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[Quick Help] Potentiometers

post #1 of 15
8/26/11 at 9:07pm
Thread Starter 

I dont mean to start to many new threads but, can we talk potentiometers?

 

I am looking for a dual channel with possible On/Off switch at about 50K Ω, but more importantly I would like to know what are good options for audio pots and should i avoid.

 

For exemple, I know Alps is a good brand and the RK097 is commonly used in Cmoys.

 

Also, i know that a potentiometer has 3 leads so it can increase resistance in one side and decrease the other (Used as potential dividers?). The mystery is why do you need to ground one side when you are not interested by this fancy option.

 

Thanks


Edited by TestSubject - 8/26/11 at 9:14pm
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post #2 of 15
8/27/11 at 11:14am
Thread Starter 

Anyone?

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post #3 of 15
8/27/11 at 11:45am

a good read

http://www.geofex.com/article_folders/potsecrets/potscret.htm

and

http://sound.westhost.com/pots.htm


Edited by jamesbobo007 - 8/27/11 at 11:48am
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post #4 of 15
8/27/11 at 11:56am

7 minute bump

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post #5 of 15
8/27/11 at 12:06pm

I generally prefer to keep the power lines as far from signal as possible, particularly where the power line has maximum current (100% of the current for the amp generally flows in this switch), and signal is at its weakest. These 2 goals are of course impossible to achieve when using a switched pot.

 

Unless you are building a portable amp, or counting pennies in a commercial product use separate power switch & pot. Maybe for certain tone controls too. 

 

 

If you do decide to go with a switched pot, be sure it can handle the appropriate voltage & current.

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post #6 of 15
8/27/11 at 2:58pm
Thread Starter 

 

pots-f6.gif

Quote:

Turning the shaft clockwise (CW - by convention, to move pin 2 physically closer to pin 3, and increase (for example) volume) will select a different point along the resistance element, and forms a voltage divider, so the attenuation of the signal is proportional to the rotation of the shaft. At the fully clockwise position, there is close to zero ohms in series with the signal, and the full resistance of the pot to earth. Attenuation at this setting is zero (assuming a zero or low impedance source - this is often overlooked!), and this is full volume (maximum signal level).

The source impedance should normally be no greater than 1/10th (0.1) of the pot's stated resistance. Further, the load resistance or impedance should be 10 times the pot's resistance to prevent the taper from being adversely affected. You may (of course) be deliberately loading the pot as described below, but the following stage must still present a high impedance unless its impedance has been included in your calculations.

The second form of connection is a variable resistor. Not usable as a volume control, but still extensively used for other applications. It is common (and preferable) to join two of the leads together - the wiper, and one end or the other. The actual connection depends on what you are trying to achieve, and since there are so many possibilities, I won't even try to explain them all. When used in this mode it is most commonly refererred to as a variable resistance or resistor - the word rheostat is somewhat dated (to put it mildly) and is not a term that I use in any of my articles.

 

I really dont see why the 3rd lead has to be grounded in the volume control.

 

Do you need to wire the 1rst pin to the wiper in the variable resistor to allow no resistance when the wiper is turned completely? Is that the only reason?

 

Thanks for helping out.

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post #7 of 15
8/27/11 at 3:23pm
Quote:
Originally Posted by TestSubject View Post

 

pots-f6.gif

 

I really dont see why the 3rd lead has to be grounded in the volume control.

 

Do you need to wire the 1rst pin to the wiper in the variable resistor to allow no resistance when the wiper is turned completely? Is that the only reason?

 

Thanks for helping out.



If you don't have the 3rd connection to ground, then a volume pot becomes the variable resistor.  What's not mentioned and you're not realizing is that situation presents a variable input load to the source, potentially altering the frequency response across the entire travel of the volume control.  You want something that does as little to affect the actual signal as possible - only to attenuate it.  Therefore, the volume pot uses the principle of the voltage divider to ground to keep the input resistance that the sources sees constant. Even then, as noted in your quoted reference - the difference in impedance between the source output and amplifier input must be great, or impedance will still come into play.

 

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post #8 of 15
8/27/11 at 3:25pm

Ground is the reference upon which all change in potential is based.

 

Without ground, there is no defined difference between potentials as no "common ground", so to speak, is established.

 

You cannot have a change in potential without some reference point. Ground provides that reference. 

 

EDIT: oops, TomBs explanation is far better (see above).  


Edited by jdkJake - 8/27/11 at 3:28pm
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post #9 of 15
8/27/11 at 6:23pm

Bon apetite

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post #10 of 15
8/27/11 at 6:37pm
Thread Starter 

"You cannot have a change in potential without some reference point. Ground provides that reference."

 

Yet i see resistance changing when i connect pin one and the wiper to my meter.

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post #11 of 15
8/27/11 at 6:40pm

Like we talked Pierre, without a complete path to ground, there would be no change in the input signal. There must be a divider for some change in the middle. 

Without ground, all voltage points would be the same as input.

 

 


Edited by digger945 - 8/27/11 at 6:42pm
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post #12 of 15
8/28/11 at 12:01am
Quote:
Originally Posted by TestSubject View Post

"You cannot have a change in potential without some reference point. Ground provides that reference."

 

Yet i see resistance changing when i connect pin one and the wiper to my meter.


The purpose of a pot isn't to provide a variable resistance. Sometimes they're used that way, but a pot qua pot is a variable voltage divider. For that, you need the third leg.

 

Watch this:

 

How a Potentiometer Works

 

After that, maybe this article will be of interest:

 

Notes on Audio Attenuators

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post #13 of 15
8/28/11 at 6:03pm
Thread Starter 

Thanks a lot, i understand potential divider better now.

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post #14 of 15
8/29/11 at 8:15pm
Thread Starter 

Now that i did my homework, not implying everything is 100% clear but i am working on it.

 

Still looking for a good Potentiometer with On/Off switch at about 50K Ω. It is hard in mouser because their is not category for Pot with switch.

 

Thanks a bunch

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post #15 of 15
8/30/11 at 6:07am

The reason I started carrying the 50K ALPS RK097 with the switch is that you couldn't get it or anything like it anywhere else. Since then, Mouser and others have also begun carrying this one pot. It remains unique, as far as I can tell.

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