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I know how to connect potentiometer, but to be honest I don't know why. I would really like to understand what I'm doing.

From what I have read the input voltage and the ground should be connected extreme terminals and the output to the middle one.

Now few questions:
1) What would happen if ground was switched with the output?
2) Why do we need all three terminals? Can't I just use two of them?

Edit 2014-07-11 9:47

So basically potentiometer is something like this. Few thoughts about potentiometer:
1) The total resistance of R1 and R2 is constant but we can split it as we want.
2) If we use just extreme terminals then it would behave like normal resistor.
3) It doesn't matter whether ground in connected to middle or extreme terminal.

Please correct me if I misunderstood the concept.

schematic

simulate this circuit – Schematic created using CircuitLab

Edit 2014-07-11 10:49
Basically what I'm asking in question one and thought 3 is whether following circuit is correct if I want to measure voltage in analog input? enter image description here

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Your assumptions from the EDIT are correct. –  Rev1.0 Jul 11 at 8:29
1  
It's wrong. the middle terminal of the pot (the wiper) should go to the analog input and the other two terminals should go to 5V and GND. –  EM Fields Jul 11 at 9:18
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Do NOT connect GND to the center terminal (wiper) and 5V to either end terminal. If you do, and then turn the pot to that end, you will have a low resistance/short between 5V and GND! –  DoxyLover Jul 11 at 9:56
    
think of the triple-pin layout, where two are 'on one side' together and one sticks out alone. the two which are on the same side together are the top and bottom of the two 'resistor' equivalents that you have shown in your digram, and the lone pin is the wiper, or the 'middle' point between the two variable resistors. you can make an adjustable resistor, or a cool built-in resistor divider (or really just a % of VCC (or whatever input voltage) which an ADC can read nicely. –  KyranF Jul 11 at 11:02
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2 Answers 2

up vote 1 down vote accepted

A potentiometer is just a variable resistor:

poti

-> The longer the "trace" from W to either A or B, the higher the resistance.

1) What would happen if ground was switched with the output?
2) Why do we need all three terminals? Can't I just use two of them?

In general nothing, a resistor has no polarity. However, it depends on how you intend to use it (connecting only two or all three terminals to essentially have two dependent resistors)

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You understand the concept.

Your questions:

1) What would happen if ground was switched with the output?

The input signal would be connected to ground through the top resistor and the wiper, and no signal would get to the output.

2) Why do we need all three terminals? Can't I just use two of them?

If you want the output signal voltage (the voltage on the wiper) to vary between the voltages on the other two terminals, you must use all three, but if all you want is a rheostat, you can use one terminal and the wiper.

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In the third point I've written that it doesn't matter whether ground is connected to the middle or extreme terminal and you answered to my first question that if ground was switched with the output then no signal would get to the output. Unless I'm missing something those two statements are in contradiction. –  Tomek Tarczynski Jul 11 at 8:37
    
You're missing the difference between a potentiometer and a rheostat, where a potentiometer is a three-terminal device and a rheostat is a two-terminal device. –  EM Fields Jul 11 at 9:02
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