3 leg potentiometer - what's the 3rd leg for?

Question

In a potentiometer, I often see the 3rd leg connected to ground.

I understand exactly what a potentiometer is physically (a wiper along a log or linear resistance), but I don't understand the benefit of connecting that 3rd leg to ground.

score 8 · Answer 1 · answered Mar 06 '15 at 05:42

8

Using the third leg turns the rheostat into a potentiometer. The whole thing acts as a voltage divider, with the moving wiper varying the ratio between the two resistances connected to the differing voltages.

answered Mar 06 '15 at 05:42

Ignacio Vazquez-Abrams

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Oh I see. I imagined another resistance in the system acting as the 3rd leg, but I can see this being very useful. Slightly ashamed for not seeing the usefulness of this :) HAHA almost made it a day without being ashamed! – SwimBikeRun Mar 06 '15 at 05:49
I don't understand how it can be a voltage divider as shown. It's just a variable resistor. Or is that what you meant to show? Look at the rheostat diagram. The one on the extreme right. The voltage at the arrow must be the same as the voltage where that leg joins the main path, towards the top of the page. Because they are joined. If that join is at earth potential, zero volts, then the wiper is at zero. Hence the whole voltage will be dropped across the first portion of the resistance, won't it? Whereas if it is not joined but goes off freely from the arrow well then it is a voltage divider – arthur Sep 27 '18 at 09:00

score 3 · Answer 2 · answered Mar 06 '15 at 05:59

schematic

^{simulate this circuit – Schematic created using CircuitLab}

A 3 terminal pot used with 3 terminals, is basically just a voltage divider. As you move the wiper, you increase one resistor in the voltage divider, while decreasing the resistance in the other. So a 3 terminal pot is a variable voltage divider.

But each section of the voltage divider can be considered to be an infinite number of resistors in series such that the total sum is the pot full scale resistance.

Now if we use this infinite resistor model, if you connect one leg of the pot and place in somewhere in within this infinite set, that means we have shorted out those resistors. The total resistance from end to end is no longer the full scale pot resistance (because we have shorted out some of those "resistors".

Connecting a wiper to one leg of the pot, is a rheostat. A rheostat is a variable resistor.

You don't have to connect one the wiper to ground. So long as the wiper is connected to one leg of the potentiometer, it will behave a variable resistor.

score 2 · Answer 3 · answered Mar 06 '15 at 05:50

Although it didn't quite make sense to me at first, i stumbled upon this page that gives information about voltage dividers. Sparkfun : Voltage dividers

Since a potentiometer consists of two resistors, just like the most basic voltage divider you see (two or three resistors along a wire, they cut voltage each time) it can be used as a voltage divider.

It makes more sense if you study the basic voltage divider before the actual potentiometer.

3 leg potentiometer - what's the 3rd leg for?

3 Answers3