# Linear output voltage using a potentiometer

I would like to use a linear 10K potentiometer to have a variable voltage output between 0 and 5V using a 5V input. The idea is to have the output voltage increase in a linear way as I'm turning the knob.

• If it's turned 0% I need 0V output.
• If it's turned 100% open I need 5V output.
• As I'm turning, I need to have a linear voltage output

If I hook up the left pin of the potentiometer to a 5V power supply and the middle pin to GND via an LED (to avoid a short when there is zero ohm between the 2 pins), I noticed that the output voltage is not linear.

I get high accuracy in the low range (0-2V) and it takes about a 70% turn to reach 2V, leaving only 30% to cover the remaining 5V.

So if I have 10 imaginary positions on my pot, the voltage doesn't jump with 0,5V on each position.

Can something like this be achieved with a 5V power supply ? And if not, what would be the simplest way to do it ?

• You cannot completely "avoid a short" by inserting a LED. It will happen when the output voltage reaches 2 V. To avoid it, you have to connect a resistor in series to the LED. – Circuit fantasist Dec 19 '19 at 22:13

## 3 Answers

Put 5V on one outer pin and 0V on the other outer pin. The middle pin (the wiper) gives you what you want. Now this only works when you draw no current from the output so, if you wish to have a circuit that can provide a few mA for a load you'll need to buffer the wiper connection (say) with an op-amp.

Your problem lies not in using a potentiometer, but in using an LED. An LED is behaving as a variable resistor. Below a certain voltage (1.6-3V depending on the LED color) no current is flowing though it. That's why you measure the correct voltage up to 2V. Above this voltage current starts to flow through the LED, so the observed resistance of the LED decreases. Measure the voltage at the wiper without connecting the LED and you will see that it changes linear. Any current drawn from there will create a voltage drop between the wiper and the positive potentiometer connection and reduced the voltage at the wiper.

• Exactly... All kinds of diodes can be thought as of "self variable" or "dynamic" resistors that decrease their static resistance R when the current I increases and v.v. Thus the product of R and I is constant; i.e., the voltage drop across them ­­V = I x R is constant... and they act as voltage stabilizers. – Circuit fantasist Dec 19 '19 at 22:24

The potentiometer works like Andy said.The LED is nonlinear like hli said.What is needed is a VCCS this is a Voltage Controled Current Source .The controling voltage is supplied by the pot wiper and the output current drives the LED.This could be implemented with a LM358 with one section used and a BJT or mosfet to drive the led .