Wire a Potentiometer As a Variable Resistor

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At some point in an electronics project you might find yourself needing a variable resistor.  Here's how to make one using a potentiometer.

Variable resistors are useful for the following:

adjustable gain of an amplifier
adjustable cutoff frequency of an RC filter
change the brightness of an LED
making measurements with a wheatstone bridge
adjusting the sensitivity of sensors wired in series/parallel to the variable resistor
testing- find the best resistance for your circuit before soldering a non-variable resistor in permanently

Normally, potentiometers are wired as variable voltage dividers:  connect +V to one side, connect the other side to ground, and the middle pin will output a voltage between 0 and +V (fig 2).
However, by only connecting two pins (one outside pin and one center pin) of a potentiometer to your circuit, you can turn a pot into a variable resistor.  Think about it this way: the potentiometer is filled with resistive material and turning the knob changes the amount of this material that the electrons must travel through before leaving the pot.  This means that the maximum resistance of this variable resistor is the total resistance of the pot.  In figure 1 the pot has a total resistance of 10kOhms, so if the pot is turned all the way to the left the resistance between the two black wires is 10kOhms.  If the pot is turned all the way to the right the amount of resistive material between the two black leads drops to zero and the resistance goes to zero as well.  Any position in the middle will give a resistance between 0 and 10kOhms.  If I'd soldered a wire to the left lead instead of the right all of this will flip: turning all the way to the left is 0Ohms and all the way to the right is 10kOhms.

One thing you might want to keep in mind when using this technique is the taper of your pot.  This pot has a B label on it, which means it is a linear taper pot.  This means the resistance of the material inside the pot is uniform for all positions of the knob.  If the knob of the pot is positioned exactly halfway between the two extremes, the resistance between the black wires equals 10kOhm/2 = 5kOhm.  Position the knob halfway between this mid point and the right extreme and the resistance between the two wires equals 10kOhm/4 = 2.5kOhm.  If the wiring is reversed (wires connected to the left and middle pin instead of right and middle) the pot is still linear but the knob positions of 0Ohm and 10kOhms have reversed.
If you have a pot with an A label on it it has a logarithmic taper: the resistive material inside the pot is not uniform.  When you move the knob from the right extreme the resistance changes dramatically then becomes almost constant as you approach the left extreme position.  So if you wire up the right and middle leads (as shown in fig 1) the dramatic change will happen as you approach 0Ohms resistance, but with the left and middle leads connected the dramatic change happens as you approach 10kOhms resistance.  See fig 3 for more detail (from http://www.beavisaudio.com/techpages/Pots/).  Depending on what you are doing you may find one preferable over the other

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    35 Discussions

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    paulanthonyrobinson

    5 weeks ago

    FYI.

    If you need to control the speed of an AC motor you can use a rheostat type device. These devices are a high voltage inductor (coil) with a wiper that controls the amount of AC power that is output through them.

    To control the speed of a DC motor, you need to use a circuit that controls the duty cycle of a given motor. This works so that the voltage going into the motor is off part of the time. The more the power going into the motor is off the slower it seems to run. This is one of the easy ways to control the speed of a DC motor. There are other ways but these are more involved than a simple 555 timer would be.

    You might have an accidental short circuit or similar that allows the current through without controlling the amount of it. this could result from several different things there in that part of the circuit. You might have the wiper and one of the other connectors backwards or you are allowing the potentiometer to go to a point where there is no resistance in that part of the circuit. To stop this from happening again all you need to do is put a resistor in the circuit just in front of the wiper of the potentiometer. This can be as small as an 8 or 10 ohm resistor so there would never be a 0 ohm resistance there.

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    This is useful information. Also, it should be noted that a potentiometer cannot be used to slow down a motor or dim an LED.

    4 replies

    A potentiometer by itself is not able to control the speed of a dc motor but if it is included in a circuit with a 555 timer then it can control a dc motor's speed through the percentage of the time that the duty cycle is high.

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    A potentiometer can slow down a motor to some extent, but still, it can not be used as a speed regulator. A pot with higher maximum resistance will stop the motor when it is just little away from the zero position.

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    ViditG4

    2 years ago

    Please help me to make a speed adjustable cpu fan please.

    1 reply
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    paulanthonyrobinsonViditG4

    Reply 5 weeks ago

    The easiest way to do this is through the use of a 555 timer circuit with a variable resistor between pins 7 and 8 on the integrated circuit. It is a circuit similar to the image included. This works to control the speed of the fan through control of the fan's duty cycle. The duty cycle controls how much of time that the fan will run or does not run. The higher the output cycle is high the faster it seems to run. The image included here is of a metronome circuit but it is still a similar principle behind what I am talking about except that the fan control circuit will run at a much higher rate.

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    dhwolfer

    2 years ago

    Here is what I need. I am trying to produce a supply that has a variable output of AC from 0 volts all the way up to 120. I would like to wire in a digital display that would actually give what the output of the terminals would be at each stage. For instance, if the POT was turned to say the middle while the input was always 125vac and then watch the output voltage on the digital display until it reaches the voltage I need at the time. I need the full spectrum of 0v to 125vac in because I will be using this as a testing platform for different things. Is there any way to do this cheaply? Thanks. I have not found anything on line that would do this. So I would appreciate your help.

    4 replies
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    paulanthonyrobinsondhwolfer

    Reply 5 weeks ago

    Your best choice is to use a rheostat type device. These devices are similar to a toroidal transformer except that they include a wiper that makes contact with the coil and uses a knob to rotate the wiper to adjust the output. Some of these units come with a voltage gauge printed on the outer shell that tells the voltage of the device and others have a built-in meter for voltage readout. If you use one of these devices then there would be no need to build one of the devices you need.

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    DavidI22dhwolfer

    Reply 2 years ago

    BTW, the unit listed only has a range of 2.5 to 30 volts so wouldn't work for your 120 volt AC application. Maybe the site has other products that would work?

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    dhwolferDavidI22

    Reply 2 years ago

    What site are you talking about? If you mean the instructables site then I have not found a solution here yet.

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    DavidI22dhwolfer

    Reply 2 years ago

    In the comments I gave a link to the meter display with a photo; the web store is called 'banggood.com'. I've ordered from them before, and have found similar parts on ebay and amazon. Good luck!

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    KishoreW2

    Question 3 months ago on Introduction

    What if your knob has 6 resistance contacts instead of 3

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    KishoreW2

    3 months ago

    Help on how to bypasss gain knob on my cerwin Vega 71000.0m single channel 1000w amp. Please help. Just want to run at lowest setting of gain

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    Yesper

    2 years ago

    Hi!

    Im want to swap two trimpots for one dual-gang potentiometer. The pot is supposed to function as a variable resistor and will need a 22k resistor in series to set a minimum resistance. How am I supposed to wire the dual-gang to have the resistor work on both "gangs"?

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    NickR63

    3 years ago

    Is there a way to wire a digital display to this so that it reads resistance if you are doing something remotely? Say if you wire it to force resistance to go up and down, but as you twist the pot, you can't see what your reading is because the device you're checking is in a different room or floor and don't want to have to go back and forth to make sure you're within a tenth of what you need to be. Kind of like one of those small LED voltmeter displays?

    1 reply
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    DavidI22NickR63

    Reply 3 years ago

    if you just want to measure a voltage, these are inexpensive: http://www.banggood.com/0_28-Inch-2_5V-30V-Mini-Digital-Voltmeter-p-974258.html

    DVM.JPG