# Multimeter and can tell which 2 pins to use to vary the resistance

1. Jan 8, 2010

### ramonegumpert

Dear experts,

I am a novice of engineering.
So, pardon my basic question about potentiometers.

I have a 3 pin pot.
I used my multimeter and can tell which 2 pins to use to vary the resistance.
To me, i see that i only need 2 pins out of the 3. I looked at the internet and it shows that the 3rd pin is connected to ground.

I am not sure must that pin be used? If not , have i wrongly used the pot?

I did not see the need for ground as a resistor has only 'in' (+) and 'out' (-) terminals.

I noticed also that i could use either of the 2 pins with the middle pin called the sweep to vary the resistance.

May i know which 2 pins to use and must the 3 pins be used if i just want to drop resistance to lower it so that the load can get the proper voltage?

sincerely
Ramone

Last edited: Jan 8, 2010
2. Jan 8, 2010

### vk6kro

Re: potentiometer

https://www.physicsforums.com/attachment.php?attachmentid=22936&d=1262955931

A potentiometer gives you much greater control than a rheostat. Not that this is always a good thing.

In the diagram above, the rheostat on the left, with the control at the top of the 10 K rheostat, the voltage across the 100 K resistor is 10 volts but at the bottom of the travel it is 9.09 volts. (100 K / 110 K ) * 10.

In the potentiometer at the right, the travel of the pot gives a range from 10 volts to zero volts.
It is a potentiometer because of the earth connection at the bottom of the 10 K pot.

Last edited: Jan 8, 2010
3. Jan 8, 2010

Re: potentiometer

Hello ramonegumpert,

Most small variable resistors are made up as three-terminal potentiometers. This enables them to be used in more different ways, in two or three terminal applications.

Note however that if your circuit does not show a connection to ground from a variable resistor, you must not connect the third terminal to ground. Doing that can cause a short-circuit.

On the other hand, where only a 2-terminal variable resistor is required, it is normally OK to connect the third pin to the wiper (= slider = sweep pin). This is sometimes recommended to help reduce problems caused by poor slider contact.

Finally, do you know how that the word potentiometer cones from a kind of slide-wire variable resistor used in a method for measuring voltages? Try looking that up on the web.

4. Jan 8, 2010

### Staff: Mentor

Re: potentiometer

Here's a very good (classic) primer on potentiometers from Bourns:

http://www.bourns.com/pdfs/trmrpmr.pdf [Broken]

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Last edited by a moderator: May 4, 2017
5. Jan 8, 2010

### ramonegumpert

Re: potentiometer

a Big thanks to all of you :
vk6kro : excellent explaination. could not be clearer, as always :) I have never knew the difference between a rheostat and a pot nor used a rheostat before.