further away as you probably don't need it all the time and 'fine' goes in the handle. Another option would be to use a multiturn pot.

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Thanks! Could you offer a schematic of the curcuit? – Tim Bo Feb 20 '13 at 11:26

Oli's answer is excellent for using a standard pot, but there are also multi-turn pots available that need from several to ten or so turns to change from one end of the scale to the other if you need additional precision. While more expensive look at the following for example:

http://www.digikey.com/product-detail/en/3590S-2-103L/3590S-2-103L-ND/1088586

That 10K pot needs ten turns to go to one end of the scale from the other, so they may be worth looking into as well. Look for parts from your preferred suppliers either listed as multi-turn or that show a rotation above 360 degrees.

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Can not use them in this application. Have to use the factory spec'd size (physical) inside the whip. Thanks though – Tim Bo Feb 21 '13 at 1:12