Ray Wilson authored this content while he was actively running MFOS as the founder and resident genius.
We retain the content because it reflects a valuable point of view representing that time and place.

Article by Ray Wilson

Changing a Pot's Adjustment Range

There are times when the range of voltage adjustment for a potentiometer needs to be adjusted. You may be powering a project with a different power supply voltage. You may wish the initial Cut Off Frequency pot went a bit higher or lower or you may want to adjust the coarse range adjust on your VCOs to your preference. I will describe four scenarios which are depicted in the image below and how to change the potentiometer's range of adjustment. The calculator will tell you the voltage range for any of the scenarios.

Scenario 1

In scenario 1 the pot presents the full range of voltage adjustment between -V and +V at the wiper terminal. There is no way to get more voltage from this arrangement however when the mixing resistor is being fed to a non-inverting op amp input it's value can be increased to lower the entire range of adjustment or decreased to increase the entire range of adjustment evenly for both the high and low range. Be careful that you don't exceed the power rating of a pot when wired in this fashion or the pot's resistive element will get hot and perhaps burn out.

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Scenario 2

Scenario 2 always supplies -V as the lowest adjustable voltage but allows adjustment of the maximum positive voltage. If you want to lower the maximum positive range of voltage presented to the pot's wiper you can add a resistor between +V and the the end of the pot as shown. If you already have a resistor in this configuration and want to increase the maximum positive voltage available to the wiper then lower the value of resistor RP. Now you have a fixed voltage divider between +V and the top of the pot. To determine the maximum voltage available to the pot's wiper terminal in this scenario take the total voltage +V + abs(-V) divided by the total resistance of the pot and the resistor in series Rs. This gives you the current through the pot and resistor in series IPRs. Taking IPRs times the resistor RP's value gives you the voltage dropped on RP (Rv). If you take +V - Rv you get the maximum positive voltage you will be able to present to the pot wiper when turned completely up (adjusted towards the resistor to +V). To use the calculator for Scenario 2 set RN to "Shorted".

Examples

Supply Voltage	Potentiometer Value	Resistor RP Value	Series Resistance Rs	Current IPRs	Voltage On Resistor RP (Rv)	Max Pos Adjustment
+/-12V (24V Rail to Rail)	100K	10K	110K	218 uA	2.18V	(12 - 2.18) = 9.82V
+/-12V (24V Rail to Rail)	100K	20K	120K	200 uA	4.00V	(12 - 4.00) = 8.00V
+/-12V (24V Rail to Rail)	100K	47K	147K	163 uA	7.67V	(12 - 7.67) = 4.33V

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Scenario 3

Scenario 3 always supplies +V as the highest adjustable voltage but allows adjustment of the maximum negative voltage. If you want to raise the minimum negative range of voltage (bring it closer to ground) presented to the pot's wiper you can add a resistor between -V and the the end of the pot as shown. If you already have a resistor in this configuration and want to lower the maximum negative voltage (bring it closer to -V) available to the wiper then lower the value of resistor RN. Now you have a fixed voltage divider between -V and the bottom of the pot. To determine the maximum negative voltage available to the pot's wiper terminal in this scenario take the total voltage +V + abs(-V) divided by the total resistance of the pot and the resistor in series Rs. This gives you the current through the pot and resistor in series IPRs. Taking IPRs times the resistor's value gives you the voltage dropped on resistor RN (Rv). If you take Rv plus -V you get the minumum negative voltage you will be able to present to the pot wiper when turned completely down (adjusted towards resistor RN to -V). To use the calculator for Scenario 3 set RP to "Shorted".

Examples

Supply Voltage	Potentiometer Value	Resistor RN Value	Series Resistance Rs	Current IPRs	Voltage On Resistor RP (Rv)	Max Neg Adjustment
+/-12V (24V Rail to Rail)	100K	10K	110K	218 uA	2.18V	(2.18 + (-12)) = -9.82V
+/-12V (24V Rail to Rail)	100K	20K	120K	200 uA	4.00V	(4.00 + (-12)) = -8.00V
+/-12V (24V Rail to Rail)	100K	47K	147K	163 uA	7.67V	(7.67 + (-12)) = -4.33V

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Scenario 4

In scenario 4 changing either resistor (RP or RN) will adjust both ends of the adjustment range. Increasing RP alone will lower the entire adjustment range. Decreasing RP alone will raise the entire adjustment range. Increasing RN alone will raise the entire adjustment range. Decreasing RN alone will lower the entire adjustment range. Ohm's law of course always applies and calculating the voltages is not rocket science but it is a pain in the nether regions so here is a a javascript program to do it for you. Better living through software! The resistor value selectors contain 1% resistor values so if you are using 5% resistors just find the closest match and you will be fine. Note that the voltages are symmetrical and the midpoint is 0V when RP and RN are equal. The calculator updates when you select new values.

Power Supply	Resistor RN Value	Potentiometer Value	Resistor RP Value
+/-9V +/-10V +/-12V +/-15V	Select Shorted 10 ohm 10.2 ohm 10.5 ohm 10.7 ohm 11 ohm 11.3 ohm 11.5 ohm 11.8 ohm 12.1 ohm 12.4 ohm 12.7 ohm 13 ohm 13.3 ohm 13.7 ohm 14 ohm 14.3 ohm 14.7 ohm 15 ohm 15.4 ohm 15.8 ohm 16.2 ohm 16.5 ohm 16.9 ohm 17.4 ohm 17.8 ohm 18.2 ohm 18.7 ohm 19.1 ohm 19.6 ohm 20 ohm 20.5 ohm 21 ohm 21.5 ohm 22.1 ohm 22.6 ohm 23.2 ohm 23.7 ohm 24.3 ohm 24.9 ohm 25.5 ohm 26.1 ohm 26.7 ohm 27.4 ohm 28 ohm 28.7 ohm 29.4 ohm 30.1 ohm 30.9 ohm 31.6 ohm 32.4 ohm 33.2 ohm 34 ohm 35.7 ohm 36.5 ohm 37.4 ohm 38.3 ohm 39.2 ohm 40.2 ohm 41.2 ohm 42.2 ohm 43.2 ohm 44.2 ohm 45.3 ohm 46.4 ohm 47.5 ohm 48.7 ohm 49.9 ohm 51.1 ohm 52.3 ohm 53.6 ohm 54.9 ohm 56.2 ohm 57.6 ohm 59 ohm 60.4 ohm 61.9 ohm 63.4 ohm 64.9 ohm 66.5 ohm 68.1 ohm 69.8 ohm 71.5 ohm 73.2 ohm 75 ohm 76.8 ohm 78.7 ohm 80.6 ohm 82.5 ohm 84.5 ohm 86.6 ohm 88.7 ohm 90.9 ohm 93.1 ohm 95.3 ohm 97.6 ohm 100 ohm 102 ohm 105 ohm 107 ohm 110 ohm 113 ohm 115 ohm 118 ohm 121 ohm 124 ohm 127 ohm 130 ohm 133 ohm 137 ohm 140 ohm 143 ohm 147 ohm 150 ohm 154 ohm 158 ohm 162 ohm 165 ohm 169 ohm 174 ohm 178 ohm 182 ohm 187 ohm 191 ohm 196 ohm 200 ohm 205 ohm 210 ohm 215 ohm 221 ohm 226 ohm 232 ohm 237 ohm 243 ohm 249 ohm 255 ohm 261 ohm 267 ohm 274 ohm 280 ohm 287 ohm 294 ohm 301 ohm 309 ohm 316 ohm 324 ohm 332 ohm 340 ohm 357 ohm 365 ohm 374 ohm 383 ohm 392 ohm 402 ohm 412 ohm 422 ohm 432 ohm 442 ohm 453 ohm 464 ohm 475 ohm 487 ohm 499 ohm 511 ohm 523 ohm 536 ohm 549 ohm 562 ohm 576 ohm 590 ohm 604 ohm 619 ohm 634 ohm 649 ohm 665 ohm 681 ohm 698 ohm 715 ohm 732 ohm 750 ohm 768 ohm 787 ohm 806 ohm 825 ohm 845 ohm 866 ohm 887 ohm 909 ohm 931 ohm 953 ohm 976 ohm 1.00K 1.02K 1.05K 1.07K 1.10K 1.13K 1.15K 1.18K 1.21K 1.24K 1.27K 1.30K 1.33K 1.37K 1.40K 1.43K 1.47K 1.50K 1.54K 1.58K 1.62K 1.65K 1.69K 1.44K 1.78K 1.82K 1.87K 1.91K 1.96K 2.00K 2.05K 2.10K 2.15K 2.21K 2.26K 2.32K 2.37K 2.43K 2.49K 2.55K 2.61K 2.67K 2.74K 2.80K 2.87K 2.94K 3.01K 3.09K 3.16K 3.24K 3.32K 3.40K 3.57K 3.65K 3.74K 3.83K 3.92K 4.02K 4.12K 4.22K 4.32K 4.42K 4.53K 4.64K 4.75K 4.87K 4.99K 5.11K 5.23K 5.36K 5.49K 5.62K 5.76K 5.90K 6.04K 6.19K 6.34K 6.49K 6.65K 6.81K 6.98K 7.15K 7.32K 7.50K 7.68K 7.87K 8.06K 8.25K 8.45K 8.66K 8.87K 9.09K 9.31K 9.53K 9.76K 10K 10.2K 10.5K 10.7K 11K 11.3K 11.5K 11.8K 12.1K 12.4K 12.7K 13K 13.3K 13.7K 14K 14.3K 14.7K 15K 15.4K 15.8K 16.2K 16.5K 16.9K 17.4K 17.8K 18.2K 18.7K 19.1K 19.6K 20K 20.5K 21K 21.5K 22.1K 22.6K 23.2K 23.7K 24.3K 24.9K 25.5K 26.1K 26.7K 27.4K 28K 28.7K 29.4K 30.1K 30.9K 31.6K 32.4K 33.2K 34K 35.7K 36.5K 37.4K 38.3K 39.2K 40.2K 41.2K 42.2K 43.2K 44.2K 45.3K 46.4K 47.5K 48.7K 49.9K 51.1K 52.3K 53.6K 54.9K 56.2K 57.6K 59K 60.4K 61.9K 63.4K 64.9K 66.5K 68.1K 69.8K 71.5K 73.2K 75K 76.8K 78.7K 80.6K 82.5K 84.5K 86.6K 88.7K 90.9K 93.1K 95.3K 97.6K 100K 102K 105K 107K 110K 113K 115K 118K 121K 124K 127K 130K 133K 137K 140K 143K 147K 150K 154K 158K 162K 165K 169K 174K 178K 182K 187K 191K 196K 200K 205K 210K 215K 221K 226K 232K 237K 243K 249K 255K 261K 267K 274K 280K 287K 294K 301K 309K 316K 324K 332K 340K 357K 365K 374K 383K 392K 402K 412K 422K 432K 442K 453K 464K 475K 487K 499K 511K 523K 536K 549K 562K 576K 590K 604K 619K 634K 649K 665K 681K 698K 715K 732K 750K 768K 787K 806K 825K 845K 866K 887K 909K 931K 953K 976K 1M	10K 20K 50K 100K 200K 500K 1M	Select Shorted 10 ohm 10.2 ohm 10.5 ohm 10.7 ohm 11 ohm 11.3 ohm 11.5 ohm 11.8 ohm 12.1 ohm 12.4 ohm 12.7 ohm 13 ohm 13.3 ohm 13.7 ohm 14 ohm 14.3 ohm 14.7 ohm 15 ohm 15.4 ohm 15.8 ohm 16.2 ohm 16.5 ohm 16.9 ohm 17.4 ohm 17.8 ohm 18.2 ohm 18.7 ohm 19.1 ohm 19.6 ohm 20 ohm 20.5 ohm 21 ohm 21.5 ohm 22.1 ohm 22.6 ohm 23.2 ohm 23.7 ohm 24.3 ohm 24.9 ohm 25.5 ohm 26.1 ohm 26.7 ohm 27.4 ohm 28 ohm 28.7 ohm 29.4 ohm 30.1 ohm 30.9 ohm 31.6 ohm 32.4 ohm 33.2 ohm 34 ohm 35.7 ohm 36.5 ohm 37.4 ohm 38.3 ohm 39.2 ohm 40.2 ohm 41.2 ohm 42.2 ohm 43.2 ohm 44.2 ohm 45.3 ohm 46.4 ohm 47.5 ohm 48.7 ohm 49.9 ohm 51.1 ohm 52.3 ohm 53.6 ohm 54.9 ohm 56.2 ohm 57.6 ohm 59 ohm 60.4 ohm 61.9 ohm 63.4 ohm 64.9 ohm 66.5 ohm 68.1 ohm 69.8 ohm 71.5 ohm 73.2 ohm 75 ohm 76.8 ohm 78.7 ohm 80.6 ohm 82.5 ohm 84.5 ohm 86.6 ohm 88.7 ohm 90.9 ohm 93.1 ohm 95.3 ohm 97.6 ohm 100 ohm 102 ohm 105 ohm 107 ohm 110 ohm 113 ohm 115 ohm 118 ohm 121 ohm 124 ohm 127 ohm 130 ohm 133 ohm 137 ohm 140 ohm 143 ohm 147 ohm 150 ohm 154 ohm 158 ohm 162 ohm 165 ohm 169 ohm 174 ohm 178 ohm 182 ohm 187 ohm 191 ohm 196 ohm 200 ohm 205 ohm 210 ohm 215 ohm 221 ohm 226 ohm 232 ohm 237 ohm 243 ohm 249 ohm 255 ohm 261 ohm 267 ohm 274 ohm 280 ohm 287 ohm 294 ohm 301 ohm 309 ohm 316 ohm 324 ohm 332 ohm 340 ohm 357 ohm 365 ohm 374 ohm 383 ohm 392 ohm 402 ohm 412 ohm 422 ohm 432 ohm 442 ohm 453 ohm 464 ohm 475 ohm 487 ohm 499 ohm 511 ohm 523 ohm 536 ohm 549 ohm 562 ohm 576 ohm 590 ohm 604 ohm 619 ohm 634 ohm 649 ohm 665 ohm 681 ohm 698 ohm 715 ohm 732 ohm 750 ohm 768 ohm 787 ohm 806 ohm 825 ohm 845 ohm 866 ohm 887 ohm 909 ohm 931 ohm 953 ohm 976 ohm 1.00K 1.02K 1.05K 1.07K 1.10K 1.13K 1.15K 1.18K 1.21K 1.24K 1.27K 1.30K 1.33K 1.37K 1.40K 1.43K 1.47K 1.50K 1.54K 1.58K 1.62K 1.65K 1.69K 1.44K 1.78K 1.82K 1.87K 1.91K 1.96K 2.00K 2.05K 2.10K 2.15K 2.21K 2.26K 2.32K 2.37K 2.43K 2.49K 2.55K 2.61K 2.67K 2.74K 2.80K 2.87K 2.94K 3.01K 3.09K 3.16K 3.24K 3.32K 3.40K 3.57K 3.65K 3.74K 3.83K 3.92K 4.02K 4.12K 4.22K 4.32K 4.42K 4.53K 4.64K 4.75K 4.87K 4.99K 5.11K 5.23K 5.36K 5.49K 5.62K 5.76K 5.90K 6.04K 6.19K 6.34K 6.49K 6.65K 6.81K 6.98K 7.15K 7.32K 7.50K 7.68K 7.87K 8.06K 8.25K 8.45K 8.66K 8.87K 9.09K 9.31K 9.53K 9.76K 10K 10.2K 10.5K 10.7K 11K 11.3K 11.5K 11.8K 12.1K 12.4K 12.7K 13K 13.3K 13.7K 14K 14.3K 14.7K 15K 15.4K 15.8K 16.2K 16.5K 16.9K 17.4K 17.8K 18.2K 18.7K 19.1K 19.6K 20K 20.5K 21K 21.5K 22.1K 22.6K 23.2K 23.7K 24.3K 24.9K 25.5K 26.1K 26.7K 27.4K 28K 28.7K 29.4K 30.1K 30.9K 31.6K 32.4K 33.2K 34K 35.7K 36.5K 37.4K 38.3K 39.2K 40.2K 41.2K 42.2K 43.2K 44.2K 45.3K 46.4K 47.5K 48.7K 49.9K 51.1K 52.3K 53.6K 54.9K 56.2K 57.6K 59K 60.4K 61.9K 63.4K 64.9K 66.5K 68.1K 69.8K 71.5K 73.2K 75K 76.8K 78.7K 80.6K 82.5K 84.5K 86.6K 88.7K 90.9K 93.1K 95.3K 97.6K 100K 102K 105K 107K 110K 113K 115K 118K 121K 124K 127K 130K 133K 137K 140K 143K 147K 150K 154K 158K 162K 165K 169K 174K 178K 182K 187K 191K 196K 200K 205K 210K 215K 221K 226K 232K 237K 243K 249K 255K 261K 267K 274K 280K 287K 294K 301K 309K 316K 324K 332K 340K 357K 365K 374K 383K 392K 402K 412K 422K 432K 442K 453K 464K 475K 487K 499K 511K 523K 536K 549K 562K 576K 590K 604K 619K 634K 649K 665K 681K 698K 715K 732K 750K 768K 787K 806K 825K 845K 866K 887K 909K 931K 953K 976K 1M
Maximum Voltage At Wiper			Volts
Voltage Wiper At Midpoint			Volts
Minimum Voltage At Wiper			Volts

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Image Showing Voltage Adjustment Pot Scenarios