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 Forum index » DIY Hardware and Software
Wiring Potentiometers
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WWBJD



Joined: Aug 14, 2006
Posts: 4
Location: Dallas, TX

PostPosted: Wed Aug 16, 2006 2:49 pm    Post subject: Wiring Potentiometers Reply with quote  Mark this post and the followings unread

I'm all confused by potentiometers.

Most of RW's schematics are easily readable, with the leads of the pots clearly labeled as to what goes where.

But, when there isn't a wire connected to the middle terminal (the sweep terminal?) what's the purpose of the pot?

Is it most common to only connect to the "input" and "output" terminals?

I've also seen situations were two of the terminals are just connected together..

Anyone out there with some kind of quick and simple explanation of wiring pots?
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bigtex



Joined: Mar 30, 2006
Posts: 321
Location: Cupertino, California

PostPosted: Wed Aug 16, 2006 3:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

Can you give an example? You are correct that there should always be something connected to the middle terminal of a potentiometer or there's no point in using one. However, it is possible that the point with no connection is one end of the potentiometer and not the wiper.

Allow me to demonstrate. First, imagine a voltage divider using two resistors as such:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

It is called a voltage divider because the two resistors are literally dividing the voltage. If they are of equal value, the output voltage will be half of the input voltage. You've probably seen such diagrams and discussion before.

If you use the same model to think of a potentiometer, you will see that it is the same as a voltage divider made of two resistors, but the two resistors are variable like so:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

As you turn the potentiometer, the wiper's connection moves along the total resistance and if R1 increases, R2 decreases, or if R1 decreases, R2 increases.

So if you see a potentiometer with one leg left unconnected, it is likely one of the ends and not the wiper arm. If you could point me to a schematic you are confused about I might be able to explain it better. In every potentiometer I've ever seen, though, it is the middle leg that is the wiper (unless it is an odd package like some of the long square precision trimmers or a slider). This is what one would look like if you opened it up:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

So if you connect the wiper to one leg, you have just the variable resistance between the two. You can ignore the other leg if all you need is a variable resistance. It would be adjusting current flow much like a faucet would adjust water flow through a pipe.

Also, I'm not sure of the theory behind connecting the wiper to one leg, but essentially it is the same as leaving one leg unconnected (but might make a small difference... not sure). You'll have a variable resistance between one end and the wiper, but not a voltage divider. In that case it is considered a rheostat and not a potentiometer, but the name isn't too important.
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Blue Hell
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Joined: Apr 03, 2004
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PostPosted: Wed Aug 16, 2006 5:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

bigtex wrote:
Also, I'm not sure of the theory behind connecting the wiper to one leg


Leaving one end unconnected or conneced to the wiper both make the potentiometer a single variable resiistor.

My theory has always been that when the wiper stops making contact at least the resistance will not jump to infinity for the case where the wiper is connected to one end.

This may help for a circuit to at least keep it sort of working even when its broken really. Imagine a situation where an infinite resistor would cause a lot of noise or hum, or where it could drive the circuit out of specs possibly damaging it.

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bigtex



Joined: Mar 30, 2006
Posts: 321
Location: Cupertino, California

PostPosted: Wed Aug 16, 2006 5:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

That makes perfect sense. I was thinking about a theoretically perfect potentiometer that wouldn't break contact at the end, but you're right, most potentiometers will lose contact right at the end. This ensures a a maximum resistance equal to the potentiometer's value, rather than the possibility of an open or infinite resistance. Good call.
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toppobrillo



Joined: Dec 10, 2005
Posts: 766
Location: oakland, ca
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PostPosted: Thu Aug 17, 2006 12:48 am    Post subject: Reply with quote  Mark this post and the followings unread

hey a good article on pots is here:

http://geofex.com/Article_Folders/potsecrets/potscret.htm

specially bout getting special tapers out of linear pots

loads of other cool stuff at this site too if you dint know it
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Uncle Krunkus
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Joined: Jul 11, 2005
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PostPosted: Thu Aug 17, 2006 5:50 am    Post subject: Reply with quote  Mark this post and the followings unread

I was thinking about that wiper to one side idea just the other day! I came up with the same answer. Cool I can't see any other possible reason for it.
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bigtex



Joined: Mar 30, 2006
Posts: 321
Location: Cupertino, California

PostPosted: Thu Aug 17, 2006 11:23 am    Post subject: Reply with quote  Mark this post and the followings unread

topp wrote:
Location: oakland, ca


Hey, you're local now. Maybe I'll run into you at a show. Such as Jean-Jaques Perrey at Recombinant Media Labs on the 29th. Yes, that Jean-Jaques Perrey. Amazing.

Um, sorry for the off-topic post... oh and that's a great article on potentiometers.
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toppobrillo



Joined: Dec 10, 2005
Posts: 766
Location: oakland, ca
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PostPosted: Thu Aug 17, 2006 1:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Hey, you're local now


yeassiree, just getting situated.. i got my workspace set up in a nice little 'nook' in my kitchen [1050$ in oakland=awesome apt] sos ill be getting back to work on my stuff real soon. met a nice gal in SF who has loaned me an oscilloscope [i sure do miss mine] and already got us a show in LA at the end of this month. lotsa stuff goin on around here.. enjoying it much!

yeah ifn i m able to make it out to the show ill let ya know, theres show this sat. here in oakland. ill get a link or somethinfg and PM it to ya

sorry for OT stuff carry on about the pots!!
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